Fixed ChildSceneLayer elevation issue on Fuchsia. (flutter/engine#18144)

* Fixed ChildSceneLayer elevation issue on Fuchsia.

ChildSceneLayers would draw on top of stacked Flutter content on
Fuchsia if not wrapped up in a Material widget, i.e. a
PhysicalShapeLayer. This patch pushes the logic from there to all
types of Layers.

engine/src/flutter/flow/layers/child_scene_layer.cc

@@ -20,6 +20,9 @@ ChildSceneLayer::ChildSceneLayer(zx_koid_t layer_id,
 void ChildSceneLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "ChildSceneLayer::Preroll");
   set_needs_system_composite(true);
+
+  CheckForChildLayerBelow(context);
+
   context->child_scene_layer_exists_below = true;
 
   // An alpha "hole punch" is required if the frame behind us is not opaque.
@@ -47,6 +50,8 @@ void ChildSceneLayer::UpdateScene(SceneUpdateContext& context) {
   TRACE_EVENT0("flutter", "ChildSceneLayer::UpdateScene");
   FML_DCHECK(needs_system_composite());
 
+  Layer::UpdateScene(context);
+
   auto* view_holder = ViewHolder::FromId(layer_id_);
   FML_DCHECK(view_holder);
 

engine/src/flutter/flow/layers/container_layer.cc

@@ -29,6 +29,11 @@ void ContainerLayer::Paint(PaintContext& context) const {
 void ContainerLayer::PrerollChildren(PrerollContext* context,
                                      const SkMatrix& child_matrix,
                                      SkRect* child_paint_bounds) {
+#if defined(OS_FUCHSIA)
+  child_layer_exists_below_ = context->child_scene_layer_exists_below;
+  context->child_scene_layer_exists_below = false;
+#endif
+
   // Platform views have no children, so context->has_platform_view should
   // always be false.
   FML_DCHECK(!context->has_platform_view);
@@ -51,6 +56,14 @@ void ContainerLayer::PrerollChildren(PrerollContext* context,
   }
 
   context->has_platform_view = child_has_platform_view;
+
+#if defined(OS_FUCHSIA)
+  if (child_layer_exists_below_) {
+    set_needs_system_composite(true);
+  }
+  context->child_scene_layer_exists_below =
+      context->child_scene_layer_exists_below || child_layer_exists_below_;
+#endif
 }
 
 void ContainerLayer::PaintChildren(PaintContext& context) const {
@@ -67,19 +80,72 @@ void ContainerLayer::PaintChildren(PaintContext& context) const {
 
 #if defined(OS_FUCHSIA)
 
+void ContainerLayer::CheckForChildLayerBelow(PrerollContext* context) {
+  // All ContainerLayers make the check in PrerollChildren.
+}
+
 void ContainerLayer::UpdateScene(SceneUpdateContext& context) {
   UpdateSceneChildren(context);
 }
 
 void ContainerLayer::UpdateSceneChildren(SceneUpdateContext& context) {
+  auto update_scene_layers = [&] {
+    // Paint all of the layers which need to be drawn into the container.
+    // These may be flattened down to a containing Scenic Frame.
+    for (auto& layer : layers_) {
+      if (layer->needs_system_composite()) {
+        layer->UpdateScene(context);
+      }
+    }
+  };
+
   FML_DCHECK(needs_system_composite());
 
