diff --git a/include/gui/ISurfaceComposer.h b/include/gui/ISurfaceComposer.h
index 824e5c4..ff04e2f 100644
--- a/include/gui/ISurfaceComposer.h
+++ b/include/gui/ISurfaceComposer.h
@@ -157,7 +157,7 @@
     virtual status_t captureScreen(const sp<IBinder>& display,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform,
             Rotation rotation = eRotateNone) = 0;
 
diff --git a/include/gui/ISurfaceComposerClient.h b/include/gui/ISurfaceComposerClient.h
index 4a4efb6..0a4d35d 100644
--- a/include/gui/ISurfaceComposerClient.h
+++ b/include/gui/ISurfaceComposerClient.h
@@ -60,6 +60,7 @@
     virtual status_t createSurface(
             const String8& name, uint32_t w, uint32_t h,
             PixelFormat format, uint32_t flags,
+            const sp<IBinder>& parent,
             sp<IBinder>* handle,
             sp<IGraphicBufferProducer>* gbp) = 0;
 
diff --git a/include/gui/SurfaceComposerClient.h b/include/gui/SurfaceComposerClient.h
index 183b42d..789dc7c 100644
--- a/include/gui/SurfaceComposerClient.h
+++ b/include/gui/SurfaceComposerClient.h
@@ -105,7 +105,8 @@
             uint32_t w,         // width in pixel
             uint32_t h,         // height in pixel
             PixelFormat format, // pixel-format desired
-            uint32_t flags = 0  // usage flags
+            uint32_t flags = 0,  // usage flags
+            SurfaceControl* parent = nullptr // parent
     );
 
     //! Create a virtual display
@@ -142,7 +143,7 @@
     status_t    show(const sp<IBinder>& id);
     status_t    setFlags(const sp<IBinder>& id, uint32_t flags, uint32_t mask);
     status_t    setTransparentRegionHint(const sp<IBinder>& id, const Region& transparent);
-    status_t    setLayer(const sp<IBinder>& id, uint32_t layer);
+    status_t    setLayer(const sp<IBinder>& id, int32_t layer);
     status_t    setLayerInfo(const sp<IBinder>& id, uint32_t type, uint32_t appid);
     status_t    setAlpha(const sp<IBinder>& id, float alpha=1.0f);
     status_t    setMatrix(const sp<IBinder>& id, float dsdx, float dtdx, float dsdy, float dtdy);
@@ -213,12 +214,12 @@
             const sp<IBinder>& display,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform);
     static status_t captureToBuffer(
             const sp<IBinder>& display,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform,
             uint32_t rotation,
             sp<GraphicBuffer>* outbuffer);
@@ -242,11 +243,11 @@
             bool useIdentityTransform);
     status_t update(const sp<IBinder>& display,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform);
     status_t update(const sp<IBinder>& display,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform, uint32_t rotation);
 
     sp<CpuConsumer> getCpuConsumer() const;
diff --git a/include/gui/SurfaceControl.h b/include/gui/SurfaceControl.h
index 456bde4..62217ad 100644
--- a/include/gui/SurfaceControl.h
+++ b/include/gui/SurfaceControl.h
@@ -61,7 +61,7 @@
     void        disconnect();
 
     status_t    setLayerStack(uint32_t layerStack);
-    status_t    setLayer(uint32_t layer);
+    status_t    setLayer(int32_t layer);
     status_t    setLayerInfo(uint32_t type, uint32_t appid);
     status_t    setPosition(float x, float y);
     status_t    setSize(uint32_t w, uint32_t h);
diff --git a/include/private/gui/LayerState.h b/include/private/gui/LayerState.h
index b648751..b3481d6 100644
--- a/include/private/gui/LayerState.h
+++ b/include/private/gui/LayerState.h
@@ -84,7 +84,7 @@
             uint32_t        what;
             float           x;
             float           y;
-            uint32_t        z;
+            int32_t         z;
             uint32_t        w;
             uint32_t        h;
             uint32_t        layerStack;
diff --git a/libs/gui/ISurfaceComposer.cpp b/libs/gui/ISurfaceComposer.cpp
index fa2f59a..74c3bed 100644
--- a/libs/gui/ISurfaceComposer.cpp
+++ b/libs/gui/ISurfaceComposer.cpp
@@ -104,7 +104,7 @@
     virtual status_t captureScreen(const sp<IBinder>& display,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform,
             ISurfaceComposer::Rotation rotation)
     {
@@ -115,8 +115,8 @@
         data.write(sourceCrop);
         data.writeUint32(reqWidth);
         data.writeUint32(reqHeight);
-        data.writeUint32(minLayerZ);
-        data.writeUint32(maxLayerZ);
+        data.writeInt32(minLayerZ);
+        data.writeInt32(maxLayerZ);
         data.writeInt32(static_cast<int32_t>(useIdentityTransform));
         data.writeInt32(static_cast<int32_t>(rotation));
         remote()->transact(BnSurfaceComposer::CAPTURE_SCREEN, data, &reply);
@@ -544,8 +544,8 @@
             data.read(sourceCrop);
             uint32_t reqWidth = data.readUint32();
             uint32_t reqHeight = data.readUint32();
-            uint32_t minLayerZ = data.readUint32();
-            uint32_t maxLayerZ = data.readUint32();
+            int32_t minLayerZ = data.readInt32();
+            int32_t maxLayerZ = data.readInt32();
             bool useIdentityTransform = static_cast<bool>(data.readInt32());
             int32_t rotation = data.readInt32();
 
diff --git a/libs/gui/ISurfaceComposerClient.cpp b/libs/gui/ISurfaceComposerClient.cpp
index 47cb047..b2036dc 100644
--- a/libs/gui/ISurfaceComposerClient.cpp
+++ b/libs/gui/ISurfaceComposerClient.cpp
@@ -56,7 +56,7 @@
 
     virtual status_t createSurface(const String8& name, uint32_t width,
             uint32_t height, PixelFormat format, uint32_t flags,
-            sp<IBinder>* handle,
+            const sp<IBinder>& parent, sp<IBinder>* handle,
             sp<IGraphicBufferProducer>* gbp) {
         Parcel data, reply;
         data.writeInterfaceToken(ISurfaceComposerClient::getInterfaceDescriptor());
@@ -65,6 +65,9 @@
         data.writeUint32(height);
         data.writeInt32(static_cast<int32_t>(format));
         data.writeUint32(flags);
+        if (parent != nullptr) {
+            data.writeStrongBinder(parent);
+        }
         remote()->transact(CREATE_SURFACE, data, &reply);
         *handle = reply.readStrongBinder();
         *gbp = interface_cast<IGraphicBufferProducer>(reply.readStrongBinder());
@@ -145,10 +148,14 @@
             uint32_t height = data.readUint32();
             PixelFormat format = static_cast<PixelFormat>(data.readInt32());
             uint32_t createFlags = data.readUint32();
+            sp<IBinder> parent = nullptr;
+            if (data.dataAvail() > 0) {
+                parent = data.readStrongBinder();
+            }
             sp<IBinder> handle;
             sp<IGraphicBufferProducer> gbp;
             status_t result = createSurface(name, width, height, format,
-                    createFlags, &handle, &gbp);
+                    createFlags, parent, &handle, &gbp);
             reply->writeStrongBinder(handle);
             reply->writeStrongBinder(IInterface::asBinder(gbp));
             reply->writeInt32(result);
diff --git a/libs/gui/LayerState.cpp b/libs/gui/LayerState.cpp
index 3949186..16bf324 100644
--- a/libs/gui/LayerState.cpp
+++ b/libs/gui/LayerState.cpp
@@ -28,7 +28,7 @@
     output.writeUint32(what);
     output.writeFloat(x);
     output.writeFloat(y);
-    output.writeUint32(z);
+    output.writeInt32(z);
     output.writeUint32(w);
     output.writeUint32(h);
     output.writeUint32(layerStack);
@@ -54,7 +54,7 @@
     what = input.readUint32();
     x = input.readFloat();
     y = input.readFloat();
-    z = input.readUint32();
+    z = input.readInt32();
     w = input.readUint32();
     h = input.readUint32();
     layerStack = input.readUint32();
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index 3346a83..c2be6eb 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -148,7 +148,7 @@
     status_t setSize(const sp<SurfaceComposerClient>& client, const sp<IBinder>& id,
             uint32_t w, uint32_t h);
     status_t setLayer(const sp<SurfaceComposerClient>& client, const sp<IBinder>& id,
-            uint32_t z);
+            int32_t z);
     status_t setLayerInfo(const sp<SurfaceComposerClient>& client, const sp<IBinder>& id,
             uint32_t type, uint32_t appid);
     status_t setFlags(const sp<SurfaceComposerClient>& client, const sp<IBinder>& id,
@@ -327,7 +327,7 @@
 }
 
 status_t Composer::setLayer(const sp<SurfaceComposerClient>& client,
-        const sp<IBinder>& id, uint32_t z) {
+        const sp<IBinder>& id, int32_t z) {
     Mutex::Autolock _l(mLock);
     layer_state_t* s = getLayerStateLocked(client, id);
     if (!s)
@@ -610,13 +610,19 @@
         uint32_t w,
         uint32_t h,
         PixelFormat format,
-        uint32_t flags)
+        uint32_t flags,
+        SurfaceControl* parent)
 {
     sp<SurfaceControl> sur;
     if (mStatus == NO_ERROR) {
         sp<IBinder> handle;
+        sp<IBinder> parentHandle;
         sp<IGraphicBufferProducer> gbp;
-        status_t err = mClient->createSurface(name, w, h, format, flags,
+
+        if (parent != nullptr) {
+            parentHandle = parent->getHandle();
+        }
+        status_t err = mClient->createSurface(name, w, h, format, flags, parentHandle,
                 &handle, &gbp);
         ALOGE_IF(err, "SurfaceComposerClient::createSurface error %s", strerror(-err));
         if (err == NO_ERROR) {
@@ -714,7 +720,7 @@
     return getComposer().setSize(this, id, w, h);
 }
 
-status_t SurfaceComposerClient::setLayer(const sp<IBinder>& id, uint32_t z) {
+status_t SurfaceComposerClient::setLayer(const sp<IBinder>& id, int32_t z) {
     return getComposer().setLayer(this, id, z);
 }
 
@@ -871,7 +877,7 @@
         const sp<IBinder>& display,
         const sp<IGraphicBufferProducer>& producer,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ, bool useIdentityTransform) {
+        int32_t minLayerZ, int32_t maxLayerZ, bool useIdentityTransform) {
     sp<ISurfaceComposer> s(ComposerService::getComposerService());
     if (s == NULL) return NO_INIT;
     return s->captureScreen(display, producer, sourceCrop,
@@ -880,7 +886,7 @@
 
 status_t ScreenshotClient::captureToBuffer(const sp<IBinder>& display,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ, bool useIdentityTransform,
+        int32_t minLayerZ, int32_t maxLayerZ, bool useIdentityTransform,
         uint32_t rotation,
         sp<GraphicBuffer>* outBuffer) {
     sp<ISurfaceComposer> s(ComposerService::getComposerService());
@@ -926,7 +932,7 @@
 
 status_t ScreenshotClient::update(const sp<IBinder>& display,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform, uint32_t rotation) {
     sp<ISurfaceComposer> s(ComposerService::getComposerService());
     if (s == NULL) return NO_INIT;
@@ -953,7 +959,7 @@
 
 status_t ScreenshotClient::update(const sp<IBinder>& display,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform) {
 
     return ScreenshotClient::update(display, sourceCrop, reqWidth, reqHeight,
@@ -962,14 +968,16 @@
 
 status_t ScreenshotClient::update(const sp<IBinder>& display, Rect sourceCrop,
         bool useIdentityTransform) {
-    return ScreenshotClient::update(display, sourceCrop, 0, 0, 0, -1U,
+    return ScreenshotClient::update(display, sourceCrop, 0, 0,
+            INT32_MIN, INT32_MAX,
             useIdentityTransform, ISurfaceComposer::eRotateNone);
 }
 
 status_t ScreenshotClient::update(const sp<IBinder>& display, Rect sourceCrop,
         uint32_t reqWidth, uint32_t reqHeight, bool useIdentityTransform) {
     return ScreenshotClient::update(display, sourceCrop, reqWidth, reqHeight,
-            0, -1U, useIdentityTransform, ISurfaceComposer::eRotateNone);
+            INT32_MIN, INT32_MAX,
+            useIdentityTransform, ISurfaceComposer::eRotateNone);
 }
 
