Dynamically determine whether to use offscreen surface based on need

ci/licenses_golden/licenses_flutter

@@ -46,6 +46,7 @@ FILE: ../../../flutter/flow/layers/layer.cc
 FILE: ../../../flutter/flow/layers/layer.h
 FILE: ../../../flutter/flow/layers/layer_tree.cc
 FILE: ../../../flutter/flow/layers/layer_tree.h
+FILE: ../../../flutter/flow/layers/layer_unittests.cc
 FILE: ../../../flutter/flow/layers/opacity_layer.cc
 FILE: ../../../flutter/flow/layers/opacity_layer.h
 FILE: ../../../flutter/flow/layers/performance_overlay_layer.cc

flow/BUILD.gn

@@ -109,6 +109,7 @@ executable("flow_unittests") {
     "flow_run_all_unittests.cc",
     "flow_test_utils.cc",
     "flow_test_utils.h",
+    "layers/layer_unittests.cc",
     "layers/performance_overlay_layer_unittests.cc",
     "layers/physical_shape_layer_unittests.cc",
     "matrix_decomposition_unittests.cc",

flow/layers/backdrop_filter_layer.cc

@@ -7,7 +7,9 @@
 namespace flutter {
 
 BackdropFilterLayer::BackdropFilterLayer(sk_sp<SkImageFilter> filter)
-    : filter_(std::move(filter)) {}
+    : filter_(std::move(filter)) {
+  set_layer_reads_surface(filter_.get() != nullptr);
+}
 
 BackdropFilterLayer::~BackdropFilterLayer() = default;
 

flow/layers/clip_path_layer.cc

@@ -15,6 +15,7 @@ namespace flutter {
 ClipPathLayer::ClipPathLayer(const SkPath& clip_path, Clip clip_behavior)
     : clip_path_(clip_path), clip_behavior_(clip_behavior) {
   FML_DCHECK(clip_behavior != Clip::none);
+  set_renders_to_save_layer(clip_behavior == Clip::antiAliasWithSaveLayer);
 }
 
 ClipPathLayer::~ClipPathLayer() = default;

flow/layers/clip_rect_layer.cc

@@ -9,6 +9,7 @@ namespace flutter {
 ClipRectLayer::ClipRectLayer(const SkRect& clip_rect, Clip clip_behavior)
     : clip_rect_(clip_rect), clip_behavior_(clip_behavior) {
   FML_DCHECK(clip_behavior != Clip::none);
+  set_renders_to_save_layer(clip_behavior == Clip::antiAliasWithSaveLayer);
 }
 
 ClipRectLayer::~ClipRectLayer() = default;

flow/layers/clip_rrect_layer.cc

@@ -9,6 +9,7 @@ namespace flutter {
 ClipRRectLayer::ClipRRectLayer(const SkRRect& clip_rrect, Clip clip_behavior)
     : clip_rrect_(clip_rrect), clip_behavior_(clip_behavior) {
   FML_DCHECK(clip_behavior != Clip::none);
+  set_renders_to_save_layer(clip_behavior == Clip::antiAliasWithSaveLayer);
 }
 
 ClipRRectLayer::~ClipRRectLayer() = default;

flow/layers/color_filter_layer.cc

@@ -7,7 +7,9 @@
 namespace flutter {
 
 ColorFilterLayer::ColorFilterLayer(sk_sp<SkColorFilter> filter)
-    : filter_(std::move(filter)) {}
+    : filter_(std::move(filter)) {
+  set_renders_to_save_layer(true);
+}
 
 ColorFilterLayer::~ColorFilterLayer() = default;
 

flow/layers/container_layer.cc

@@ -6,13 +6,46 @@
 
 namespace flutter {
 
-ContainerLayer::ContainerLayer() {}
+ContainerLayer::ContainerLayer()
+    : child_needs_screen_readback_(false), renders_to_save_layer_(false) {}
 
 ContainerLayer::~ContainerLayer() = default;
 
 void ContainerLayer::Add(std::shared_ptr<Layer> layer) {
-  layer->set_parent(this);
+  Layer* the_layer = layer.get();
+  the_layer->set_parent(this);
   layers_.push_back(std::move(layer));
+  if (the_layer->tree_reads_surface()) {
+    NotifyChildReadback(the_layer);
+  }
+}
+
+void ContainerLayer::ClearChildren() {
+  layers_.clear();
+  if (child_needs_screen_readback_) {
+    child_needs_screen_readback_ = false;
+    UpdateTreeReadsSurface();
+  }
+}
+
+void ContainerLayer::set_renders_to_save_layer(bool value) {
+  if (renders_to_save_layer_ != value) {
+    renders_to_save_layer_ = value;
+    UpdateTreeReadsSurface();
+  }
+}
+
+void ContainerLayer::NotifyChildReadback(const Layer* layer) {
+  if (child_needs_screen_readback_ || !layer->tree_reads_surface()) {
+    return;
+  }
+  child_needs_screen_readback_ = true;
+  UpdateTreeReadsSurface();
+}
+
+bool ContainerLayer::ComputeTreeReadsSurface() const {
+  return layer_reads_surface() ||
+         (!renders_to_save_layer_ && child_needs_screen_readback_);
 }
 
 void ContainerLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {

flow/layers/container_layer.h

@@ -25,22 +25,43 @@ class ContainerLayer : public Layer {
 
   const std::vector<std::shared_ptr<Layer>>& layers() const { return layers_; }
 
+  // Called when the layer, which must be a child of this container,
+  // changes its tree_reads_surface() result from false to true.
+  void NotifyChildReadback(const Layer* layer);
+
  protected:
   void PrerollChildren(PrerollContext* context,
                        const SkMatrix& child_matrix,
                        SkRect* child_paint_bounds);
   void PaintChildren(PaintContext& context) const;
 
