Fix child caching in opacity_layer (flutter#17914)

Choose a child more likely to remain stable from frame to frame as the target to cache in the OpacityLayer.

flow/layers/opacity_layer.cc

@@ -55,7 +55,7 @@ void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
 #ifndef SUPPORT_FRACTIONAL_TRANSLATION
     child_matrix = RasterCache::GetIntegralTransCTM(child_matrix);
 #endif
-    TryToPrepareRasterCache(context, container, child_matrix);
+    TryToPrepareRasterCache(context, GetCacheableChild(), child_matrix);
   }
 
   // Restore cull_rect
@@ -78,7 +78,7 @@ void OpacityLayer::Paint(PaintContext& context) const {
 #endif
 
   if (context.raster_cache &&
-      context.raster_cache->Draw(GetChildContainer(),
+      context.raster_cache->Draw(GetCacheableChild(),
                                  *context.leaf_nodes_canvas, &paint)) {
     return;
   }
@@ -118,4 +118,13 @@ ContainerLayer* OpacityLayer::GetChildContainer() const {
   return static_cast<ContainerLayer*>(layers()[0].get());
 }
 
+Layer* OpacityLayer::GetCacheableChild() const {
+  ContainerLayer* child_container = GetChildContainer();
+  if (child_container->layers().size() == 1) {
+    return child_container->layers()[0].get();
+  }
+
+  return child_container;
+}
+
 }  // namespace flutter

flow/layers/opacity_layer.h

@@ -38,8 +38,58 @@ class OpacityLayer : public ContainerLayer {
 #endif  // defined(OS_FUCHSIA)
 
  private:
+  /**
+   * @brief Returns the ContainerLayer used to hold all of the children
+   * of the OpacityLayer.
+   *
+   * Often opacity layers will only have a single child since the associated
+   * Flutter widget is specified with only a single child widget pointer.
+   * But depending on the structure of the child tree that single widget at
+   * the framework level can turn into multiple children at the engine
+   * API level since there is no guarantee of a 1:1 correspondence of widgets
+   * to engine layers. This synthetic child container layer is established to
+   * hold all of the children in a single layer so that we can cache their
+   * output, but this synthetic layer will typically not be the best choice
+   * for the layer cache since the synthetic container is created fresh with
+   * each new OpacityLayer, and so may not be stable from frame to frame.
+   *
+   * @see GetCacheableChild()
+   * @return the ContainerLayer child used to hold the children
+   */
   ContainerLayer* GetChildContainer() const;
 
+  /**
+   * @brief Returns the best choice for a Layer object that can be used
+   * in RasterCache operations to cache the children of the OpacityLayer.
+   *
+   * The returned Layer must represent all children and try to remain stable
+   * if the OpacityLayer is reconstructed in subsequent frames of the scene.
+   *
+   * Note that since the synthetic child container returned from the
+   * GetChildContainer() method is created fresh with each new OpacityLayer,
+   * its return value will not be a good candidate for caching. But if the
+   * standard recommendations for animations are followed and the child widget
+   * is wrapped with a RepaintBoundary widget at the framework level, then
+   * the synthetic child container should contain the same single child layer
+   * on each frame. Under those conditions, that single child of the child
+   * container will be the best candidate for caching in the RasterCache
+   * and this method will return that single child if possible to improve
+   * the performance of caching the children.
+   *
+   * Note that if GetCacheableChild() does not find a single stable child of
+   * the child container it will return the child container as a fallback.
+   * Even though that child is new in each frame of an animation and thus we
+   * cannot reuse the cached layer raster between animation frames, the single
+   * container child will allow us to paint the child onto an offscreen buffer
+   * during Preroll() which reduces one render target switch compared to
+   * painting the child on the fly via an AutoSaveLayer in Paint() and thus
+   * still improves our performance.
+   *
+   * @see GetChildContainer()
+   * @return the best candidate Layer for caching the children
+   */
+  Layer* GetCacheableChild() const;
+
   SkAlpha alpha_;
   SkPoint offset_;
   SkRRect frameRRect_;