 void ScreenshotClient::release() {
diff --git a/libs/gui/SurfaceControl.cpp b/libs/gui/SurfaceControl.cpp
index b47e434..c82f6a7 100644
--- a/libs/gui/SurfaceControl.cpp
+++ b/libs/gui/SurfaceControl.cpp
@@ -102,7 +102,7 @@
     if (err < 0) return err;
     return mClient->setLayerStack(mHandle, layerStack);
 }
-status_t SurfaceControl::setLayer(uint32_t layer) {
+status_t SurfaceControl::setLayer(int32_t layer) {
     status_t err = validate();
     if (err < 0) return err;
     return mClient->setLayer(mHandle, layer);
diff --git a/libs/gui/tests/Surface_test.cpp b/libs/gui/tests/Surface_test.cpp
index e40b4eb..7b78175 100644
--- a/libs/gui/tests/Surface_test.cpp
+++ b/libs/gui/tests/Surface_test.cpp
@@ -398,7 +398,7 @@
     status_t captureScreen(const sp<IBinder>& /*display*/,
             const sp<IGraphicBufferProducer>& /*producer*/,
             Rect /*sourceCrop*/, uint32_t /*reqWidth*/, uint32_t /*reqHeight*/,
-            uint32_t /*minLayerZ*/, uint32_t /*maxLayerZ*/,
+            int32_t /*minLayerZ*/, int32_t /*maxLayerZ*/,
             bool /*useIdentityTransform*/,
             Rotation /*rotation*/) override { return NO_ERROR; }
     status_t clearAnimationFrameStats() override { return NO_ERROR; }
diff --git a/services/surfaceflinger/Android.mk b/services/surfaceflinger/Android.mk
index fcc9241..1cd9215 100644
--- a/services/surfaceflinger/Android.mk
+++ b/services/surfaceflinger/Android.mk
@@ -15,6 +15,7 @@
     Layer.cpp \
     LayerDim.cpp \
     LayerRejecter.cpp \
+    LayerVector.cpp \
     MessageQueue.cpp \
     MonitoredProducer.cpp \
     SurfaceFlingerConsumer.cpp \
diff --git a/services/surfaceflinger/Client.cpp b/services/surfaceflinger/Client.cpp
index e14a59b..7e04fda 100644
--- a/services/surfaceflinger/Client.cpp
+++ b/services/surfaceflinger/Client.cpp
@@ -106,14 +106,22 @@
 status_t Client::createSurface(
         const String8& name,
         uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
+        const sp<IBinder>& parentHandle,
         sp<IBinder>* handle,
         sp<IGraphicBufferProducer>* gbp)
 {
+    sp<Layer> parent = nullptr;
+    if (parentHandle != nullptr) {
+        parent = getLayerUser(parentHandle);
+        if (parent == nullptr) {
+            return NAME_NOT_FOUND;
+        }
+    }
+
     /*
      * createSurface must be called from the GL thread so that it can
      * have access to the GL context.
      */
-
     class MessageCreateLayer : public MessageBase {
         SurfaceFlinger* flinger;
         Client* client;
@@ -124,26 +132,29 @@
         uint32_t w, h;
         PixelFormat format;
         uint32_t flags;
+        sp<Layer>* parent;
     public:
         MessageCreateLayer(SurfaceFlinger* flinger,
                 const String8& name, Client* client,
                 uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
                 sp<IBinder>* handle,
-                sp<IGraphicBufferProducer>* gbp)
+                sp<IGraphicBufferProducer>* gbp,
+                sp<Layer>* parent)
             : flinger(flinger), client(client),
               handle(handle), gbp(gbp), result(NO_ERROR),
-              name(name), w(w), h(h), format(format), flags(flags) {
+              name(name), w(w), h(h), format(format), flags(flags),
+              parent(parent) {
         }
         status_t getResult() const { return result; }
         virtual bool handler() {
             result = flinger->createLayer(name, client, w, h, format, flags,
-                    handle, gbp);
+                    handle, gbp, parent);
             return true;
         }
     };
 
     sp<MessageBase> msg = new MessageCreateLayer(mFlinger.get(),
-            name, this, w, h, format, flags, handle, gbp);
+            name, this, w, h, format, flags, handle, gbp, &parent);
     mFlinger->postMessageSync(msg);
     return static_cast<MessageCreateLayer*>( msg.get() )->getResult();
 }
diff --git a/services/surfaceflinger/Client.h b/services/surfaceflinger/Client.h
index 9c7d050..2a9825c 100644
--- a/services/surfaceflinger/Client.h
+++ b/services/surfaceflinger/Client.h
@@ -55,6 +55,7 @@
     virtual status_t createSurface(
             const String8& name,
             uint32_t w, uint32_t h,PixelFormat format, uint32_t flags,
+            const sp<IBinder>& parent,
             sp<IBinder>* handle,
             sp<IGraphicBufferProducer>* gbp);
 
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index b388448..6fd9cd7 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -280,6 +280,9 @@
 // it's removed from the drawing state list)
 void Layer::onRemoved() {
     mSurfaceFlingerConsumer->abandon();
+    for (const auto& child : mCurrentChildren) {
+        child->onRemoved();
+    }
 }
 
 // ---------------------------------------------------------------------------
@@ -362,6 +365,40 @@
     return Region(win).subtract(exclude).getBounds();
 }
 
+Rect Layer::computeScreenBounds(bool reduceTransparentRegion) const {
+    const Layer::State& s(getDrawingState());
+    Rect win(s.active.w, s.active.h);
+
+    if (!s.crop.isEmpty()) {
+        win.intersect(s.crop, &win);
+    }
+
+    Transform t = getTransform();
+    win = t.transform(win);
+
+    const sp<Layer>& p = getParent();
+    // Now we need to calculate the parent bounds, so we can clip ourselves to those.
+    // When calculating the parent bounds for purposes of clipping,
+    // we don't need to constrain the parent to its transparent region.
+    // The transparent region is an optimization based on the
+    // buffer contents of the layer, but does not affect the space allocated to
+    // it by policy, and thus children should be allowed to extend into the
+    // parent's transparent region. In fact one of the main uses, is to reduce
+    // buffer allocation size in cases where a child window sits behind a main window
+    // (by marking the hole in the parent window as a transparent region)
+    if (p != nullptr) {
+        Rect bounds = p->computeScreenBounds(false);
+        bounds.intersect(win, &win);
+    }
+
+    if (reduceTransparentRegion) {
+        auto const screenTransparentRegion = t.transform(s.activeTransparentRegion);
+        win = reduce(win, screenTransparentRegion);
+    }
+
+    return win;
+}
+
 Rect Layer::computeBounds() const {
     const Layer::State& s(getDrawingState());
     return computeBounds(s.activeTransparentRegion);
@@ -374,15 +411,25 @@
     if (!s.crop.isEmpty()) {
         win.intersect(s.crop, &win);
     }
+
+    Rect bounds = win;
+    const auto& p = getParent();
+    if (p != nullptr) {
+        bounds = p->computeScreenBounds();
+    }
+
+    Transform t = getTransform();
+    if (p != nullptr) {
+        win = t.transform(win);
+        win.intersect(bounds, &win);
+        win = t.inverse().transform(win);
+    }
+
     // subtract the transparent region and snap to the bounds
     return reduce(win, activeTransparentRegion);
 }
 
-gfx::FloatRect Layer::computeCrop(const sp<const DisplayDevice>& hw) const {
-    // the content crop is the area of the content that gets scaled to the
-    // layer's size.
-    gfx::FloatRect crop = getContentCrop().toFloatRect();
-
+Rect Layer::computeInitialCrop(const sp<const DisplayDevice>& hw) const {
     // the crop is the area of the window that gets cropped, but not
     // scaled in any ways.
     const State& s(getDrawingState());
@@ -399,7 +446,8 @@
         activeCrop = s.crop;
     }
 
-    activeCrop = s.active.transform.transform(activeCrop);
+    Transform t = getTransform();
+    activeCrop = t.transform(activeCrop);
     if (!activeCrop.intersect(hw->getViewport(), &activeCrop)) {
         activeCrop.clear();
     }
@@ -408,7 +456,27 @@
             activeCrop.clear();
         }
     }
-    activeCrop = s.active.transform.inverse().transform(activeCrop);
+    return activeCrop;
+}
+
+gfx::FloatRect Layer::computeCrop(const sp<const DisplayDevice>& hw) const {
+    // the content crop is the area of the content that gets scaled to the
+    // layer's size. This is in buffer space.
+    gfx::FloatRect crop = getContentCrop().toFloatRect();
+
+    // In addition there is a WM-specified crop we pull from our drawing state.
+    const State& s(getDrawingState());
+
+    // Screen space to make reduction to parent crop clearer.
+    Rect activeCrop = computeInitialCrop(hw);
+    const auto& p = getParent();
+    if (p != nullptr) {
+        auto parentCrop = p->computeInitialCrop(hw);
+        activeCrop.intersect(parentCrop, &activeCrop);
+    }
+    Transform t = getTransform();
+    // Back to layer space to work with the content crop.
+    activeCrop = t.inverse().transform(activeCrop);
 
     // This needs to be here as transform.transform(Rect) computes the
     // transformed rect and then takes the bounding box of the result before
@@ -482,7 +550,7 @@
 }
 
 #ifdef USE_HWC2
-void Layer::setGeometry(const sp<const DisplayDevice>& displayDevice)
+void Layer::setGeometry(const sp<const DisplayDevice>& displayDevice, uint32_t z)
 #else
 void Layer::setGeometry(
     const sp<const DisplayDevice>& hw,
@@ -535,9 +603,10 @@
     // apply the layer's transform, followed by the display's global transform
     // here we're guaranteed that the layer's transform preserves rects
     Region activeTransparentRegion(s.activeTransparentRegion);
+    Transform t = getTransform();
     if (!s.crop.isEmpty()) {
         Rect activeCrop(s.crop);
-        activeCrop = s.active.transform.transform(activeCrop);
+        activeCrop = t.transform(activeCrop);
 #ifdef USE_HWC2
         if(!activeCrop.intersect(displayDevice->getViewport(), &activeCrop)) {
 #else
@@ -545,7 +614,7 @@
 #endif
             activeCrop.clear();
         }
-        activeCrop = s.active.transform.inverse().transform(activeCrop, true);
+        activeCrop = t.inverse().transform(activeCrop, true);
         // This needs to be here as transform.transform(Rect) computes the
         // transformed rect and then takes the bounding box of the result before
         // returning. This means
@@ -564,7 +633,8 @@
         activeTransparentRegion.orSelf(Rect(activeCrop.right, activeCrop.top,
                 s.active.w, activeCrop.bottom));
     }
-    Rect frame(s.active.transform.transform(computeBounds(activeTransparentRegion)));
+
+    Rect frame(t.transform(computeBounds(activeTransparentRegion)));
     if (!s.finalCrop.isEmpty()) {
         if(!frame.intersect(s.finalCrop, &frame)) {
             frame.clear();
@@ -602,9 +672,9 @@
             "%s (%d)", mName.string(), s.alpha, to_string(error).c_str(),
             static_cast<int32_t>(error));
 
-    error = hwcLayer->setZOrder(s.z);
+    error = hwcLayer->setZOrder(z);
     ALOGE_IF(error != HWC2::Error::None, "[%s] Failed to set Z %u: %s (%d)",
-            mName.string(), s.z, to_string(error).c_str(),
+            mName.string(), z, to_string(error).c_str(),
             static_cast<int32_t>(error));
 #else
     if (!frame.intersect(hw->getViewport(), &frame)) {
@@ -625,7 +695,7 @@
      */
 
     const Transform bufferOrientation(mCurrentTransform);
-    Transform transform(tr * s.active.transform * bufferOrientation);
+    Transform transform(tr * t * bufferOrientation);
 
     if (mSurfaceFlingerConsumer->getTransformToDisplayInverse()) {
         /*
@@ -811,7 +881,7 @@
     }
     // Subtract the transparent region and snap to the bounds
     Rect bounds = reduce(win, s.activeTransparentRegion);
-    Rect frame(s.active.transform.transform(bounds));
+    Rect frame(getTransform().transform(bounds));
     frame.intersect(displayDevice->getViewport(), &frame);
     if (!s.finalCrop.isEmpty()) {
         frame.intersect(s.finalCrop, &frame);
@@ -860,7 +930,7 @@
     }
     // subtract the transparent region and snap to the bounds
     Rect bounds = reduce(win, s.activeTransparentRegion);
-    Rect frame(s.active.transform.transform(bounds));
+    Rect frame(getTransform().transform(bounds));
     frame.intersect(hw->getViewport(), &frame);
     if (!s.finalCrop.isEmpty()) {
         frame.intersect(s.finalCrop, &frame);
@@ -903,15 +973,14 @@
 
         // figure out if there is something below us
         Region under;
-        const SurfaceFlinger::LayerVector& drawingLayers(
-                mFlinger->mDrawingState.layersSortedByZ);
-        const size_t count = drawingLayers.size();
-        for (size_t i=0 ; i<count ; ++i) {
-            const sp<Layer>& layer(drawingLayers[i]);
-            if (layer.get() == static_cast<Layer const*>(this))
-                break;
+        bool finished = false;
+        mFlinger->mDrawingState.layersSortedByZ.traverseInZOrder([&](Layer* layer) {
+            if (finished || layer == static_cast<Layer const*>(this)) {
+                finished = true;
+                return;
+            }
             under.orSelf( hw->getTransform().transform(layer->visibleRegion) );
-        }
+        });
         // if not everything below us is covered, we plug the holes!
         Region holes(clip.subtract(under));
         if (!holes.isEmpty()) {
@@ -1023,12 +1092,13 @@
      */
     Rect win(computeBounds());
 
+    Transform t = getTransform();
     if (!s.finalCrop.isEmpty()) {
-        win = s.active.transform.transform(win);
+        win = t.transform(win);
         if (!win.intersect(s.finalCrop, &win)) {
             win.clear();
         }
-        win = s.active.transform.inverse().transform(win);
+        win = t.inverse().transform(win);
         if (!win.intersect(computeBounds(), &win)) {
             win.clear();
         }
@@ -1228,25 +1298,21 @@
         bool useIdentityTransform) const
 {
     const Layer::State& s(getDrawingState());
-    const Transform tr(hw->getTransform());
+    const Transform hwTransform(hw->getTransform());
     const uint32_t hw_h = hw->getHeight();
-    Rect win(s.active.w, s.active.h);
-    if (!s.crop.isEmpty()) {
-        win.intersect(s.crop, &win);
-    }
-    // subtract the transparent region and snap to the bounds
-    win = reduce(win, s.activeTransparentRegion);
+    Rect win = computeBounds();
 
     vec2 lt = vec2(win.left, win.top);
     vec2 lb = vec2(win.left, win.bottom);
     vec2 rb = vec2(win.right, win.bottom);
     vec2 rt = vec2(win.right, win.top);
 