+  virtual bool ComputeTreeReadsSurface() const override;
+
 #if defined(OS_FUCHSIA)
   void UpdateSceneChildren(SceneUpdateContext& context);
 #endif  // defined(OS_FUCHSIA)
 
+  // Specify whether or not the container has its children render
+  // to a SaveLayer which will prevent many rendering anomalies
+  // from propagating to the parent - such as if the children
+  // read back from the surface on which they render, or if the
+  // children perform non-associative rendering. Those children
+  // will now be performing those operations on the SaveLayer
+  // rather than the layer that this container renders onto.
+  void set_renders_to_save_layer(bool value);
+
   // For OpacityLayer to restructure to have a single child.
-  void ClearChildren() { layers_.clear(); }
+  void ClearChildren();
 
  private:
   std::vector<std::shared_ptr<Layer>> layers_;
 
+  // child_needs_screen_readback_ is maintained even if the
+  // renders_to_save_layer_ property is set in case both
+  // parameters are dynamically and independently determined.
+  bool child_needs_screen_readback_;
+  bool renders_to_save_layer_;
+
   FML_DISALLOW_COPY_AND_ASSIGN(ContainerLayer);
 };
 

flow/layers/layer.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "flutter/flow/layers/layer.h"
+#include "flutter/flow/layers/container_layer.h"
 
 #include "flutter/flow/paint_utils.h"
 #include "third_party/skia/include/core/SkColorFilter.h"
@@ -13,7 +14,9 @@ Layer::Layer()
     : parent_(nullptr),
       needs_system_composite_(false),
       paint_bounds_(SkRect::MakeEmpty()),
-      unique_id_(NextUniqueID()) {}
+      unique_id_(NextUniqueID()),
+      tree_reads_surface_(false),
+      layer_reads_surface_(false) {}
 
 Layer::~Layer() = default;
 
@@ -26,6 +29,28 @@ uint64_t Layer::NextUniqueID() {
   return id;
 }
 
+void Layer::set_layer_reads_surface(bool value) {
+  if (layer_reads_surface_ != value) {
+    layer_reads_surface_ = value;
+    UpdateTreeReadsSurface();
+  }
+}
+
+bool Layer::ComputeTreeReadsSurface() const {
+  return layer_reads_surface_;
+}
+
+void Layer::UpdateTreeReadsSurface() {
+  bool new_tree_reads_surface = ComputeTreeReadsSurface();
+
+  if (tree_reads_surface_ != new_tree_reads_surface) {
+    tree_reads_surface_ = new_tree_reads_surface;
+    if (parent_ != nullptr) {
+      parent_->NotifyChildReadback(this);
+    }
+  }
+}
+
 void Layer::Preroll(PrerollContext* context, const SkMatrix& matrix) {}
 
 #if defined(OS_FUCHSIA)

flow/layers/layer.h

@@ -145,14 +145,47 @@ class Layer {
 
   bool needs_painting() const { return !paint_bounds_.isEmpty(); }
 
+  // True iff the layer, or some descendant of the layer, performs an
+  // operation which depends on (i.e. must read back from) the surface
+  // on which it is rendered in any way beyond the functionality of
+  // BlendMode. This value has no setter as it is computed from other
+  // flags and properties on the layer.
+  // For an example see |BackdropFilterLayer|.
+  //
+  // See |UpdateTreeReadsSurface|
+  // See |set_layer_reads_surface|
+  // See |ContainerLayer::set_renders_to_save_layer|
+  // See |ContainerLayer::NotifyChildReadback|
+  bool tree_reads_surface() const { return tree_reads_surface_; }
+
   uint64_t unique_id() const { return unique_id_; }
 
+ protected:
+  // Compute a new value for tree_reads_surface_ from all of the various
+  // properties of this layer.
+  // Used by |UpdateTreeReadsSurface|
+  virtual bool ComputeTreeReadsSurface() const;
+
+  // Update the tree_reads_surface_ value and propagate changes to
+  // ancestors if needed.
+  // Uses |ComputeTreeReadsSurface|
+  void UpdateTreeReadsSurface();
+
+  // True iff the layer itself (not a child or other descendant) performs
+  // an operation which reads from the surface on which it is rendered.
+  bool layer_reads_surface() const { return layer_reads_surface_; }
+
+  void set_layer_reads_surface(bool value);
+
  private:
   ContainerLayer* parent_;
   bool needs_system_composite_;
   SkRect paint_bounds_;
   uint64_t unique_id_;
 
+  bool tree_reads_surface_;
+  bool layer_reads_surface_;
+
   static uint64_t NextUniqueID();
 
   FML_DISALLOW_COPY_AND_ASSIGN(Layer);

flow/layers/layer_tree.h

@@ -43,6 +43,10 @@ class LayerTree {
     root_layer_ = std::move(root_layer);
   }
 
+  bool root_needs_screen_readback() const {
+    return root_layer_ && root_layer_->tree_reads_surface();
+  }
+
   const SkISize& frame_size() const { return frame_size_; }
 
   void set_frame_size(const SkISize& frame_size) { frame_size_ = frame_size; }