flow/layers/opacity_layer_unittests.cc

@@ -53,6 +53,71 @@ TEST_F(OpacityLayerTest, PaintBeforePreollDies) {
 }
 #endif
 
+TEST_F(OpacityLayerTest, ChildIsCached) {
+  const SkAlpha alpha_half = 255 / 2;
+  auto initial_transform = SkMatrix::MakeTrans(50.0, 25.5);
+  auto other_transform = SkMatrix::MakeScale(1.0, 2.0);
+  const SkPath child_path = SkPath().addRect(SkRect::MakeWH(5.0f, 5.0f));
+  auto mock_layer = std::make_shared<MockLayer>(child_path);
+  auto layer =
+      std::make_shared<OpacityLayer>(alpha_half, SkPoint::Make(0.0f, 0.0f));
+  layer->Add(mock_layer);
+
+  SkMatrix cache_ctm = initial_transform;
+  SkCanvas cache_canvas;
+  cache_canvas.setMatrix(cache_ctm);
+  SkCanvas other_canvas;
+  other_canvas.setMatrix(other_transform);
+
+  use_mock_raster_cache();
+
+  EXPECT_EQ(raster_cache()->GetLayerCachedEntriesCount(), (size_t)0);
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer.get(), other_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer.get(), cache_canvas));
+
+  layer->Preroll(preroll_context(), initial_transform);
+
+  EXPECT_EQ(raster_cache()->GetLayerCachedEntriesCount(), (size_t)1);
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer.get(), other_canvas));
+  EXPECT_TRUE(raster_cache()->Draw(mock_layer.get(), cache_canvas));
+}
+
+TEST_F(OpacityLayerTest, ChildrenNotCached) {
+  const SkAlpha alpha_half = 255 / 2;
+  auto initial_transform = SkMatrix::MakeTrans(50.0, 25.5);
+  auto other_transform = SkMatrix::MakeScale(1.0, 2.0);
+  const SkPath child_path1 = SkPath().addRect(SkRect::MakeWH(5.0f, 5.0f));
+  const SkPath child_path2 = SkPath().addRect(SkRect::MakeWH(5.0f, 5.0f));
+  auto mock_layer1 = std::make_shared<MockLayer>(child_path1);
+  auto mock_layer2 = std::make_shared<MockLayer>(child_path2);
+  auto layer =
+      std::make_shared<OpacityLayer>(alpha_half, SkPoint::Make(0.0f, 0.0f));
+  layer->Add(mock_layer1);
+  layer->Add(mock_layer2);
+
+  SkMatrix cache_ctm = initial_transform;
+  SkCanvas cache_canvas;
+  cache_canvas.setMatrix(cache_ctm);
+  SkCanvas other_canvas;
+  other_canvas.setMatrix(other_transform);
+
+  use_mock_raster_cache();
+
+  EXPECT_EQ(raster_cache()->GetLayerCachedEntriesCount(), (size_t)0);
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer1.get(), other_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer1.get(), cache_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer2.get(), other_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer2.get(), cache_canvas));
+
+  layer->Preroll(preroll_context(), initial_transform);
+
+  EXPECT_EQ(raster_cache()->GetLayerCachedEntriesCount(), (size_t)1);
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer1.get(), other_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer1.get(), cache_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer2.get(), other_canvas));
+  EXPECT_FALSE(raster_cache()->Draw(mock_layer2.get(), cache_canvas));
+}
+
 TEST_F(OpacityLayerTest, FullyOpaque) {
   const SkPath child_path = SkPath().addRect(SkRect::MakeWH(5.0f, 5.0f));
   const SkPoint layer_offset = SkPoint::Make(0.5f, 1.5f);

flow/raster_cache.cc

@@ -86,7 +86,7 @@ static bool IsPictureWorthRasterizing(SkPicture* picture,
 }
 
 /// @note Procedure doesn't copy all closures.
-static RasterCacheResult Rasterize(
+static std::unique_ptr<RasterCacheResult> Rasterize(
     GrContext* context,
     const SkMatrix& ctm,
     SkColorSpace* dst_color_space,
@@ -105,7 +105,7 @@ static RasterCacheResult Rasterize(
           : SkSurface::MakeRaster(image_info);
 
   if (!surface) {
-    return {};
+    return nullptr;
   }
 
   SkCanvas* canvas = surface->getCanvas();
@@ -118,14 +118,16 @@ static RasterCacheResult Rasterize(
     DrawCheckerboard(canvas, logical_rect);
   }
 
-  return {surface->makeImageSnapshot(), logical_rect};
+  return std::make_unique<RasterCacheResult>(surface->makeImageSnapshot(),
+                                             logical_rect);
 }
 
-RasterCacheResult RasterizePicture(SkPicture* picture,
-                                   GrContext* context,
-                                   const SkMatrix& ctm,
-                                   SkColorSpace* dst_color_space,
-                                   bool checkerboard) {
+std::unique_ptr<RasterCacheResult> RasterCache::RasterizePicture(
+    SkPicture* picture,
+    GrContext* context,
+    const SkMatrix& ctm,
+    SkColorSpace* dst_color_space,
+    bool checkerboard) const {
   return Rasterize(context, ctm, dst_color_space, checkerboard,
                    picture->cullRect(),
                    [=](SkCanvas* canvas) { canvas->drawPicture(picture); });
@@ -138,34 +140,41 @@ void RasterCache::Prepare(PrerollContext* context,
   Entry& entry = layer_cache_[cache_key];
   entry.access_count++;
   entry.used_this_frame = true;
-  if (!entry.image.is_valid()) {
-    entry.image = Rasterize(
-        context->gr_context, ctm, context->dst_color_space,
-        checkerboard_images_, layer->paint_bounds(),
-        [layer, context](SkCanvas* canvas) {
-          SkISize canvas_size = canvas->getBaseLayerSize();
-          SkNWayCanvas internal_nodes_canvas(canvas_size.width(),
-                                             canvas_size.height());
-          internal_nodes_canvas.addCanvas(canvas);
-          Layer::PaintContext paintContext = {
-              (SkCanvas*)&internal_nodes_canvas,
-              canvas,
-              context->gr_context,
-              nullptr,
-              context->raster_time,
-              context->ui_time,
-              context->texture_registry,
-              context->has_platform_view ? nullptr : context->raster_cache,
-              context->checkerboard_offscreen_layers,
-              context->frame_physical_depth,
-              context->frame_device_pixel_ratio};
-          if (layer->needs_painting()) {
-            layer->Paint(paintContext);
-          }
-        });
+  if (!entry.image) {
+    entry.image = RasterizeLayer(context, layer, ctm, checkerboard_images_);
   }
 }
 