-  // Paint all of the layers which need to be drawn into the container.
-  // These may be flattened down to a containing
-  for (auto& layer : layers_) {
-    if (layer->needs_system_composite()) {
-      layer->UpdateScene(context);
+  // If there is embedded Fuchsia content in the scene (a ChildSceneLayer),
+  // PhysicalShapeLayers that appear above the embedded content will be turned
+  // into their own Scenic layers.
+  if (child_layer_exists_below_) {
+    float global_scenic_elevation =
+        context.GetGlobalElevationForNextScenicLayer();
+    float local_scenic_elevation =
+        global_scenic_elevation - context.scenic_elevation();
+    float z_translation = -local_scenic_elevation;
+
+    // Retained rendering: speedup by reusing a retained entity node if
+    // possible. When an entity node is reused, no paint layer is added to the
+    // frame so we won't call PhysicalShapeLayer::Paint.
+    LayerRasterCacheKey key(unique_id(), context.Matrix());
+    if (context.HasRetainedNode(key)) {
+      TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
+      scenic::EntityNode* retained_node = context.GetRetainedNode(key);
+      FML_DCHECK(context.top_entity());
+      FML_DCHECK(retained_node->session() == context.session());
+
+      // Re-adjust the elevation.
+      retained_node->SetTranslation(0.f, 0.f, z_translation);
+
+      context.top_entity()->entity_node().AddChild(*retained_node);
+      return;
     }
+
+    TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
+    // If we can't find an existing retained surface, create one.
+    SceneUpdateContext::Frame frame(
+        context, SkRRect::MakeRect(paint_bounds()), SK_ColorTRANSPARENT,
+        SkScalarRoundToInt(context.alphaf() * 255),
+        "flutter::PhysicalShapeLayer", z_translation, this);
+
+    frame.AddPaintLayer(this);
+
+    // Node: UpdateSceneChildren needs to be called here so that |frame| is
+    // still in scope (and therefore alive) while UpdateSceneChildren is being
+    // called.
+    float scenic_elevation = context.scenic_elevation();
+    context.set_scenic_elevation(scenic_elevation + local_scenic_elevation);
+    update_scene_layers();
+    context.set_scenic_elevation(scenic_elevation);
+  } else {
+    update_scene_layers();
   }
 }
 

engine/src/flutter/flow/layers/container_layer.h

@@ -19,6 +19,7 @@ class ContainerLayer : public Layer {
   void Preroll(PrerollContext* context, const SkMatrix& matrix) override;
   void Paint(PaintContext& context) const override;
 #if defined(OS_FUCHSIA)
+  void CheckForChildLayerBelow(PrerollContext* context) override;
   void UpdateScene(SceneUpdateContext& context) override;
 #endif  // defined(OS_FUCHSIA)
 

engine/src/flutter/flow/layers/layer.cc

@@ -56,7 +56,53 @@ Layer::AutoPrerollSaveLayerState::~AutoPrerollSaveLayerState() {
 }
 
 #if defined(OS_FUCHSIA)
-void Layer::UpdateScene(SceneUpdateContext& context) {}
+
+void Layer::CheckForChildLayerBelow(PrerollContext* context) {
+  child_layer_exists_below_ = context->child_scene_layer_exists_below;
+  if (child_layer_exists_below_) {
+    set_needs_system_composite(true);
+  }
+}
+
+void Layer::UpdateScene(SceneUpdateContext& context) {
+  // If there is embedded Fuchsia content in the scene (a ChildSceneLayer),
+  // PhysicalShapeLayers that appear above the embedded content will be turned
+  // into their own Scenic layers.
+  if (child_layer_exists_below_) {
+    float global_scenic_elevation =
+        context.GetGlobalElevationForNextScenicLayer();
+    float local_scenic_elevation =
+        global_scenic_elevation - context.scenic_elevation();
+    float z_translation = -local_scenic_elevation;
+
+    // Retained rendering: speedup by reusing a retained entity node if
+    // possible. When an entity node is reused, no paint layer is added to the
+    // frame so we won't call PhysicalShapeLayer::Paint.
+    LayerRasterCacheKey key(unique_id(), context.Matrix());
+    if (context.HasRetainedNode(key)) {
+      TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
+      scenic::EntityNode* retained_node = context.GetRetainedNode(key);
+      FML_DCHECK(context.top_entity());
+      FML_DCHECK(retained_node->session() == context.session());
+
+      // Re-adjust the elevation.
+      retained_node->SetTranslation(0.f, 0.f, z_translation);
+
+      context.top_entity()->entity_node().AddChild(*retained_node);
+      return;
+    }
+
+    TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
+    // If we can't find an existing retained surface, create one.
+    SceneUpdateContext::Frame frame(
+        context, SkRRect::MakeRect(paint_bounds()), SK_ColorTRANSPARENT,
+        SkScalarRoundToInt(context.alphaf() * 255),
+        "flutter::PhysicalShapeLayer", z_translation, this);
+
+    frame.AddPaintLayer(this);
+  }
+}
+
 #endif  // defined(OS_FUCHSIA)
 