+    Transform layerTransform = getTransform();
     if (!useIdentityTransform) {
-        lt = s.active.transform.transform(lt);
-        lb = s.active.transform.transform(lb);
-        rb = s.active.transform.transform(rb);
-        rt = s.active.transform.transform(rt);
+        lt = layerTransform.transform(lt);
+        lb = layerTransform.transform(lb);
+        rb = layerTransform.transform(rb);
+        rt = layerTransform.transform(rt);
     }
 
     if (!s.finalCrop.isEmpty()) {
@@ -1257,10 +1323,10 @@
     }
 
     Mesh::VertexArray<vec2> position(mesh.getPositionArray<vec2>());
-    position[0] = tr.transform(lt);
-    position[1] = tr.transform(lb);
-    position[2] = tr.transform(rb);
-    position[3] = tr.transform(rt);
+    position[0] = hwTransform.transform(lt);
+    position[1] = hwTransform.transform(lb);
+    position[2] = hwTransform.transform(rb);
+    position[3] = hwTransform.transform(rt);
     for (size_t i=0 ; i<4 ; i++) {
         position[i].y = hw_h - position[i].y;
     }
@@ -1542,11 +1608,7 @@
     // that transactions for layers depending on this layer's frames becoming
     // visible are not blocked
     if (c.flags & layer_state_t::eLayerHidden) {
-        Mutex::Autolock lock(mLocalSyncPointMutex);
-        for (auto& point : mLocalSyncPoints) {
-            point->setFrameAvailable();
-        }
-        mLocalSyncPoints.clear();
+        clearSyncPoints();
     }
 
     // Commit the transaction
@@ -1585,7 +1647,19 @@
     return true;
 }
 
-bool Layer::setLayer(uint32_t z) {
+bool Layer::setChildLayer(const sp<Layer>& childLayer, int32_t z) {
+    ssize_t idx = mCurrentChildren.indexOf(childLayer);
+    if (idx < 0) {
+        return false;
+    }
+    if (childLayer->setLayer(z)) {
+        mCurrentChildren.removeAt(idx);
+        mCurrentChildren.add(childLayer);
+    }
+    return true;
+}
+
+bool Layer::setLayer(int32_t z) {
     if (mCurrentState.z == z)
         return false;
     mCurrentState.sequence++;
@@ -1594,6 +1668,7 @@
     setTransactionFlags(eTransactionNeeded);
     return true;
 }
+
 bool Layer::setSize(uint32_t w, uint32_t h) {
     if (mCurrentState.requested.w == w && mCurrentState.requested.h == h)
         return false;
@@ -1699,6 +1774,14 @@
     return true;
 }
 
+uint32_t Layer::getLayerStack() const {
+    auto p = getParent();
+    if (p == nullptr) {
+        return getDrawingState().layerStack;
+    }
+    return p->getLayerStack();
+}
+
 void Layer::deferTransactionUntil(const sp<IBinder>& handle,
         uint64_t frameNumber) {
     mCurrentState.handle = handle;
@@ -1827,13 +1910,22 @@
 }
 #endif
 
+bool Layer::isHiddenByPolicy() const {
+    const Layer::State& s(mDrawingState);
+    const auto& parent = getParent();
+    if (parent != nullptr && parent->isHiddenByPolicy()) {
+        return true;
+    }
+    return s.flags & layer_state_t::eLayerHidden;
+}
+
 bool Layer::isVisible() const {
     const Layer::State& s(mDrawingState);
 #ifdef USE_HWC2
-    return !(s.flags & layer_state_t::eLayerHidden) && s.alpha > 0.0f
+    return !(isHiddenByPolicy()) && s.alpha > 0.0f
             && (mActiveBuffer != NULL || mSidebandStream != NULL);
 #else
-    return !(s.flags & layer_state_t::eLayerHidden) && s.alpha
+    return !(isHiddenByPolicy()) && s.alpha
             && (mActiveBuffer != NULL || mSidebandStream != NULL);
 #endif
 }
@@ -1878,7 +1970,7 @@
         recomputeVisibleRegions = true;
 
         const State& s(getDrawingState());
-        return s.active.transform.transform(Region(Rect(s.active.w, s.active.h)));
+        return getTransform().transform(Region(Rect(s.active.w, s.active.h)));
     }
 
     Region outDirtyRegion;
@@ -2067,7 +2159,7 @@
     Region dirtyRegion(Rect(s.active.w, s.active.h));
 
     // transform the dirty region to window-manager space
-    outDirtyRegion = (s.active.transform.transform(dirtyRegion));
+    outDirtyRegion = (getTransform().transform(dirtyRegion));
 
     return outDirtyRegion;
 }
@@ -2130,7 +2222,9 @@
             "alpha=0x%02x, flags=0x%08x, tr=[%.2f, %.2f][%.2f, %.2f]\n"
 #endif
             "      client=%p\n",
-            s.layerStack, s.z, s.active.transform.tx(), s.active.transform.ty(), s.active.w, s.active.h,
+            getLayerStack(), s.z,
+            s.active.transform.tx(), s.active.transform.ty(),
+            s.active.w, s.active.h,
             s.crop.left, s.crop.top,
             s.crop.right, s.crop.bottom,
             s.finalCrop.left, s.finalCrop.top,
@@ -2263,6 +2357,90 @@
     return mSurfaceFlingerConsumer->getTransformToDisplayInverse();
 }
 
+void Layer::addChild(const sp<Layer>& layer) {
+    mCurrentChildren.add(layer);
+    layer->setParent(this);
+}
+
+ssize_t Layer::removeChild(const sp<Layer>& layer) {
+    layer->setParent(nullptr);
+    return mCurrentChildren.remove(layer);
+}
+
+void Layer::setParent(const sp<Layer>& layer) {
+    mParent = layer;
+}
+
+void Layer::clearSyncPoints() {
+    for (const auto& child : mCurrentChildren) {
+        child->clearSyncPoints();
+    }
+
+    Mutex::Autolock lock(mLocalSyncPointMutex);
+    for (auto& point : mLocalSyncPoints) {
+        point->setFrameAvailable();
+    }
+    mLocalSyncPoints.clear();
+}
+
+int32_t Layer::getZ() const {
+    return mDrawingState.z;
+}
+
+/**
+ * Negatively signed children are before 'this' in Z-order.
+ */
+void Layer::traverseInZOrder(const std::function<void(Layer*)>& exec) {
+    size_t i = 0;
+    for (; i < mDrawingChildren.size(); i++) {
+        const auto& child = mDrawingChildren[i];
+        if (child->getZ() >= 0)
+            break;
+        child->traverseInZOrder(exec);
+    }
+    exec(this);
+    for (; i < mDrawingChildren.size(); i++) {
+        const auto& child = mDrawingChildren[i];
+        child->traverseInZOrder(exec);
+    }
+}
+
+/**
+ * Positively signed children are before 'this' in reverse Z-order.
+ */
+void Layer::traverseInReverseZOrder(const std::function<void(Layer*)>& exec) {
+    int32_t i = 0;
+    for (i = mDrawingChildren.size()-1; i>=0; i--) {
+        const auto& child = mDrawingChildren[i];
+        if (child->getZ() < 0) {
+            break;
+        }
+        child->traverseInReverseZOrder(exec);
+    }
+    exec(this);
+    for (; i>=0; i--) {
+        const auto& child = mDrawingChildren[i];
+        child->traverseInReverseZOrder(exec);
+    }
+}
+
+Transform Layer::getTransform() const {
+    Transform t;
+    const auto& p = getParent();
+    if (p != nullptr) {
+        t = p->getTransform();
+    }
+    return t * getDrawingState().active.transform;
+}
+
+void Layer::commitChildList() {
+    for (size_t i = 0; i < mCurrentChildren.size(); i++) {
+        const auto& child = mCurrentChildren[i];
+        child->commitChildList();
+    }
+    mDrawingChildren = mCurrentChildren;
+}
+
 // ---------------------------------------------------------------------------
 
 }; // namespace android
diff --git a/services/surfaceflinger/Layer.h b/services/surfaceflinger/Layer.h
index 1654a51..bf224ae 100644
--- a/services/surfaceflinger/Layer.h
+++ b/services/surfaceflinger/Layer.h
@@ -42,6 +42,7 @@
 
 #include "FrameTracker.h"
 #include "Client.h"
+#include "LayerVector.h"
 #include "MonitoredProducer.h"
 #include "SurfaceFlinger.h"
 #include "SurfaceFlingerConsumer.h"
@@ -109,7 +110,7 @@
     struct State {
         Geometry active;
         Geometry requested;
-        uint32_t z;
+        int32_t z;
         uint32_t layerStack;
 #ifdef USE_HWC2
         float alpha;
@@ -154,7 +155,7 @@
 
     // modify current state
     bool setPosition(float x, float y, bool immediate);
-    bool setLayer(uint32_t z);
+    bool setLayer(int32_t z);
     bool setSize(uint32_t w, uint32_t h);
 #ifdef USE_HWC2
     bool setAlpha(float alpha);
@@ -168,6 +169,7 @@
     bool setFinalCrop(const Rect& crop);
     bool setLayerStack(uint32_t layerStack);
     bool setDataSpace(android_dataspace dataSpace);
+    uint32_t getLayerStack() const;
     void deferTransactionUntil(const sp<IBinder>& handle, uint64_t frameNumber);
     bool setOverrideScalingMode(int32_t overrideScalingMode);
 
@@ -219,6 +221,14 @@
     virtual bool isVisible() const;
 
     /*
+     * isHiddenByPolicy - true if this layer has been forced invisible.
+     * just because this is false, doesn't mean isVisible() is true.
+     * For example if this layer has no active buffer, it may not be hidden by
+     * policy, but it still can not be visible.
+     */
+    virtual bool isHiddenByPolicy() const;
+
+    /*
      * isFixedSize - true if content has a fixed size
      */
     virtual bool isFixedSize() const;
@@ -234,7 +244,7 @@
     // -----------------------------------------------------------------------
 
 #ifdef USE_HWC2
-    void setGeometry(const sp<const DisplayDevice>& displayDevice);
+    void setGeometry(const sp<const DisplayDevice>& displayDevice, uint32_t z);
     void forceClientComposition(int32_t hwcId);
     void setPerFrameData(const sp<const DisplayDevice>& displayDevice);
 
@@ -422,6 +432,26 @@
 
     bool getTransformToDisplayInverse() const;
 
+    Transform getTransform() const;
+
+    void traverseInReverseZOrder(const std::function<void(Layer*)>& exec);
+    void traverseInZOrder(const std::function<void(Layer*)>& exec);
+
+    void addChild(const sp<Layer>& layer);
+    // Returns index if removed, or negative value otherwise
+    // for symmetry with Vector::remove
+    ssize_t removeChild(const sp<Layer>& layer);
+    sp<Layer> getParent() const { return mParent.promote(); }
+    bool hasParent() const { return getParent() != nullptr; }
+
+    Rect computeScreenBounds(bool reduceTransparentRegion = true) const;
+    bool setChildLayer(const sp<Layer>& childLayer, int32_t z);
+
+    // Copy the current list of children to the drawing state. Called by
+    // SurfaceFlinger to complete a transaction.
+    void commitChildList();
+
+    int32_t getZ() const;
 protected:
     // constant
     sp<SurfaceFlinger> mFlinger;
@@ -462,7 +492,12 @@
     bool needsFiltering(const sp<const DisplayDevice>& hw) const;
 
     uint32_t getEffectiveUsage(uint32_t usage) const;
+
     gfx::FloatRect computeCrop(const sp<const DisplayDevice>& hw) const;
+    // Compute the initial crop as specified by parent layers and the SurfaceControl
+    // for this layer. Does not include buffer crop from the IGraphicBufferProducer
+    // client, as that should not affect child clipping. Returns in screen space.
+    Rect computeInitialCrop(const sp<const DisplayDevice>& hw) const;
     bool isCropped() const;
     static bool getOpacityForFormat(uint32_t format);
 
@@ -478,6 +513,8 @@
     // Loads the corresponding system property once per process
     static bool latchUnsignaledBuffers();
 
+    void setParent(const sp<Layer>& layer);
+
     // -----------------------------------------------------------------------
 
     class SyncPoint
@@ -532,6 +569,8 @@
     void popPendingState(State* stateToCommit);
     bool applyPendingStates(State* stateToCommit);
 
+    void clearSyncPoints();
+
     // Returns mCurrentScaling mode (originating from the
     // Client) or mOverrideScalingMode mode (originating from
     // the Surface Controller) if set.
@@ -662,6 +701,13 @@
 
     bool mAutoRefresh;
     bool mFreezePositionUpdates;
+
+    // Child list about to be committed/used for editing.
+    LayerVector mCurrentChildren;
+    // Child list used for rendering.
+    LayerVector mDrawingChildren;
+
+    wp<Layer> mParent;
 };
 
