Ensure OpacityLayer to have a single child (#8362)

It ensures that every OpacityLayer can be optimized by retained
rendering.

Tested with `flutter test --local-engine=host_debug_unopt` in
flutter/flutter/packages/flutter.

Related issues: #21756 #23535

flow/layers/container_layer.h

@@ -35,6 +35,9 @@ class ContainerLayer : public Layer {
   void UpdateSceneChildren(SceneUpdateContext& context);
 #endif  // defined(OS_FUCHSIA)
 
+  // For OpacityLayer to restructure to have a single child.
+  void ClearChildren() { layers_.clear(); }
+
  private:
   std::vector<std::shared_ptr<Layer>> layers_;
 

flow/layers/opacity_layer.cc

@@ -4,18 +4,38 @@
 
 #include "flutter/flow/layers/opacity_layer.h"
 
+#include "flutter/flow/layers/transform_layer.h"
+
 namespace flow {
 
 OpacityLayer::OpacityLayer() = default;
 
 OpacityLayer::~OpacityLayer() = default;
 
+void OpacityLayer::EnsureSingleChild() {
+  FML_DCHECK(layers().size() > 0);  // OpacityLayer should never be a leaf
+
+  if (layers().size() == 1) {
+    return;
+  }
+
+  auto new_child = std::make_shared<flow::TransformLayer>();
+  for (auto& child : layers()) {
+    new_child->Add(child);
+  }
+  ClearChildren();
+  Add(new_child);
+}
+
 void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
+  EnsureSingleChild();
   SkMatrix child_matrix = matrix;
   child_matrix.postTranslate(offset_.fX, offset_.fY);
   ContainerLayer::Preroll(context, child_matrix);
   set_paint_bounds(paint_bounds().makeOffset(offset_.fX, offset_.fY));
-  if (context->raster_cache && layers().size() == 1 &&
+  // See |EnsureSingleChild|.
+  FML_DCHECK(layers().size() == 1);
+  if (context->raster_cache &&
       SkRect::Intersects(context->cull_rect, paint_bounds())) {
     Layer* child = layers()[0].get();
     SkMatrix ctm = child_matrix;
@@ -41,12 +61,14 @@ void OpacityLayer::Paint(PaintContext& context) const {
       context.leaf_nodes_canvas->getTotalMatrix()));
 #endif
 
+  // See |EnsureSingleChild|.
+  FML_DCHECK(layers().size() == 1);
+
   // Embedded platform views are changing the canvas in the middle of the paint
   // traversal. To make sure we paint on the right canvas, when the embedded
   // platform views preview is enabled (context.view_embedded is not null) we
   // don't use the cache.
-  if (context.view_embedder == nullptr && layers().size() == 1 &&
-      context.raster_cache) {
+  if (context.view_embedder == nullptr && context.raster_cache) {
     const SkMatrix& ctm = context.leaf_nodes_canvas->getTotalMatrix();
     RasterCacheResult child_cache =
         context.raster_cache->Get(layers()[0].get(), ctm);

flow/layers/opacity_layer.h

@@ -28,6 +28,16 @@ class OpacityLayer : public ContainerLayer {
   int alpha_;
   SkPoint offset_;
 
+  // Restructure (if necessary) OpacityLayer to have only one child.
+  //
+  // This is needed to ensure that retained rendering can always be applied to
+  // save the costly saveLayer.
+  //
+  // If there are multiple children, this creates a new identity TransformLayer,
+  // sets all children to be the TransformLayer's children, and sets that
+  // TransformLayer as the single child of this OpacityLayer.
+  void EnsureSingleChild();
+
   FML_DISALLOW_COPY_AND_ASSIGN(OpacityLayer);
 };