Reduce number of surfaces required when presenting platform views (flutter#43301)

Fixes flutter#129710
Fixes flutter#138936

Implements https://flutter.dev/go/optimized-platform-view-layers

## Example

This scene would normally require 5 surfaces, but with the PR it comes
down to 2 (when drawing over platform views) and 1 when all drawing is
outside of platform views).


https://github.com/flutter/flutter/assets/96958/091da832-bfbc-44a2-8da5-d55d84024c96

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link to the migration guide as per the [breaking change policy].*

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responsibilities.
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Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
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Author: knopp

flow/embedded_views.cc

@@ -21,9 +21,8 @@ void DisplayListEmbedderViewSlice::end_recording() {
   builder_ = nullptr;
 }
 
-std::list<SkRect> DisplayListEmbedderViewSlice::searchNonOverlappingDrawnRects(
-    const SkRect& query) const {
-  return display_list_->rtree()->searchAndConsolidateRects(query);
+const DlRegion& DisplayListEmbedderViewSlice::getRegion() const {
+  return display_list_->rtree()->region();
 }
 
 void DisplayListEmbedderViewSlice::render_into(DlCanvas* canvas) {

flow/embedded_views.h

@@ -338,8 +338,11 @@ class EmbedderViewSlice {
   virtual ~EmbedderViewSlice() = default;
   virtual DlCanvas* canvas() = 0;
   virtual void end_recording() = 0;
-  virtual std::list<SkRect> searchNonOverlappingDrawnRects(
-      const SkRect& query) const = 0;
+  virtual const DlRegion& getRegion() const = 0;
+  DlRegion region(const SkRect& query) const {
+    return DlRegion::MakeIntersection(getRegion(), DlRegion(query.roundOut()));
+  }
+
   virtual void render_into(DlCanvas* canvas) = 0;
 };
 
@@ -350,8 +353,8 @@ class DisplayListEmbedderViewSlice : public EmbedderViewSlice {
 
   DlCanvas* canvas() override;
   void end_recording() override;
-  std::list<SkRect> searchNonOverlappingDrawnRects(
-      const SkRect& query) const override;
+  const DlRegion& getRegion() const override;
+
   void render_into(DlCanvas* canvas) override;
   void dispatch(DlOpReceiver& receiver);
   bool is_empty();

shell/platform/android/external_view_embedder/external_view_embedder.cc

@@ -104,15 +104,15 @@ void AndroidExternalViewEmbedder::SubmitFrame(
       // The rect above the `current_view_rect`
       SkRect partial_joined_rect = SkRect::MakeEmpty();
       // Each rect corresponds to a native view that renders Flutter UI.
-      std::list<SkRect> intersection_rects =
-          slice->searchNonOverlappingDrawnRects(current_view_rect);
+      std::vector<SkIRect> intersection_rects =
+          slice->region(current_view_rect).getRects();
 
       // Limit the number of native views, so it doesn't grow forever.
       //
       // In this case, the rects are merged into a single one that is the union
       // of all the rects.
-      for (const SkRect& rect : intersection_rects) {
-        partial_joined_rect.join(rect);
+      for (const SkIRect& rect : intersection_rects) {
+        partial_joined_rect.join(SkRect::Make(rect));
       }
       // Get the intersection rect with the `current_view_rect`,
       partial_joined_rect.intersect(current_view_rect);

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews.mm

@@ -705,8 +705,7 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
     for (size_t j = i + 1; j > 0; j--) {
       int64_t current_platform_view_id = composition_order_[j - 1];
       SkRect platform_view_rect = GetPlatformViewRect(current_platform_view_id);
-      std::list<SkRect> intersection_rects =
-          slice->searchNonOverlappingDrawnRects(platform_view_rect);
+      std::vector<SkIRect> intersection_rects = slice->region(platform_view_rect).getRects();
       auto allocation_size = intersection_rects.size();
 