 // ---------------------------------------------------------------------------
diff --git a/services/surfaceflinger/LayerDim.cpp b/services/surfaceflinger/LayerDim.cpp
index 4d5b3db..daebf8a 100644
--- a/services/surfaceflinger/LayerDim.cpp
+++ b/services/surfaceflinger/LayerDim.cpp
@@ -59,7 +59,7 @@
 
 bool LayerDim::isVisible() const {
     const Layer::State& s(getDrawingState());
-    return !(s.flags & layer_state_t::eLayerHidden) && s.alpha;
+    return !isHiddenByPolicy() && s.alpha;
 }
 
 
diff --git a/services/surfaceflinger/LayerVector.cpp b/services/surfaceflinger/LayerVector.cpp
new file mode 100644
index 0000000..7ba6ad3
--- /dev/null
+++ b/services/surfaceflinger/LayerVector.cpp
@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "LayerVector.h"
+#include "Layer.h"
+
+namespace android {
+LayerVector::LayerVector(const LayerVector& rhs) : SortedVector<sp<Layer>>(rhs) {
+}
+
+int LayerVector::do_compare(const void* lhs, const void* rhs) const
+{
+    // sort layers per layer-stack, then by z-order and finally by sequence
+    const auto& l = *reinterpret_cast<const sp<Layer>*>(lhs);
+    const auto& r = *reinterpret_cast<const sp<Layer>*>(rhs);
+
+    uint32_t ls = l->getCurrentState().layerStack;
+    uint32_t rs = r->getCurrentState().layerStack;
+    if (ls != rs)
+        return ls - rs;
+
+    uint32_t lz = l->getCurrentState().z;
+    uint32_t rz = r->getCurrentState().z;
+    if (lz != rz)
+        return lz - rz;
+
+    return l->sequence - r->sequence;
+}
+
+void LayerVector::traverseInZOrder(const std::function<void(Layer*)>& consume) const {
+    for (size_t i = 0; i < size(); i++) {
+        (*this)[i]->traverseInZOrder(consume);
+    }
+}
+
+void LayerVector::traverseInReverseZOrder(const std::function<void(Layer*)>& consume) const {
+    for (auto i = static_cast<int64_t>(size()) - 1; i >= 0; i--) {
+        (*this)[i]->traverseInReverseZOrder(consume);
+     }
+}
+} // namespace android
diff --git a/services/surfaceflinger/LayerVector.h b/services/surfaceflinger/LayerVector.h
new file mode 100644
index 0000000..7dfa4a0
--- /dev/null
+++ b/services/surfaceflinger/LayerVector.h
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2016 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef ANDROID_SURFACE_FLINGER_LAYER_VECTOR_H
+#define ANDROID_SURFACE_FLINGER_LAYER_VECTOR_H
+
+#include <utils/SortedVector.h>
+#include <utils/RefBase.h>
+
+#include <functional>
+
+namespace android {
+class Layer;
+
+/*
+ * Used by the top-level SurfaceFlinger state and individual layers
+ * to track layers they own sorted according to Z-order. Provides traversal
+ * functions for traversing all owned layers, and their descendents.
+ */
+class LayerVector : public SortedVector<sp<Layer>> {
+public:
+    LayerVector() = default;
+    LayerVector(const LayerVector& rhs);
+    // Sorts layer by layer-stack, Z order, and finally creation order (sequence).
+    int do_compare(const void* lhs, const void* rhs) const override;
+
+    void traverseInReverseZOrder(const std::function<void(Layer*)>& consume) const;
+    void traverseInZOrder(const std::function<void(Layer*)>& consume) const;
+};
+}
+
+#endif /* ANDROID_SURFACE_FLINGER_LAYER_VECTOR_H_ */
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 5f25ef5..3480d83 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -69,6 +69,7 @@
 #include "EventControlThread.h"
 #include "EventThread.h"
 #include "Layer.h"
+#include "LayerVector.h"
 #include "LayerDim.h"
 #include "SurfaceFlinger.h"
 
@@ -133,6 +134,7 @@
         mTransactionPending(false),
         mAnimTransactionPending(false),
         mLayersRemoved(false),
+        mLayersAdded(false),
         mRepaintEverything(0),
         mRenderEngine(NULL),
         mBootTime(systemTime()),
@@ -158,7 +160,8 @@
         mHasPoweredOff(false),
         mFrameBuckets(),
         mTotalTime(0),
-        mLastSwapTime(0)
+        mLastSwapTime(0),
+        mNumLayers(0)
 {
     ALOGI("SurfaceFlinger is starting");
 
@@ -1211,13 +1214,12 @@
     ALOGV("preComposition");
 
     bool needExtraInvalidate = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        if (layers[i]->onPreComposition(refreshStartTime)) {
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
+        if (layer->onPreComposition(refreshStartTime)) {
             needExtraInvalidate = true;
         }
-    }
+    });
+
     if (needExtraInvalidate) {
         signalLayerUpdate();
     }
@@ -1258,18 +1260,14 @@
     } else {
         retireFenceTime = &displayFenceTime;
     }
-
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        bool frameLatched =
-                layers[i]->onPostComposition(glCompositionDoneFenceTime,
-                        *presentFenceTime, *retireFenceTime);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
+        bool frameLatched = layer->onPostComposition(glCompositionDoneFenceTime,
+                *presentFenceTime, *retireFenceTime);
         if (frameLatched) {
-            recordBufferingStats(layers[i]->getName().string(),
-                    layers[i]->getOccupancyHistory(false));
+            recordBufferingStats(layer->getName().string(),
+                    layer->getOccupancyHistory(false));
         }
-    }
+    });
 
     if (displayFence->isValid()) {
         if (mPrimaryDispSync.addPresentFence(displayFence)) {
@@ -1332,7 +1330,6 @@
         mVisibleRegionsDirty = false;
         invalidateHwcGeometry();
 
-        const LayerVector& layers(mDrawingState.layersSortedByZ);
         for (size_t dpy=0 ; dpy<mDisplays.size() ; dpy++) {
             Region opaqueRegion;
             Region dirtyRegion;
@@ -1341,15 +1338,12 @@
             const Transform& tr(displayDevice->getTransform());
             const Rect bounds(displayDevice->getBounds());
             if (displayDevice->isDisplayOn()) {
-                SurfaceFlinger::computeVisibleRegions(layers,
+                computeVisibleRegions(
                         displayDevice->getLayerStack(), dirtyRegion,
                         opaqueRegion);
 
-                const size_t count = layers.size();
-                for (size_t i=0 ; i<count ; i++) {
-                    const sp<Layer>& layer(layers[i]);
-                    const Layer::State& s(layer->getDrawingState());
-                    if (s.layerStack == displayDevice->getLayerStack()) {
+                mDrawingState.traverseInZOrder([&](Layer* layer) {
+                    if (layer->getLayerStack() == displayDevice->getLayerStack()) {
                         Region drawRegion(tr.transform(
                                 layer->visibleNonTransparentRegion));
                         drawRegion.andSelf(bounds);
@@ -1362,7 +1356,7 @@
                                     nullptr);
                         }
                     }
-                }
+                });
             }
             displayDevice->setVisibleLayersSortedByZ(layersSortedByZ);
             displayDevice->undefinedRegion.set(bounds);
@@ -1416,7 +1410,8 @@
                 const Vector<sp<Layer>>& currentLayers(
                         displayDevice->getVisibleLayersSortedByZ());
                 bool foundLayerWithoutHwc = false;
-                for (auto& layer : currentLayers) {
+                for (size_t i = 0; i < currentLayers.size(); i++) {
+                    const auto& layer = currentLayers[i];
                     if (!layer->hasHwcLayer(hwcId)) {
                         auto hwcLayer = mHwc->createLayer(hwcId);
                         if (hwcLayer) {
@@ -1428,7 +1423,7 @@
                         }
                     }
 
-                    layer->setGeometry(displayDevice);
+                    layer->setGeometry(displayDevice, i);
                     if (mDebugDisableHWC || mDebugRegion) {
                         layer->forceClientComposition(hwcId);
                     }
@@ -1567,13 +1562,10 @@
 
 void SurfaceFlinger::handleTransactionLocked(uint32_t transactionFlags)
 {
-    const LayerVector& currentLayers(mCurrentState.layersSortedByZ);
-    const size_t count = currentLayers.size();
-
     // Notify all layers of available frames
-    for (size_t i = 0; i < count; ++i) {
-        currentLayers[i]->notifyAvailableFrames();
-    }
+    mCurrentState.traverseInZOrder([](Layer* layer) {
+        layer->notifyAvailableFrames();
+    });
 
     /*
      * Traversal of the children
@@ -1581,15 +1573,14 @@
      */
 
     if (transactionFlags & eTraversalNeeded) {
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(currentLayers[i]);
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             uint32_t trFlags = layer->getTransactionFlags(eTransactionNeeded);
-            if (!trFlags) continue;
+            if (!trFlags) return;
 
             const uint32_t flags = layer->doTransaction(0);
             if (flags & Layer::eVisibleRegion)
                 mVisibleRegionsDirty = true;
-        }
+        });
     }
 
     /*
@@ -1782,13 +1773,13 @@
         //
         sp<const DisplayDevice> disp;
         uint32_t currentlayerStack = 0;
-        for (size_t i=0; i<count; i++) {
+        bool first = true;
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             // NOTE: we rely on the fact that layers are sorted by
             // layerStack first (so we don't have to traverse the list
             // of displays for every layer).
-            const sp<Layer>& layer(currentLayers[i]);
-            uint32_t layerStack = layer->getDrawingState().layerStack;
-            if (i==0 || currentlayerStack != layerStack) {
+            uint32_t layerStack = layer->getLayerStack();
+            if (first || currentlayerStack != layerStack) {
                 currentlayerStack = layerStack;
                 // figure out if this layerstack is mirrored
                 // (more than one display) if so, pick the default display,
@@ -1816,7 +1807,9 @@
                 disp = getDefaultDisplayDevice();
             }
             layer->updateTransformHint(disp);
-        }
+
+            first = false;
+        });
     }
 
 
@@ -1824,9 +1817,9 @@
      * Perform our own transaction if needed
      */
 
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    if (currentLayers.size() > layers.size()) {
-        // layers have been added
+    if (mLayersAdded) {
+        mLayersAdded = false;
+        // Layers have been added.
         mVisibleRegionsDirty = true;
     }
 
@@ -1835,20 +1828,17 @@
     if (mLayersRemoved) {
         mLayersRemoved = false;
         mVisibleRegionsDirty = true;
-        const size_t count = layers.size();
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(layers[i]);
-            if (currentLayers.indexOf(layer) < 0) {
+        mDrawingState.traverseInZOrder([&](Layer* layer) {
+            if (mLayersPendingRemoval.indexOf(layer) >= 0) {
                 // this layer is not visible anymore
                 // TODO: we could traverse the tree from front to back and
                 //       compute the actual visible region
                 // TODO: we could cache the transformed region
-                const Layer::State& s(layer->getDrawingState());
-                Region visibleReg = s.active.transform.transform(
-                        Region(Rect(s.active.w, s.active.h)));
-                invalidateLayerStack(s.layerStack, visibleReg);
+                Region visibleReg;
+                visibleReg.set(layer->computeScreenBounds());
+                invalidateLayerStack(layer->getLayerStack(), visibleReg);
             }
-        }
+        });
     }
 
     commitTransaction();
@@ -1874,10 +1864,10 @@
 {
     if (!mLayersPendingRemoval.isEmpty()) {
         // Notify removed layers now that they can't be drawn from
-        for (size_t i = 0; i < mLayersPendingRemoval.size(); i++) {
-            recordBufferingStats(mLayersPendingRemoval[i]->getName().string(),
-                    mLayersPendingRemoval[i]->getOccupancyHistory(true));
-            mLayersPendingRemoval[i]->onRemoved();
+        for (const auto& l : mLayersPendingRemoval) {
+            recordBufferingStats(l->getName().string(),
+                    l->getOccupancyHistory(true));
+            l->onRemoved();
         }
         mLayersPendingRemoval.clear();
     }
@@ -1887,13 +1877,15 @@
     mAnimCompositionPending = mAnimTransactionPending;
 
     mDrawingState = mCurrentState;
+    mDrawingState.traverseInZOrder([](Layer* layer) {
+        layer->commitChildList();
+    });
     mTransactionPending = false;
     mAnimTransactionPending = false;
     mTransactionCV.broadcast();
 }
 
-void SurfaceFlinger::computeVisibleRegions(
-        const LayerVector& currentLayers, uint32_t layerStack,
+void SurfaceFlinger::computeVisibleRegions(uint32_t layerStack,
         Region& outDirtyRegion, Region& outOpaqueRegion)
 {
     ATRACE_CALL();
@@ -1905,16 +1897,13 @@
 
     outDirtyRegion.clear();
 
-    size_t i = currentLayers.size();
-    while (i--) {
-        const sp<Layer>& layer = currentLayers[i];
-
+    mDrawingState.traverseInReverseZOrder([&](Layer* layer) {
         // start with the whole surface at its current location
         const Layer::State& s(layer->getDrawingState());
 
         // only consider the layers on the given layer stack
-        if (s.layerStack != layerStack)
-            continue;
+        if (layer->getLayerStack() != layerStack)
+            return;
 
         /*
          * opaqueRegion: area of a surface that is fully opaque.
@@ -1949,12 +1938,12 @@
         // handle hidden surfaces by setting the visible region to empty
         if (CC_LIKELY(layer->isVisible())) {
             const bool translucent = !layer->isOpaque(s);
-            Rect bounds(s.active.transform.transform(layer->computeBounds()));
+            Rect bounds(layer->computeScreenBounds());
             visibleRegion.set(bounds);
+            Transform tr = layer->getTransform();
             if (!visibleRegion.isEmpty()) {
                 // Remove the transparent area from the visible region
                 if (translucent) {
-                    const Transform tr(s.active.transform);
                     if (tr.preserveRects()) {
                         // transform the transparent region
                         transparentRegion = tr.transform(s.activeTransparentRegion);
@@ -1966,7 +1955,7 @@
                 }
 
                 // compute the opaque region
-                const int32_t layerOrientation = s.active.transform.getOrientation();
+                const int32_t layerOrientation = tr.getOrientation();
                 if (s.alpha == 1.0f && !translucent &&
                         ((layerOrientation & Transform::ROT_INVALID) == false)) {
                     // the opaque region is the layer's footprint
@@ -2023,7 +2012,7 @@
         layer->setCoveredRegion(coveredRegion);
         layer->setVisibleNonTransparentRegion(
                 visibleRegion.subtract(transparentRegion));
-    }
+    });
 
     outOpaqueRegion = aboveOpaqueLayers;
 }
@@ -2045,7 +2034,6 @@
     nsecs_t latchTime = systemTime();
 
     bool visibleRegions = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
     bool frameQueued = false;
 