+std::unique_ptr<RasterCacheResult> RasterCache::RasterizeLayer(
+    PrerollContext* context,
+    Layer* layer,
+    const SkMatrix& ctm,
+    bool checkerboard) const {
+  return Rasterize(
+      context->gr_context, ctm, context->dst_color_space, checkerboard,
+      layer->paint_bounds(), [layer, context](SkCanvas* canvas) {
+        SkISize canvas_size = canvas->getBaseLayerSize();
+        SkNWayCanvas internal_nodes_canvas(canvas_size.width(),
+                                           canvas_size.height());
+        internal_nodes_canvas.addCanvas(canvas);
+        Layer::PaintContext paintContext = {
+            (SkCanvas*)&internal_nodes_canvas,  // internal_nodes_canvas
+            canvas,                             // leaf_nodes_canvas
+            context->gr_context,                // gr_context
+            nullptr,                            // view_embedder
+            context->raster_time,
+            context->ui_time,
+            context->texture_registry,
+            context->has_platform_view ? nullptr : context->raster_cache,
+            context->checkerboard_offscreen_layers,
+            context->frame_physical_depth,
+            context->frame_device_pixel_ratio};
+        if (layer->needs_painting()) {
+          layer->Paint(paintContext);
+        }
+      });
+}
+
 bool RasterCache::Prepare(GrContext* context,
                           SkPicture* picture,
                           const SkMatrix& transformation_matrix,
@@ -202,7 +211,7 @@ bool RasterCache::Prepare(GrContext* context,
     return false;
   }
 
-  if (!entry.image.is_valid()) {
+  if (!entry.image) {
     entry.image = RasterizePicture(picture, context, transformation_matrix,
                                    dst_color_space, checkerboard_images_);
     picture_cached_this_frame_++;
@@ -221,8 +230,8 @@ bool RasterCache::Draw(const SkPicture& picture, SkCanvas& canvas) const {
   entry.access_count++;
   entry.used_this_frame = true;
 
-  if (entry.image.is_valid()) {
-    entry.image.draw(canvas);
+  if (entry.image) {
+    entry.image->draw(canvas);
     return true;
   }
 
@@ -242,8 +251,8 @@ bool RasterCache::Draw(const Layer* layer,
   entry.access_count++;
   entry.used_this_frame = true;
 
-  if (entry.image.is_valid()) {
-    entry.image.draw(canvas, paint);
+  if (entry.image) {
+    entry.image->draw(canvas, paint);
     return true;
   }
 
@@ -266,6 +275,14 @@ size_t RasterCache::GetCachedEntriesCount() const {
   return layer_cache_.size() + picture_cache_.size();
 }
 
+size_t RasterCache::GetLayerCachedEntriesCount() const {
+  return layer_cache_.size();
+}
+
+size_t RasterCache::GetPictureCachedEntriesCount() const {
+  return picture_cache_.size();
+}
+
 void RasterCache::SetCheckboardCacheImages(bool checkerboard) {
   if (checkerboard_images_ == checkerboard) {
     return;
@@ -287,15 +304,17 @@ void RasterCache::TraceStatsToTimeline() const {
   size_t picture_cache_bytes = 0;
 
   for (const auto& item : layer_cache_) {
-    const auto dimensions = item.second.image.image_dimensions();
     layer_cache_count++;
-    layer_cache_bytes += dimensions.width() * dimensions.height() * 4;
+    if (item.second.image) {
+      layer_cache_bytes += item.second.image->image_bytes();
+    }
   }
 
   for (const auto& item : picture_cache_) {
-    const auto dimensions = item.second.image.image_dimensions();
     picture_cache_count++;
-    picture_cache_bytes += dimensions.width() * dimensions.height() * 4;
+    if (item.second.image) {
+      picture_cache_bytes += item.second.image->image_bytes();
+    }
   }
 
   FML_TRACE_COUNTER("flutter", "RasterCache",