 Layer::AutoSaveLayer::AutoSaveLayer(const PaintContext& paint_context,

engine/src/flutter/flow/layers/layer.h

@@ -165,6 +165,7 @@ class Layer {
 #if defined(OS_FUCHSIA)
   // Updates the system composited scene.
   virtual void UpdateScene(SceneUpdateContext& context);
+  virtual void CheckForChildLayerBelow(PrerollContext* context);
 #endif
 
   bool needs_system_composite() const { return needs_system_composite_; }
@@ -184,6 +185,11 @@ class Layer {
 
   uint64_t unique_id() const { return unique_id_; }
 
+ protected:
+#if defined(OS_FUCHSIA)
+  bool child_layer_exists_below_ = false;
+#endif
+
  private:
   SkRect paint_bounds_;
   uint64_t unique_id_;

engine/src/flutter/flow/layers/physical_shape_layer.cc

@@ -52,21 +52,10 @@ void PhysicalShapeLayer::Preroll(PrerollContext* context,
 
   context->total_elevation += elevation_;
   total_elevation_ = context->total_elevation;
-#if defined(OS_FUCHSIA)
-  child_layer_exists_below_ = context->child_scene_layer_exists_below;
-  context->child_scene_layer_exists_below = false;
-#endif
 
   SkRect child_paint_bounds;
   PrerollChildren(context, matrix, &child_paint_bounds);
 
-#if defined(OS_FUCHSIA)
-  if (child_layer_exists_below_) {
-    set_needs_system_composite(true);
-  }
-  context->child_scene_layer_exists_below =
-      context->child_scene_layer_exists_below || child_layer_exists_below_;
-#endif
   context->total_elevation -= elevation_;
 
   if (elevation_ == 0) {
@@ -80,62 +69,6 @@ void PhysicalShapeLayer::Preroll(PrerollContext* context,
   }
 }
 
-#if defined(OS_FUCHSIA)
-
-void PhysicalShapeLayer::UpdateScene(SceneUpdateContext& context) {
-  FML_DCHECK(needs_system_composite());
-  TRACE_EVENT0("flutter", "PhysicalShapeLayer::UpdateScene");
-
-  // If there is embedded Fuchsia content in the scene (a ChildSceneLayer),
-  // PhysicalShapeLayers that appear above the embedded content will be turned
-  // into their own Scenic layers.
-  if (child_layer_exists_below_) {
-    float global_scenic_elevation =
-        context.GetGlobalElevationForNextScenicLayer();
-    float local_scenic_elevation =
-        global_scenic_elevation - context.scenic_elevation();
-    float z_translation = -local_scenic_elevation;
-
-    // Retained rendering: speedup by reusing a retained entity node if
-    // possible. When an entity node is reused, no paint layer is added to the
-    // frame so we won't call PhysicalShapeLayer::Paint.
-    LayerRasterCacheKey key(unique_id(), context.Matrix());
-    if (context.HasRetainedNode(key)) {
-      TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
-      scenic::EntityNode* retained_node = context.GetRetainedNode(key);
-      FML_DCHECK(context.top_entity());
-      FML_DCHECK(retained_node->session() == context.session());
-
-      // Re-adjust the elevation.
-      retained_node->SetTranslation(0.f, 0.f, z_translation);
-
-      context.top_entity()->entity_node().AddChild(*retained_node);
-      return;
-    }
-
-    TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
-    // If we can't find an existing retained surface, create one.
-    SceneUpdateContext::Frame frame(context, frameRRect_, SK_ColorTRANSPARENT,
-                                    SkScalarRoundToInt(context.alphaf() * 255),
-                                    "flutter::PhysicalShapeLayer",
-                                    z_translation, this);
-
-    frame.AddPaintLayer(this);
-
-    // Node: UpdateSceneChildren needs to be called here so that |frame| is
-    // still in scope (and therefore alive) while UpdateSceneChildren is being
-    // called.
-    float scenic_elevation = context.scenic_elevation();
-    context.set_scenic_elevation(scenic_elevation + local_scenic_elevation);
-    ContainerLayer::UpdateSceneChildren(context);
-    context.set_scenic_elevation(scenic_elevation);
-  } else {
-    ContainerLayer::UpdateSceneChildren(context);
-  }
-}
-
-#endif  // defined(OS_FUCHSIA)
-
 void PhysicalShapeLayer::Paint(PaintContext& context) const {
   TRACE_EVENT0("flutter", "PhysicalShapeLayer::Paint");
   FML_DCHECK(needs_painting());