     // Store the set of layers that need updates. This set must not change as
@@ -2057,24 +2045,23 @@
     // 3.) Layer 1 is latched.
     // Display is now waiting on Layer 1's frame, which is behind layer 0's
     // second frame. But layer 0's second frame could be waiting on display.
-    for (size_t i = 0, count = layers.size(); i<count ; i++) {
-        const sp<Layer>& layer(layers[i]);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
         if (layer->hasQueuedFrame()) {
             frameQueued = true;
             if (layer->shouldPresentNow(mPrimaryDispSync)) {
-                mLayersWithQueuedFrames.push_back(layer.get());
+                mLayersWithQueuedFrames.push_back(layer);
             } else {
                 layer->useEmptyDamage();
             }
         } else {
             layer->useEmptyDamage();
         }
-    }
+    });
+
     for (auto& layer : mLayersWithQueuedFrames) {
         const Region dirty(layer->latchBuffer(visibleRegions, latchTime));
         layer->useSurfaceDamage();
-        const Layer::State& s(layer->getDrawingState());
-        invalidateLayerStack(s.layerStack, dirty);
+        invalidateLayerStack(layer->getLayerStack(), dirty);
     }
 
     mVisibleRegionsDirty |= visibleRegions;
@@ -2296,16 +2283,23 @@
 status_t SurfaceFlinger::addClientLayer(const sp<Client>& client,
         const sp<IBinder>& handle,
         const sp<IGraphicBufferProducer>& gbc,
-        const sp<Layer>& lbc)
+        const sp<Layer>& lbc,
+        const sp<Layer>& parent)
 {
     // add this layer to the current state list
     {
         Mutex::Autolock _l(mStateLock);
-        if (mCurrentState.layersSortedByZ.size() >= MAX_LAYERS) {
+        if (mNumLayers >= MAX_LAYERS) {
             return NO_MEMORY;
         }
-        mCurrentState.layersSortedByZ.add(lbc);
+        if (parent == nullptr) {
+            mCurrentState.layersSortedByZ.add(lbc);
+        } else {
+            parent->addChild(lbc);
+        }
         mGraphicBufferProducerList.add(IInterface::asBinder(gbc));
+        mLayersAdded = true;
+        mNumLayers++;
     }
 
     // attach this layer to the client
@@ -2322,14 +2316,22 @@
         return NO_ERROR;
     }
 
-    ssize_t index = mCurrentState.layersSortedByZ.remove(layer);
-    if (index >= 0) {
-        mLayersPendingRemoval.push(layer);
-        mLayersRemoved = true;
-        setTransactionFlags(eTransactionNeeded);
-        return NO_ERROR;
+    const auto& p = layer->getParent();
+    const ssize_t index = (p != nullptr) ? p->removeChild(layer) :
+        mCurrentState.layersSortedByZ.remove(layer);
+
+    if (index < 0) {
+        ALOGE("Failed to find layer (%s) in layer parent (%s).",
+                layer->getName().string(),
+                (p != nullptr) ? p->getName().string() : "no-parent");
+        return BAD_VALUE;
     }
-    return status_t(index);
+
+    mLayersPendingRemoval.add(layer);
+    mLayersRemoved = true;
+    mNumLayers--;
+    setTransactionFlags(eTransactionNeeded);
+    return NO_ERROR;
 }
 
 uint32_t SurfaceFlinger::peekTransactionFlags() {
@@ -2504,13 +2506,20 @@
         }
         if (what & layer_state_t::eLayerChanged) {
             // NOTE: index needs to be calculated before we update the state
-            ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
-            if (layer->setLayer(s.z) && idx >= 0) {
-                mCurrentState.layersSortedByZ.removeAt(idx);
-                mCurrentState.layersSortedByZ.add(layer);
-                // we need traversal (state changed)
-                // AND transaction (list changed)
-                flags |= eTransactionNeeded|eTraversalNeeded;
+            const auto& p = layer->getParent();
+            if (p == nullptr) {
+                ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
+                if (layer->setLayer(s.z) && idx >= 0) {
+                    mCurrentState.layersSortedByZ.removeAt(idx);
+                    mCurrentState.layersSortedByZ.add(layer);
+                    // we need traversal (state changed)
+                    // AND transaction (list changed)
+                    flags |= eTransactionNeeded|eTraversalNeeded;
+                }
+            } else {
+                if (p->setChildLayer(layer, s.z)) {
+                    flags |= eTransactionNeeded|eTraversalNeeded;
+                }
             }
         }
         if (what & layer_state_t::eSizeChanged) {
@@ -2543,9 +2552,17 @@
                 flags |= eTraversalNeeded;
         }
         if (what & layer_state_t::eLayerStackChanged) {
-            // NOTE: index needs to be calculated before we update the state
             ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
-            if (layer->setLayerStack(s.layerStack) && idx >= 0) {
+            // We only allow setting layer stacks for top level layers,
+            // everything else inherits layer stack from its parent.
+            if (layer->hasParent()) {
+                ALOGE("Attempt to set layer stack on layer with parent (%s) is invalid",
+                        layer->getName().string());
+            } else if (idx < 0) {
+                ALOGE("Attempt to set layer stack on layer without parent (%s) that "
+                        "that also does not appear in the top level layer list. Something"
+                        " has gone wrong.", layer->getName().string());
+            } else if (layer->setLayerStack(s.layerStack)) {
                 mCurrentState.layersSortedByZ.removeAt(idx);
                 mCurrentState.layersSortedByZ.add(layer);
                 // we need traversal (state changed)
@@ -2571,7 +2588,8 @@
         const String8& name,
         const sp<Client>& client,
         uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
-        sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp)
+        sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp,
+        sp<Layer>* parent)
 {
     if (int32_t(w|h) < 0) {
         ALOGE("createLayer() failed, w or h is negative (w=%d, h=%d)",
@@ -2603,7 +2621,7 @@
         return result;
     }
 
-    result = addClientLayer(client, *handle, *gbp, layer);
+    result = addClientLayer(client, *handle, *gbp, layer, *parent);
     if (result != NO_ERROR) {
         return result;
     }
@@ -2891,12 +2909,9 @@
 void SurfaceFlinger::listLayersLocked(const Vector<String16>& /* args */,
         size_t& /* index */, String8& result) const
 {
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         result.appendFormat("%s\n", layer->getName().string());
-    }
+    });
 }
 
 void SurfaceFlinger::dumpStatsLocked(const Vector<String16>& args, size_t& index,
@@ -2915,14 +2930,11 @@
     if (name.isEmpty()) {
         mAnimFrameTracker.dumpStats(result);
     } else {
-        const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-        const size_t count = currentLayers.size();
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(currentLayers[i]);
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             if (name == layer->getName()) {
                 layer->dumpFrameStats(result);
             }
-        }
+        });
     }
 }
 
@@ -2935,14 +2947,11 @@
         index++;
     }
 
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         if (name.isEmpty() || (name == layer->getName())) {
             layer->clearFrameStats();
         }
-    }
+    });
 
     mAnimFrameTracker.clearStats();
 }
@@ -2950,12 +2959,9 @@
 // This should only be called from the main thread.  Otherwise it would need
 // the lock and should use mCurrentState rather than mDrawingState.
 void SurfaceFlinger::logFrameStats() {
-    const LayerVector& drawingLayers = mDrawingState.layersSortedByZ;
-    const size_t count = drawingLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(drawingLayers[i]);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
         layer->logFrameStats();
-    }
+    });
 
     mAnimFrameTracker.logAndResetStats(String8("<win-anim>"));
 }
@@ -3112,15 +3118,12 @@
     /*
      * Dump the visible layer list
      */
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
     colorizer.bold(result);
-    result.appendFormat("Visible layers (count = %zu)\n", count);
+    result.appendFormat("Visible layers (count = %zu)\n", mNumLayers);
     colorizer.reset(result);
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         layer->dump(result, colorizer);
-    }
+    });
 
     /*
      * Dump Display state
@@ -3198,10 +3201,9 @@
 
         result.appendFormat("Display %d HWC layers:\n", hwcId);
         Layer::miniDumpHeader(result);
-        for (size_t l = 0; l < count; l++) {
-            const sp<Layer>& layer(currentLayers[l]);
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             layer->miniDump(result, hwcId);
-        }
+        });
         result.append("\n");
     }
 
@@ -3582,7 +3584,7 @@
 status_t SurfaceFlinger::captureScreen(const sp<IBinder>& display,
         const sp<IGraphicBufferProducer>& producer,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform, ISurfaceComposer::Rotation rotation) {
 
     if (CC_UNLIKELY(display == 0))
@@ -3633,7 +3635,7 @@
                 const sp<IBinder>& display,
                 const sp<IGraphicBufferProducer>& producer,
                 Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-                uint32_t minLayerZ, uint32_t maxLayerZ,
+                int32_t minLayerZ, int32_t maxLayerZ,
                 bool useIdentityTransform,
                 Transform::orientation_flags rotation,
                 bool isLocalScreenshot)
@@ -3682,7 +3684,7 @@
 void SurfaceFlinger::renderScreenImplLocked(
         const sp<const DisplayDevice>& hw,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool yswap, bool useIdentityTransform, Transform::orientation_flags rotation)
 {
     ATRACE_CALL();
@@ -3726,20 +3728,24 @@
     // redraw the screen entirely...
     engine.clearWithColor(0, 0, 0, 1);
 
-    const LayerVector& layers( mDrawingState.layersSortedByZ );
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; ++i) {
-        const sp<Layer>& layer(layers[i]);
-        const Layer::State& state(layer->getDrawingState());
-        if (state.layerStack == hw->getLayerStack()) {
-            if (state.z >= minLayerZ && state.z <= maxLayerZ) {
-                if (layer->isVisible()) {
-                    if (filtering) layer->setFiltering(true);
-                    layer->draw(hw, useIdentityTransform);
-                    if (filtering) layer->setFiltering(false);
-                }
-            }
+    // We loop through the first level of layers without traversing,
+    // as we need to interpret min/max layer Z in the top level Z space.
+    for (const auto& layer : mDrawingState.layersSortedByZ) {
+        if (layer->getLayerStack() != hw->getLayerStack()) {
+            continue;
         }
+        const Layer::State& state(layer->getDrawingState());
+        if (state.z < minLayerZ || state.z > maxLayerZ) {
+            continue;
+        }
+        layer->traverseInZOrder([&](Layer* layer) {
+            if (!layer->isVisible()) {
+                return;
+            }
+            if (filtering) layer->setFiltering(true);
+            layer->draw(hw, useIdentityTransform);
+            if (filtering) layer->setFiltering(false);
+        });
     }
 
     hw->setViewportAndProjection();
@@ -3750,7 +3756,7 @@
         const sp<const DisplayDevice>& hw,
         const sp<IGraphicBufferProducer>& producer,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform, Transform::orientation_flags rotation,
         bool isLocalScreenshot)
 {
@@ -3774,16 +3780,16 @@
     reqHeight = (!reqHeight) ? hw_h : reqHeight;
 
     bool secureLayerIsVisible = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i = 0 ; i < count ; ++i) {
-        const sp<Layer>& layer(layers[i]);
+    for (const auto& layer : mDrawingState.layersSortedByZ) {
         const Layer::State& state(layer->getDrawingState());
-        if (state.layerStack == hw->getLayerStack() && state.z >= minLayerZ &&
-                state.z <= maxLayerZ && layer->isVisible() &&
-                layer->isSecure()) {
-            secureLayerIsVisible = true;
+        if ((layer->getLayerStack() != hw->getLayerStack()) ||
+                (state.z < minLayerZ || state.z > maxLayerZ)) {
+            continue;
         }
+        layer->traverseInZOrder([&](Layer *layer) {
+            secureLayerIsVisible = secureLayerIsVisible || (layer->isVisible() &&
+                    layer->isSecure());
+        });
     }
 
     if (!isLocalScreenshot && secureLayerIsVisible) {
@@ -3911,7 +3917,7 @@
 }
 
 void SurfaceFlinger::checkScreenshot(size_t w, size_t s, size_t h, void const* vaddr,
-        const sp<const DisplayDevice>& hw, uint32_t minLayerZ, uint32_t maxLayerZ) {
+        const sp<const DisplayDevice>& hw, int32_t minLayerZ, int32_t maxLayerZ) {
     if (DEBUG_SCREENSHOTS) {
         for (size_t y=0 ; y<h ; y++) {
             uint32_t const * p = (uint32_t const *)vaddr + y*s;
@@ -3922,49 +3928,32 @@
         ALOGE("*** we just took a black screenshot ***\n"
                 "requested minz=%d, maxz=%d, layerStack=%d",
                 minLayerZ, maxLayerZ, hw->getLayerStack());
-        const LayerVector& layers( mDrawingState.layersSortedByZ );
-        const size_t count = layers.size();
-        for (size_t i=0 ; i<count ; ++i) {
-            const sp<Layer>& layer(layers[i]);
+
+        size_t i = 0;
+        for (const auto& layer : mDrawingState.layersSortedByZ) {
             const Layer::State& state(layer->getDrawingState());
-            const bool visible = (state.layerStack == hw->getLayerStack())
-                                && (state.z >= minLayerZ && state.z <= maxLayerZ)
-                                && (layer->isVisible());
-            ALOGE("%c index=%zu, name=%s, layerStack=%d, z=%d, visible=%d, flags=%x, alpha=%.3f",
-                    visible ? '+' : '-',
-                            i, layer->getName().string(), state.layerStack, state.z,
+            if (layer->getLayerStack() == hw->getLayerStack() && state.z >= minLayerZ &&
+                    state.z <= maxLayerZ) {
+                layer->traverseInZOrder([&](Layer* layer) {
+                    ALOGE("%c index=%zu, name=%s, layerStack=%d, z=%d, visible=%d, flags=%x, alpha=%.3f",
+                            layer->isVisible() ? '+' : '-',
+                            i, layer->getName().string(), layer->getLayerStack(), state.z,
                             layer->isVisible(), state.flags, state.alpha);
+                    i++;
+                });
+            }
         }
     }
 }
 
 // ---------------------------------------------------------------------------
 
-SurfaceFlinger::LayerVector::LayerVector() {
+void SurfaceFlinger::State::traverseInZOrder(const std::function<void(Layer*)>& consume) const {
+    layersSortedByZ.traverseInZOrder(consume);
 }
 
-SurfaceFlinger::LayerVector::LayerVector(const LayerVector& rhs)
-    : SortedVector<sp<Layer> >(rhs) {
-}
-
-int SurfaceFlinger::LayerVector::do_compare(const void* lhs,
-    const void* rhs) const
-{
-    // sort layers per layer-stack, then by z-order and finally by sequence
-    const sp<Layer>& l(*reinterpret_cast<const sp<Layer>*>(lhs));
-    const sp<Layer>& r(*reinterpret_cast<const sp<Layer>*>(rhs));
-
-    uint32_t ls = l->getCurrentState().layerStack;
-    uint32_t rs = r->getCurrentState().layerStack;
-    if (ls != rs)
-        return ls - rs;
-
-    uint32_t lz = l->getCurrentState().z;
-    uint32_t rz = r->getCurrentState().z;
-    if (lz != rz)
-        return lz - rz;
-
-    return l->sequence - r->sequence;
+void SurfaceFlinger::State::traverseInReverseZOrder(const std::function<void(Layer*)>& consume) const {
+    layersSortedByZ.traverseInReverseZOrder(consume);
 }
 
 }; // namespace android
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index f7f9ef5..4f3ee74 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -55,6 +55,7 @@
 #include "DisplayDevice.h"
 #include "DispSync.h"
 #include "FrameTracker.h"
+#include "LayerVector.h"
 #include "MessageQueue.h"
 #include "SurfaceInterceptor.h"
 
@@ -168,16 +169,13 @@
      * Internal data structures
      */
 
-    class LayerVector : public SortedVector< sp<Layer> > {
+    class State {
     public:
-        LayerVector();
-        LayerVector(const LayerVector& rhs);
-        virtual int do_compare(const void* lhs, const void* rhs) const;
-    };
-
-    struct State {
         LayerVector layersSortedByZ;
         DefaultKeyedVector< wp<IBinder>, DisplayDeviceState> displays;
+
+        void traverseInZOrder(const std::function<void(Layer*)>& consume) const;
+        void traverseInReverseZOrder(const std::function<void(Layer*)>& consume) const;
     };
 
     /* ------------------------------------------------------------------------
@@ -206,7 +204,7 @@
     virtual status_t captureScreen(const sp<IBinder>& display,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform, ISurfaceComposer::Rotation rotation);
     virtual status_t getDisplayStats(const sp<IBinder>& display,
             DisplayStatInfo* stats);
@@ -295,7 +293,8 @@
      */
     status_t createLayer(const String8& name, const sp<Client>& client,
             uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
-            sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp);
+            sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp,
+            sp<Layer>* parent);
 
     status_t createNormalLayer(const sp<Client>& client, const String8& name,
             uint32_t w, uint32_t h, uint32_t flags, PixelFormat& format,
@@ -322,7 +321,8 @@
     status_t addClientLayer(const sp<Client>& client,
             const sp<IBinder>& handle,
             const sp<IGraphicBufferProducer>& gbc,
-            const sp<Layer>& lbc);
+            const sp<Layer>& lbc,
+            const sp<Layer>& parent);
 
     /* ------------------------------------------------------------------------
      * Boot animation, on/off animations and screen capture
@@ -333,14 +333,14 @@
     void renderScreenImplLocked(
             const sp<const DisplayDevice>& hw,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool yswap, bool useIdentityTransform, Transform::orientation_flags rotation);
 
     status_t captureScreenImplLocked(
             const sp<const DisplayDevice>& hw,
             const sp<IGraphicBufferProducer>& producer,
             Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-            uint32_t minLayerZ, uint32_t maxLayerZ,
+            int32_t minLayerZ, int32_t maxLayerZ,
             bool useIdentityTransform, Transform::orientation_flags rotation,
             bool isLocalScreenshot);
 
@@ -397,8 +397,7 @@
      * Compositing
      */
     void invalidateHwcGeometry();
-    static void computeVisibleRegions(
-            const LayerVector& currentLayers, uint32_t layerStack,
+    void computeVisibleRegions(uint32_t layerStack,
             Region& dirtyRegion, Region& opaqueRegion);
 
     void preComposition(nsecs_t refreshStartTime);
@@ -426,6 +425,7 @@
      void enableHardwareVsync();
      void resyncToHardwareVsync(bool makeAvailable);
      void disableHardwareVsync(bool makeUnavailable);
+
 public:
      void resyncWithRateLimit();
 private:
@@ -441,7 +441,7 @@
     void appendSfConfigString(String8& result) const;
     void checkScreenshot(size_t w, size_t s, size_t h, void const* vaddr,
             const sp<const DisplayDevice>& hw,
-            uint32_t minLayerZ, uint32_t maxLayerZ);
+            int32_t minLayerZ, int32_t maxLayerZ);
 
     void logFrameStats();
 
@@ -467,11 +467,12 @@
     Condition mTransactionCV;
     bool mTransactionPending;
     bool mAnimTransactionPending;
-    Vector< sp<Layer> > mLayersPendingRemoval;
+    SortedVector< sp<Layer> > mLayersPendingRemoval;
     SortedVector< wp<IBinder> > mGraphicBufferProducerList;
 
     // protected by mStateLock (but we could use another lock)
     bool mLayersRemoved;
+    bool mLayersAdded;
 
     // access must be protected by mInvalidateLock
     volatile int32_t mRepaintEverything;
@@ -568,6 +569,8 @@
     nsecs_t mTotalTime;
     std::atomic<nsecs_t> mLastSwapTime;
 
+    size_t mNumLayers;
+
     // Double- vs. triple-buffering stats
     struct BufferingStats {
         BufferingStats()
diff --git a/services/surfaceflinger/SurfaceFlinger_hwc1.cpp b/services/surfaceflinger/SurfaceFlinger_hwc1.cpp
index fcf0185..1976d2e 100644
--- a/services/surfaceflinger/SurfaceFlinger_hwc1.cpp
+++ b/services/surfaceflinger/SurfaceFlinger_hwc1.cpp
@@ -72,6 +72,7 @@
 #include "EventControlThread.h"
 #include "EventThread.h"
 #include "Layer.h"
+#include "LayerVector.h"
 #include "LayerDim.h"
 #include "SurfaceFlinger.h"
 
@@ -136,6 +137,7 @@
         mTransactionPending(false),
         mAnimTransactionPending(false),
         mLayersRemoved(false),
+        mLayersAdded(false),
         mRepaintEverything(0),
         mRenderEngine(NULL),
         mBootTime(systemTime()),
@@ -161,7 +163,8 @@
         mHasPoweredOff(false),
         mFrameBuckets(),
         mTotalTime(0),
-        mLastSwapTime(0)
+        mLastSwapTime(0),
+        mNumLayers(0)
 {
     ALOGI("SurfaceFlinger is starting");
 
@@ -1126,13 +1129,12 @@
 void SurfaceFlinger::preComposition(nsecs_t refreshStartTime)
 {
     bool needExtraInvalidate = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        if (layers[i]->onPreComposition(refreshStartTime)) {
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
+        if (layer->onPreComposition(refreshStartTime)) {
             needExtraInvalidate = true;
         }
-    }
+    });
+
     if (needExtraInvalidate) {
         signalLayerUpdate();
     }
@@ -1159,16 +1161,15 @@
     mDisplayTimeline.push(retireFenceTime);
     mDisplayTimeline.updateSignalTimes();
 
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        bool frameLatched = layers[i]->onPostComposition(
-                glCompositionDoneFenceTime, presentFenceTime, retireFenceTime);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
+        bool frameLatched = layer->onPostComposition(glCompositionDoneFenceTime,
+                presentFenceTime, retireFenceTime);
+
         if (frameLatched) {
-            recordBufferingStats(layers[i]->getName().string(),
-                    layers[i]->getOccupancyHistory(false));
+            recordBufferingStats(layer->getName().string(),
+                    layer->getOccupancyHistory(false));
         }
-    }
+    });
 
     if (displayFence->isValid()) {
         if (mPrimaryDispSync.addPresentFence(displayFence)) {
@@ -1226,7 +1227,6 @@
         mVisibleRegionsDirty = false;
         invalidateHwcGeometry();
 
-        const LayerVector& layers(mDrawingState.layersSortedByZ);
         for (size_t dpy=0 ; dpy<mDisplays.size() ; dpy++) {
             Region opaqueRegion;
             Region dirtyRegion;
@@ -1235,14 +1235,11 @@
             const Transform& tr(hw->getTransform());
             const Rect bounds(hw->getBounds());
             if (hw->isDisplayOn()) {
-                SurfaceFlinger::computeVisibleRegions(layers,
-                        hw->getLayerStack(), dirtyRegion, opaqueRegion);
+                computeVisibleRegions(hw->getLayerStack(), dirtyRegion,
+                        opaqueRegion);
 
-                const size_t count = layers.size();
-                for (size_t i=0 ; i<count ; i++) {
-                    const sp<Layer>& layer(layers[i]);
-                    const Layer::State& s(layer->getDrawingState());
-                    if (s.layerStack == hw->getLayerStack()) {
+                mDrawingState.traverseInZOrder([&](Layer* layer) {
+                    if (layer->getLayerStack() == hw->getLayerStack()) {
                         Region drawRegion(tr.transform(
                                 layer->visibleNonTransparentRegion));
                         drawRegion.andSelf(bounds);
@@ -1250,7 +1247,7 @@
                             layersSortedByZ.add(layer);
                         }
                     }
-                }
+                });
             }
             hw->setVisibleLayersSortedByZ(layersSortedByZ);
             hw->undefinedRegion.set(bounds);
@@ -1473,13 +1470,10 @@
 
 void SurfaceFlinger::handleTransactionLocked(uint32_t transactionFlags)
 {
-    const LayerVector& currentLayers(mCurrentState.layersSortedByZ);
-    const size_t count = currentLayers.size();
-
     // Notify all layers of available frames
-    for (size_t i = 0; i < count; ++i) {
-        currentLayers[i]->notifyAvailableFrames();
-    }
+    mCurrentState.traverseInZOrder([](Layer* layer) {
+        layer->notifyAvailableFrames();
+    });
 
     /*
      * Traversal of the children
@@ -1487,15 +1481,14 @@
      */
 
     if (transactionFlags & eTraversalNeeded) {
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(currentLayers[i]);
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             uint32_t trFlags = layer->getTransactionFlags(eTransactionNeeded);
-            if (!trFlags) continue;
+            if (!trFlags) return;
 
             const uint32_t flags = layer->doTransaction(0);
             if (flags & Layer::eVisibleRegion)
                 mVisibleRegionsDirty = true;
-        }
+        });
     }
 
     /*
@@ -1682,13 +1675,13 @@
         //
         sp<const DisplayDevice> disp;
         uint32_t currentlayerStack = 0;
-        for (size_t i=0; i<count; i++) {
+        bool first = true;
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             // NOTE: we rely on the fact that layers are sorted by
             // layerStack first (so we don't have to traverse the list
             // of displays for every layer).
-            const sp<Layer>& layer(currentLayers[i]);
-            uint32_t layerStack = layer->getDrawingState().layerStack;
-            if (i==0 || currentlayerStack != layerStack) {
+            uint32_t layerStack = layer->getLayerStack();
+            if (first || currentlayerStack != layerStack) {
                 currentlayerStack = layerStack;
                 // figure out if this layerstack is mirrored
                 // (more than one display) if so, pick the default display,
@@ -1716,7 +1709,9 @@
                 disp = getDefaultDisplayDevice();
             }
             layer->updateTransformHint(disp);
-        }
+
+            first = false;
+        });
     }
 
 
@@ -1724,9 +1719,9 @@
      * Perform our own transaction if needed
      */
 
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    if (currentLayers.size() > layers.size()) {
-        // layers have been added
+    if (mLayersAdded) {
+        mLayersAdded = false;
+        // Layers have been added.
         mVisibleRegionsDirty = true;
     }
 
@@ -1735,20 +1730,17 @@
     if (mLayersRemoved) {
         mLayersRemoved = false;
         mVisibleRegionsDirty = true;
-        const size_t count = layers.size();
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(layers[i]);
-            if (currentLayers.indexOf(layer) < 0) {
+        mDrawingState.traverseInZOrder([&](Layer* layer) {
+            if (mLayersPendingRemoval.indexOf(layer) >= 0) {
                 // this layer is not visible anymore
                 // TODO: we could traverse the tree from front to back and
                 //       compute the actual visible region
                 // TODO: we could cache the transformed region
-                const Layer::State& s(layer->getDrawingState());
-                Region visibleReg = s.active.transform.transform(
-                        Region(Rect(s.active.w, s.active.h)));
-                invalidateLayerStack(s.layerStack, visibleReg);
+                Region visibleReg;
+                visibleReg.set(layer->computeScreenBounds());
+                invalidateLayerStack(layer->getLayerStack(), visibleReg);
             }
-        }
+        });
     }
 
     commitTransaction();
@@ -1786,10 +1778,10 @@
 {
     if (!mLayersPendingRemoval.isEmpty()) {
         // Notify removed layers now that they can't be drawn from
-        for (size_t i = 0; i < mLayersPendingRemoval.size(); i++) {
-            recordBufferingStats(mLayersPendingRemoval[i]->getName().string(),
-                    mLayersPendingRemoval[i]->getOccupancyHistory(true));
-            mLayersPendingRemoval[i]->onRemoved();
+        for (const auto& l : mLayersPendingRemoval) {
+            recordBufferingStats(l->getName().string(),
+                    l->getOccupancyHistory(true));
+            l->onRemoved();
         }
         mLayersPendingRemoval.clear();
     }
@@ -1799,13 +1791,15 @@
     mAnimCompositionPending = mAnimTransactionPending;
 
     mDrawingState = mCurrentState;
+    mDrawingState.traverseInZOrder([](Layer* layer) {
+        layer->commitChildList();
+    });
     mTransactionPending = false;
     mAnimTransactionPending = false;
     mTransactionCV.broadcast();
 }
 