engine/src/flutter/flow/layers/physical_shape_layer.h

@@ -35,16 +35,9 @@ class PhysicalShapeLayer : public ContainerLayer {
     return clip_behavior_ == Clip::antiAliasWithSaveLayer;
   }
 
-#if defined(OS_FUCHSIA)
-  void UpdateScene(SceneUpdateContext& context) override;
-#endif  // defined(OS_FUCHSIA)
-
   float total_elevation() const { return total_elevation_; }
 
  private:
-#if defined(OS_FUCHSIA)
-  bool child_layer_exists_below_ = false;
-#endif
   SkColor color_;
   SkColor shadow_color_;
   float elevation_ = 0.0f;

engine/src/flutter/flow/layers/picture_layer.cc

@@ -19,6 +19,11 @@ PictureLayer::PictureLayer(const SkPoint& offset,
 
 void PictureLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "PictureLayer::Preroll");
+
+#if defined(OS_FUCHSIA)
+  CheckForChildLayerBelow(context);
+#endif
+
   SkPicture* sk_picture = picture();
 
   if (auto* cache = context->raster_cache) {

engine/src/flutter/flow/layers/platform_view_layer.cc

@@ -13,6 +13,10 @@ PlatformViewLayer::PlatformViewLayer(const SkPoint& offset,
 
 void PlatformViewLayer::Preroll(PrerollContext* context,
                                 const SkMatrix& matrix) {
+#if defined(OS_FUCHSIA)
+  CheckForChildLayerBelow(context);
+#endif
+
   set_paint_bounds(SkRect::MakeXYWH(offset_.x(), offset_.y(), size_.width(),
                                     size_.height()));
 

engine/src/flutter/flow/layers/shader_mask_layer.cc

@@ -12,6 +12,10 @@ ShaderMaskLayer::ShaderMaskLayer(sk_sp<SkShader> shader,
     : shader_(shader), mask_rect_(mask_rect), blend_mode_(blend_mode) {}
 
 void ShaderMaskLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
+#if defined(OS_FUCHSIA)
+  CheckForChildLayerBelow(context);
+#endif
+
   Layer::AutoPrerollSaveLayerState save =
       Layer::AutoPrerollSaveLayerState::Create(context);
   ContainerLayer::Preroll(context, matrix);

engine/src/flutter/flow/layers/texture_layer.cc

@@ -17,6 +17,10 @@ TextureLayer::TextureLayer(const SkPoint& offset,
 void TextureLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "TextureLayer::Preroll");
 
+#if defined(OS_FUCHSIA)
+  CheckForChildLayerBelow(context);
+#endif
+
   set_paint_bounds(SkRect::MakeXYWH(offset_.x(), offset_.y(), size_.width(),
                                     size_.height()));
 }