-void SurfaceFlinger::computeVisibleRegions(
-        const LayerVector& currentLayers, uint32_t layerStack,
+void SurfaceFlinger::computeVisibleRegions(uint32_t layerStack,
         Region& outDirtyRegion, Region& outOpaqueRegion)
 {
     ATRACE_CALL();
@@ -1816,16 +1810,13 @@
 
     outDirtyRegion.clear();
 
-    size_t i = currentLayers.size();
-    while (i--) {
-        const sp<Layer>& layer = currentLayers[i];
-
+    mDrawingState.traverseInReverseZOrder([&](Layer* layer) {
         // start with the whole surface at its current location
         const Layer::State& s(layer->getDrawingState());
 
         // only consider the layers on the given layer stack
-        if (s.layerStack != layerStack)
-            continue;
+        if (layer->getLayerStack() != layerStack)
+            return;
 
         /*
          * opaqueRegion: area of a surface that is fully opaque.
@@ -1860,12 +1851,12 @@
         // handle hidden surfaces by setting the visible region to empty
         if (CC_LIKELY(layer->isVisible())) {
             const bool translucent = !layer->isOpaque(s);
-            Rect bounds(s.active.transform.transform(layer->computeBounds()));
+            Rect bounds(layer->computeScreenBounds());
             visibleRegion.set(bounds);
+            Transform tr = layer->getTransform();
             if (!visibleRegion.isEmpty()) {
                 // Remove the transparent area from the visible region
                 if (translucent) {
-                    const Transform tr(s.active.transform);
                     if (tr.preserveRects()) {
                         // transform the transparent region
                         transparentRegion = tr.transform(s.activeTransparentRegion);
@@ -1877,7 +1868,7 @@
                 }
 
                 // compute the opaque region
-                const int32_t layerOrientation = s.active.transform.getOrientation();
+                const int32_t layerOrientation = tr.getOrientation();
                 if (s.alpha==255 && !translucent &&
                         ((layerOrientation & Transform::ROT_INVALID) == false)) {
                     // the opaque region is the layer's footprint
@@ -1934,7 +1925,7 @@
         layer->setCoveredRegion(coveredRegion);
         layer->setVisibleNonTransparentRegion(
                 visibleRegion.subtract(transparentRegion));
-    }
+    });
 
     outOpaqueRegion = aboveOpaqueLayers;
 }
@@ -1955,7 +1946,6 @@
     Region dirtyRegion;
 
     bool visibleRegions = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
     bool frameQueued = false;
 
     // Store the set of layers that need updates. This set must not change as
@@ -1968,25 +1958,23 @@
     // Display is now waiting on Layer 1's frame, which is behind layer 0's
     // second frame. But layer 0's second frame could be waiting on display.
     Vector<Layer*> layersWithQueuedFrames;
-    for (size_t i = 0, count = layers.size(); i<count ; i++) {
-        const sp<Layer>& layer(layers[i]);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
         if (layer->hasQueuedFrame()) {
             frameQueued = true;
             if (layer->shouldPresentNow(mPrimaryDispSync)) {
-                layersWithQueuedFrames.push_back(layer.get());
+                layersWithQueuedFrames.push_back(layer);
             } else {
                 layer->useEmptyDamage();
             }
         } else {
             layer->useEmptyDamage();
         }
-    }
+    });
     for (size_t i = 0, count = layersWithQueuedFrames.size() ; i<count ; i++) {
         Layer* layer = layersWithQueuedFrames[i];
         const Region dirty(layer->latchBuffer(visibleRegions, latchTime));
         layer->useSurfaceDamage();
-        const Layer::State& s(layer->getDrawingState());
-        invalidateLayerStack(s.layerStack, dirty);
+        invalidateLayerStack(layer->getLayerStack(), dirty);
     }
 
     mVisibleRegionsDirty |= visibleRegions;
@@ -2202,16 +2190,23 @@
 status_t SurfaceFlinger::addClientLayer(const sp<Client>& client,
         const sp<IBinder>& handle,
         const sp<IGraphicBufferProducer>& gbc,
-        const sp<Layer>& lbc)
+        const sp<Layer>& lbc,
+        const sp<Layer>& parent)
 {
     // add this layer to the current state list
     {
         Mutex::Autolock _l(mStateLock);
-        if (mCurrentState.layersSortedByZ.size() >= MAX_LAYERS) {
+        if (mNumLayers >= MAX_LAYERS) {
             return NO_MEMORY;
         }
-        mCurrentState.layersSortedByZ.add(lbc);
+        if (parent == nullptr) {
+            mCurrentState.layersSortedByZ.add(lbc);
+        } else {
+            parent->addChild(lbc);
+        }
         mGraphicBufferProducerList.add(IInterface::asBinder(gbc));
+        mLayersAdded = true;
+        mNumLayers++;
     }
 
     // attach this layer to the client
@@ -2228,14 +2223,22 @@
         return NO_ERROR;
     }
 
-    ssize_t index = mCurrentState.layersSortedByZ.remove(layer);
-    if (index >= 0) {
-        mLayersPendingRemoval.push(layer);
-        mLayersRemoved = true;
-        setTransactionFlags(eTransactionNeeded);
-        return NO_ERROR;
+    const auto& p = layer->getParent();
+    const ssize_t index = (p != nullptr) ? p->removeChild(layer) :
+             mCurrentState.layersSortedByZ.remove(layer);
+
+    if (index < 0) {
+        ALOGE("Failed to find layer (%s) in layer parent (%s).",
+                layer->getName().string(),
+                (p != nullptr) ? p->getName().string() : "no-parent");
+        return BAD_VALUE;
     }
-    return status_t(index);
+
+    mLayersPendingRemoval.add(layer);
+    mLayersRemoved = true;
+    mNumLayers--;
+    setTransactionFlags(eTransactionNeeded);
+    return NO_ERROR;
 }
 
 uint32_t SurfaceFlinger::peekTransactionFlags() {
@@ -2410,13 +2413,20 @@
         }
         if (what & layer_state_t::eLayerChanged) {
             // NOTE: index needs to be calculated before we update the state
-            ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
-            if (layer->setLayer(s.z) && idx >= 0) {
-                mCurrentState.layersSortedByZ.removeAt(idx);
-                mCurrentState.layersSortedByZ.add(layer);
-                // we need traversal (state changed)
-                // AND transaction (list changed)
-                flags |= eTransactionNeeded|eTraversalNeeded;
+            const auto& p = layer->getParent();
+            if (p == nullptr) {
+                ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
+                if (layer->setLayer(s.z) && idx >= 0) {
+                    mCurrentState.layersSortedByZ.removeAt(idx);
+                    mCurrentState.layersSortedByZ.add(layer);
+                    // we need traversal (state changed)
+                    // AND transaction (list changed)
+                    flags |= eTransactionNeeded|eTraversalNeeded;
+                }
+            } else {
+                if (p->setChildLayer(layer, s.z)) {
+                    flags |= eTransactionNeeded|eTraversalNeeded;
+                }
             }
         }
         if (what & layer_state_t::eSizeChanged) {
@@ -2449,9 +2459,17 @@
                 flags |= eTraversalNeeded;
         }
         if (what & layer_state_t::eLayerStackChanged) {
-            // NOTE: index needs to be calculated before we update the state
             ssize_t idx = mCurrentState.layersSortedByZ.indexOf(layer);
-            if (layer->setLayerStack(s.layerStack) && idx >= 0) {
+            // We only allow setting layer stacks for top level layers,
+            // everything else inherits layer stack from its parent.
+            if (layer->hasParent()) {
+                ALOGE("Attempt to set layer stack on layer with parent (%s) is invalid",
+                        layer->getName().string());
+            } else if (idx < 0) {
+                ALOGE("Attempt to set layer stack on layer without parent (%s) that "
+                        "that also does not appear in the top level layer list. Something"
+                        " has gone wrong.", layer->getName().string());
+            } else if (layer->setLayerStack(s.layerStack)) {
                 mCurrentState.layersSortedByZ.removeAt(idx);
                 mCurrentState.layersSortedByZ.add(layer);
                 // we need traversal (state changed)
@@ -2477,7 +2495,8 @@
         const String8& name,
         const sp<Client>& client,
         uint32_t w, uint32_t h, PixelFormat format, uint32_t flags,
-        sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp)
+        sp<IBinder>* handle, sp<IGraphicBufferProducer>* gbp,
+        sp<Layer>* parent)
 {
     if (int32_t(w|h) < 0) {
         ALOGE("createLayer() failed, w or h is negative (w=%d, h=%d)",
@@ -2509,7 +2528,7 @@
         return result;
     }
 
-    result = addClientLayer(client, *handle, *gbp, layer);
+    result = addClientLayer(client, *handle, *gbp, layer, *parent);
     if (result != NO_ERROR) {
         return result;
     }
@@ -2797,12 +2816,9 @@
 void SurfaceFlinger::listLayersLocked(const Vector<String16>& /* args */,
         size_t& /* index */, String8& result) const
 {
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         result.appendFormat("%s\n", layer->getName().string());
-    }
+    });
 }
 
 void SurfaceFlinger::dumpStatsLocked(const Vector<String16>& args, size_t& index,
@@ -2821,14 +2837,11 @@
     if (name.isEmpty()) {
         mAnimFrameTracker.dumpStats(result);
     } else {
-        const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-        const size_t count = currentLayers.size();
-        for (size_t i=0 ; i<count ; i++) {
-            const sp<Layer>& layer(currentLayers[i]);
+        mCurrentState.traverseInZOrder([&](Layer* layer) {
             if (name == layer->getName()) {
                 layer->dumpFrameStats(result);
             }
-        }
+        });
     }
 }
 
@@ -2841,14 +2854,11 @@
         index++;
     }
 
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         if (name.isEmpty() || (name == layer->getName())) {
             layer->clearFrameStats();
         }
-    }
+    });
 
     mAnimFrameTracker.clearStats();
 }
@@ -2856,12 +2866,9 @@
 // This should only be called from the main thread.  Otherwise it would need
 // the lock and should use mCurrentState rather than mDrawingState.
 void SurfaceFlinger::logFrameStats() {
-    const LayerVector& drawingLayers = mDrawingState.layersSortedByZ;
-    const size_t count = drawingLayers.size();
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(drawingLayers[i]);
+    mDrawingState.traverseInZOrder([&](Layer* layer) {
         layer->logFrameStats();
-    }
+    });
 
     mAnimFrameTracker.logAndResetStats(String8("<win-anim>"));
 }
@@ -3015,15 +3022,12 @@
     /*
      * Dump the visible layer list
      */
-    const LayerVector& currentLayers = mCurrentState.layersSortedByZ;
-    const size_t count = currentLayers.size();
     colorizer.bold(result);
-    result.appendFormat("Visible layers (count = %zu)\n", count);
+    result.appendFormat("Visible layers (count = %zu)\n", mNumLayers);
     colorizer.reset(result);
-    for (size_t i=0 ; i<count ; i++) {
-        const sp<Layer>& layer(currentLayers[i]);
+    mCurrentState.traverseInZOrder([&](Layer* layer) {
         layer->dump(result, colorizer);
-    }
+    });
 
     /*
      * Dump Display state
@@ -3459,7 +3463,7 @@
 status_t SurfaceFlinger::captureScreen(const sp<IBinder>& display,
         const sp<IGraphicBufferProducer>& producer,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform, ISurfaceComposer::Rotation rotation) {
 
     if (CC_UNLIKELY(display == 0))
@@ -3500,7 +3504,7 @@
         sp<IGraphicBufferProducer> producer;
         Rect sourceCrop;
         uint32_t reqWidth, reqHeight;
-        uint32_t minLayerZ,maxLayerZ;
+        int32_t minLayerZ,maxLayerZ;
         bool useIdentityTransform;
         Transform::orientation_flags rotation;
         status_t result;
@@ -3510,7 +3514,7 @@
                 const sp<IBinder>& display,
                 const sp<IGraphicBufferProducer>& producer,
                 Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-                uint32_t minLayerZ, uint32_t maxLayerZ,
+                int32_t minLayerZ, int32_t maxLayerZ,
                 bool useIdentityTransform,
                 Transform::orientation_flags rotation,
                 bool isLocalScreenshot)
@@ -3559,7 +3563,7 @@
 void SurfaceFlinger::renderScreenImplLocked(
         const sp<const DisplayDevice>& hw,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool yswap, bool useIdentityTransform, Transform::orientation_flags rotation)
 {
     ATRACE_CALL();
@@ -3603,20 +3607,24 @@
     // redraw the screen entirely...
     engine.clearWithColor(0, 0, 0, 1);
 
-    const LayerVector& layers( mDrawingState.layersSortedByZ );
-    const size_t count = layers.size();
-    for (size_t i=0 ; i<count ; ++i) {
-        const sp<Layer>& layer(layers[i]);
-        const Layer::State& state(layer->getDrawingState());
-        if (state.layerStack == hw->getLayerStack()) {
-            if (state.z >= minLayerZ && state.z <= maxLayerZ) {
-                if (layer->isVisible()) {
-                    if (filtering) layer->setFiltering(true);
-                    layer->draw(hw, useIdentityTransform);
-                    if (filtering) layer->setFiltering(false);
-                }
-            }
+    // We loop through the first level of layers without traversing,
+    // as we need to interpret min/max layer Z in the top level Z space.
+    for (const auto& layer : mDrawingState.layersSortedByZ) {
+        if (layer->getLayerStack() != hw->getLayerStack()) {
+            continue;
         }
+        const Layer::State& state(layer->getDrawingState());
+        if (state.z < minLayerZ || state.z > maxLayerZ) {
+            continue;
+        }
+        layer->traverseInZOrder([&](Layer* layer) {
+            if (!layer->isVisible()) {
+                return;
+            }
+            if (filtering) layer->setFiltering(true);
+            layer->draw(hw, useIdentityTransform);
+            if (filtering) layer->setFiltering(false);
+        });
     }
 
     // compositionComplete is needed for older driver
@@ -3629,7 +3637,7 @@
         const sp<const DisplayDevice>& hw,
         const sp<IGraphicBufferProducer>& producer,
         Rect sourceCrop, uint32_t reqWidth, uint32_t reqHeight,
-        uint32_t minLayerZ, uint32_t maxLayerZ,
+        int32_t minLayerZ, int32_t maxLayerZ,
         bool useIdentityTransform, Transform::orientation_flags rotation,
         bool isLocalScreenshot)
 {
@@ -3653,16 +3661,16 @@
     reqHeight = (!reqHeight) ? hw_h : reqHeight;
 
     bool secureLayerIsVisible = false;
-    const LayerVector& layers(mDrawingState.layersSortedByZ);
-    const size_t count = layers.size();
-    for (size_t i = 0 ; i < count ; ++i) {
-        const sp<Layer>& layer(layers[i]);
+    for (const auto& layer : mDrawingState.layersSortedByZ) {
         const Layer::State& state(layer->getDrawingState());
-        if (state.layerStack == hw->getLayerStack() && state.z >= minLayerZ &&
-                state.z <= maxLayerZ && layer->isVisible() &&
-                layer->isSecure()) {
-            secureLayerIsVisible = true;
+        if ((layer->getLayerStack() != hw->getLayerStack()) ||
+                (state.z < minLayerZ || state.z > maxLayerZ)) {
+            continue;
         }
+        layer->traverseInZOrder([&](Layer *layer) {
+            secureLayerIsVisible = secureLayerIsVisible || (layer->isVisible() &&
+                    layer->isSecure());
+        });
     }
 
     if (!isLocalScreenshot && secureLayerIsVisible) {
@@ -3782,7 +3790,7 @@
 }
 
 void SurfaceFlinger::checkScreenshot(size_t w, size_t s, size_t h, void const* vaddr,
-        const sp<const DisplayDevice>& hw, uint32_t minLayerZ, uint32_t maxLayerZ) {
+        const sp<const DisplayDevice>& hw, int32_t minLayerZ, int32_t maxLayerZ) {
     if (DEBUG_SCREENSHOTS) {
         for (size_t y=0 ; y<h ; y++) {
             uint32_t const * p = (uint32_t const *)vaddr + y*s;
@@ -3793,49 +3801,31 @@
         ALOGE("*** we just took a black screenshot ***\n"
                 "requested minz=%d, maxz=%d, layerStack=%d",
                 minLayerZ, maxLayerZ, hw->getLayerStack());
-        const LayerVector& layers( mDrawingState.layersSortedByZ );
-        const size_t count = layers.size();
-        for (size_t i=0 ; i<count ; ++i) {
-            const sp<Layer>& layer(layers[i]);
+        size_t i = 0;
+        for (const auto& layer : mDrawingState.layersSortedByZ) {
             const Layer::State& state(layer->getDrawingState());
-            const bool visible = (state.layerStack == hw->getLayerStack())
-                                && (state.z >= minLayerZ && state.z <= maxLayerZ)
-                                && (layer->isVisible());
-            ALOGE("%c index=%zu, name=%s, layerStack=%d, z=%d, visible=%d, flags=%x, alpha=%x",
-                    visible ? '+' : '-',
-                            i, layer->getName().string(), state.layerStack, state.z,
+            if (layer->getLayerStack() == hw->getLayerStack() && state.z >= minLayerZ &&
+                    state.z <= maxLayerZ) {
+                layer->traverseInZOrder([&](Layer* layer) {
+                    ALOGE("%c index=%zu, name=%s, layerStack=%d, z=%d, visible=%d, flags=%x, alpha=%x",
+                            layer->isVisible() ? '+' : '-',
+                            i, layer->getName().string(), layer->getLayerStack(), state.z,
                             layer->isVisible(), state.flags, state.alpha);
+                    i++;
+                });
+            }
         }
     }
 }
 
 // ---------------------------------------------------------------------------
 
-SurfaceFlinger::LayerVector::LayerVector() {
+void SurfaceFlinger::State::traverseInZOrder(const std::function<void(Layer*)>& consume) const {
+    layersSortedByZ.traverseInZOrder(consume);
 }
 
-SurfaceFlinger::LayerVector::LayerVector(const LayerVector& rhs)
-    : SortedVector<sp<Layer> >(rhs) {
-}
-
-int SurfaceFlinger::LayerVector::do_compare(const void* lhs,
-    const void* rhs) const
-{
-    // sort layers per layer-stack, then by z-order and finally by sequence
-    const sp<Layer>& l(*reinterpret_cast<const sp<Layer>*>(lhs));
-    const sp<Layer>& r(*reinterpret_cast<const sp<Layer>*>(rhs));
-
-    uint32_t ls = l->getCurrentState().layerStack;
-    uint32_t rs = r->getCurrentState().layerStack;
-    if (ls != rs)
-        return ls - rs;
-
-    uint32_t lz = l->getCurrentState().z;
-    uint32_t rz = r->getCurrentState().z;
-    if (lz != rz)
-        return lz - rz;
-
-    return l->sequence - r->sequence;
+void SurfaceFlinger::State::traverseInReverseZOrder(const std::function<void(Layer*)>& consume) const {
+    layersSortedByZ.traverseInReverseZOrder(consume);
 }
 
 }; // namespace android
diff --git a/services/surfaceflinger/SurfaceInterceptor.cpp b/services/surfaceflinger/SurfaceInterceptor.cpp
index 60bc7f3..2d6472a 100644
--- a/services/surfaceflinger/SurfaceInterceptor.cpp
+++ b/services/surfaceflinger/SurfaceInterceptor.cpp
@@ -74,9 +74,11 @@
 
 void SurfaceInterceptor::saveExistingSurfacesLocked(const SortedVector<sp<Layer>>& layers) {
     ATRACE_CALL();
-    for (const auto& layer : layers) {
-        addSurfaceCreationLocked(createTraceIncrementLocked(), layer);
-        addInitialSurfaceStateLocked(createTraceIncrementLocked(), layer);
+    for (const auto& l : layers) {
+        l->traverseInZOrder([this](Layer* layer) {
+            addSurfaceCreationLocked(createTraceIncrementLocked(), layer);
+            addInitialSurfaceStateLocked(createTraceIncrementLocked(), layer);
+        });
     }
 }
 
diff --git a/services/surfaceflinger/tests/Android.mk b/services/surfaceflinger/tests/Android.mk
index 4f1a8e6..f93d918 100644
--- a/services/surfaceflinger/tests/Android.mk
+++ b/services/surfaceflinger/tests/Android.mk
@@ -20,6 +20,7 @@
     libprotobuf-cpp-full \
     libui \
     libutils \
+    liblog
 
 LOCAL_STATIC_LIBRARIES := libtrace_proto
 
diff --git a/services/surfaceflinger/tests/Transaction_test.cpp b/services/surfaceflinger/tests/Transaction_test.cpp
index 79cd245..d9f1438 100644
--- a/services/surfaceflinger/tests/Transaction_test.cpp
+++ b/services/surfaceflinger/tests/Transaction_test.cpp
@@ -84,6 +84,18 @@
         }
     }
 
+    void expectFGColor(uint32_t x, uint32_t y) {
+        checkPixel(x, y, 195, 63, 63);
+    }
+
+    void expectBGColor(uint32_t x, uint32_t y) {
+        checkPixel(x, y, 63, 63, 195);
+    }
+
+    void expectChildColor(uint32_t x, uint32_t y) {
+        checkPixel(x, y, 200, 200, 200);
+    }
+
 private:
     ScreenCapture(const sp<CpuConsumer>& cc) :
         mCC(cc) {
@@ -140,14 +152,14 @@
 
         mComposerClient->setDisplayLayerStack(display, 0);
 
-        ASSERT_EQ(NO_ERROR, mBGSurfaceControl->setLayer(INT_MAX-2));
+        ASSERT_EQ(NO_ERROR, mBGSurfaceControl->setLayer(INT32_MAX-2));
         ASSERT_EQ(NO_ERROR, mBGSurfaceControl->show());
 
-        ASSERT_EQ(NO_ERROR, mFGSurfaceControl->setLayer(INT_MAX-1));
+        ASSERT_EQ(NO_ERROR, mFGSurfaceControl->setLayer(INT32_MAX-1));
         ASSERT_EQ(NO_ERROR, mFGSurfaceControl->setPosition(64, 64));
         ASSERT_EQ(NO_ERROR, mFGSurfaceControl->show());
 
-        ASSERT_EQ(NO_ERROR, mSyncSurfaceControl->setLayer(INT_MAX-1));
+        ASSERT_EQ(NO_ERROR, mSyncSurfaceControl->setLayer(INT32_MAX-1));
         ASSERT_EQ(NO_ERROR, mSyncSurfaceControl->setPosition(displayWidth-2,
                 displayHeight-2));
         ASSERT_EQ(NO_ERROR, mSyncSurfaceControl->show());
@@ -517,4 +529,105 @@
     }
 }
 
+class ChildLayerTest : public LayerUpdateTest {
+protected:
+    void SetUp() override {
+        LayerUpdateTest::SetUp();
+        mChild = mComposerClient->createSurface(
+                String8("Child surface"),
+                10, 10, PIXEL_FORMAT_RGBA_8888,
+                0, mFGSurfaceControl.get());
+        fillSurfaceRGBA8(mChild, 200, 200, 200);
+
+        {
+            SCOPED_TRACE("before anything");
+            ScreenCapture::captureScreen(&mCapture);
+            mCapture->expectChildColor(64, 64);
+        }
+    }
+    void TearDown() override {
+        LayerUpdateTest::TearDown();
+        mChild = 0;
+    }
+
+    sp<SurfaceControl> mChild;
+    sp<ScreenCapture> mCapture;
+};
+
+TEST_F(ChildLayerTest, ChildLayerPositioning) {
+    SurfaceComposerClient::openGlobalTransaction();
+    mChild->show();
+    mChild->setPosition(10, 10);
+    mFGSurfaceControl->setPosition(64, 64);
+    SurfaceComposerClient::closeGlobalTransaction(true);
+
+    {
+        ScreenCapture::captureScreen(&mCapture);
+        // Top left of foreground must now be visible
+        mCapture->expectFGColor(64, 64);
+        // But 10 pixels in we should see the child surface
+        mCapture->expectChildColor(74, 74);
+        // And 10 more pixels we should be back to the foreground surface
+        mCapture->expectFGColor(84, 84);
+    }
+
+    SurfaceComposerClient::openGlobalTransaction();
+    ASSERT_EQ(NO_ERROR, mFGSurfaceControl->setPosition(0, 0));
+    SurfaceComposerClient::closeGlobalTransaction(true);
+
+    {
+        ScreenCapture::captureScreen(&mCapture);
+        // Top left of foreground should now be at 0, 0
+        mCapture->expectFGColor(0, 0);
+        // But 10 pixels in we should see the child surface
+        mCapture->expectChildColor(10, 10);
+        // And 10 more pixels we should be back to the foreground surface
+        mCapture->expectFGColor(20, 20);
+    }
+}
+
+TEST_F(ChildLayerTest, ChildLayerConstraints) {
+    SurfaceComposerClient::openGlobalTransaction();
+    mChild->show();
+    mFGSurfaceControl->setPosition(0, 0);
+    mChild->setPosition(63, 63);
+    SurfaceComposerClient::closeGlobalTransaction(true);
+
+    {
+        ScreenCapture::captureScreen(&mCapture);
+        mCapture->expectFGColor(0, 0);
+        // Last pixel in foreground should now be the child.
+        mCapture->expectChildColor(63, 63);
+        // But the child should be constrained and the next pixel
+        // must be the background
+        mCapture->expectBGColor(64, 64);
+    }
+}
+
+TEST_F(ChildLayerTest, ChildLayerScaling) {
+    SurfaceComposerClient::openGlobalTransaction();
+    mFGSurfaceControl->setPosition(0, 0);
+    SurfaceComposerClient::closeGlobalTransaction(true);
+
+    // Find the boundary between the parent and child
+    {
+        ScreenCapture::captureScreen(&mCapture);
+        mCapture->expectChildColor(9, 9);
+        mCapture->expectFGColor(10, 10);
+    }
+
+    SurfaceComposerClient::openGlobalTransaction();
+    mFGSurfaceControl->setMatrix(2.0, 0, 0, 2.0);
+    SurfaceComposerClient::closeGlobalTransaction(true);
+
+    // The boundary should be twice as far from the origin now.
+    // The pixels from the last test should all be child now
+    {
+        ScreenCapture::captureScreen(&mCapture);
+        mCapture->expectChildColor(9, 9);
+        mCapture->expectChildColor(10, 10);
+        mCapture->expectChildColor(19, 19);
+        mCapture->expectFGColor(20, 20);
+    }
+}
 }