       // For testing purposes, the overlay id is used to find the overlay view.
@@ -719,27 +718,22 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
       // TODO(egarciad): Consider making this configurable.
       // https://github.com/flutter/flutter/issues/52510
       if (allocation_size > kMaxLayerAllocations) {
-        SkRect joined_rect = SkRect::MakeEmpty();
-        for (const SkRect& rect : intersection_rects) {
+        SkIRect joined_rect = SkIRect::MakeEmpty();
+        for (const SkIRect& rect : intersection_rects) {
           joined_rect.join(rect);
         }
         // Replace the rects in the intersection rects list for a single rect that is
         // the union of all the rects in the list.
         intersection_rects.clear();
         intersection_rects.push_back(joined_rect);
       }
-      for (SkRect& joined_rect : intersection_rects) {
+      for (SkIRect& joined_rect : intersection_rects) {
         // Get the intersection rect between the current rect
         // and the platform view rect.
-        joined_rect.intersect(platform_view_rect);
-        // Subpixels in the platform may not align with the canvas subpixels.
-        // To workaround it, round the floating point bounds and make the rect slightly larger.
-        // For example, {0.3, 0.5, 3.1, 4.7} becomes {0, 0, 4, 5}.
-        joined_rect.setLTRB(std::floor(joined_rect.left()), std::floor(joined_rect.top()),
-                            std::ceil(joined_rect.right()), std::ceil(joined_rect.bottom()));
+        joined_rect.intersect(platform_view_rect.roundOut());
         // Clip the background canvas, so it doesn't contain any of the pixels drawn
         // on the overlay layer.
-        background_canvas->ClipRect(joined_rect, DlCanvas::ClipOp::kDifference);
+        background_canvas->ClipRect(SkRect::Make(joined_rect), DlCanvas::ClipOp::kDifference);
         // Get a new host layer.
         std::shared_ptr<FlutterPlatformViewLayer> layer =
             GetLayer(gr_context,                                      //
@@ -817,7 +811,7 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
     GrDirectContext* gr_context,
     const std::shared_ptr<IOSContext>& ios_context,
     EmbedderViewSlice* slice,
-    SkRect rect,
+    SkIRect rect,
     int64_t view_id,
     int64_t overlay_id,
     MTLPixelFormat pixel_format) {
@@ -852,7 +846,7 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
   DlCanvas* overlay_canvas = frame->Canvas();
   int restore_count = overlay_canvas->GetSaveCount();
   overlay_canvas->Save();
-  overlay_canvas->ClipRect(rect);
+  overlay_canvas->ClipRect(SkRect::Make(rect));
   overlay_canvas->Clear(DlColor::kTransparent());
   slice->render_into(overlay_canvas);
   overlay_canvas->RestoreToCount(restore_count);

shell/platform/darwin/ios/framework/Source/FlutterPlatformViews_Internal.h

@@ -319,7 +319,7 @@ class FlutterPlatformViewsController {
   std::shared_ptr<FlutterPlatformViewLayer> GetLayer(GrDirectContext* gr_context,
                                                      const std::shared_ptr<IOSContext>& ios_context,
                                                      EmbedderViewSlice* slice,
-                                                     SkRect rect,
+                                                     SkIRect rect,
                                                      int64_t view_id,
                                                      int64_t overlay_id,
                                                      MTLPixelFormat pixel_format);

shell/platform/embedder/embedder_external_view.cc

@@ -43,7 +43,7 @@ EmbedderExternalView::~EmbedderExternalView() = default;
 
 EmbedderExternalView::RenderTargetDescriptor
 EmbedderExternalView::CreateRenderTargetDescriptor() const {
-  return {view_identifier_, render_surface_size_};
+  return RenderTargetDescriptor(render_surface_size_);
 }
 
 DlCanvas* EmbedderExternalView::GetCanvas() {
@@ -62,9 +62,8 @@ bool EmbedderExternalView::HasPlatformView() const {
   return view_identifier_.platform_view_id.has_value();
 }
 
-std::list<SkRect> EmbedderExternalView::GetEngineRenderedContentsRegion(
-    const SkRect& query) const {
-  return slice_->searchNonOverlappingDrawnRects(query);
+const DlRegion& EmbedderExternalView::GetDlRegion() const {
+  return slice_->getRegion();
 }
 
 bool EmbedderExternalView::HasEngineRenderedContents() {
@@ -87,7 +86,8 @@ const EmbeddedViewParams* EmbedderExternalView::GetEmbeddedViewParams() const {
   return embedded_view_params_.get();
 }
 
-bool EmbedderExternalView::Render(const EmbedderRenderTarget& render_target) {
+bool EmbedderExternalView::Render(const EmbedderRenderTarget& render_target,
+                                  bool clear_surface) {
   TRACE_EVENT0("flutter", "EmbedderExternalView::Render");
   TryEndRecording();
   FML_DCHECK(HasEngineRenderedContents())
@@ -124,7 +124,9 @@ bool EmbedderExternalView::Render(const EmbedderRenderTarget& render_target) {
   DlSkCanvasAdapter dl_canvas(canvas);
   int restore_count = dl_canvas.GetSaveCount();
   dl_canvas.SetTransform(surface_transformation_);
-  dl_canvas.Clear(DlColor::kTransparent());
+  if (clear_surface) {
+    dl_canvas.Clear(DlColor::kTransparent());
+  }
   slice_->render_into(&dl_canvas);
   dl_canvas.RestoreToCount(restore_count);
   dl_canvas.Flush();

shell/platform/embedder/embedder_external_view.h

@@ -46,28 +46,23 @@ class EmbedderExternalView {
   };
 
   struct RenderTargetDescriptor {
-    ViewIdentifier view_identifier;
     SkISize surface_size;
 
-    RenderTargetDescriptor(ViewIdentifier p_view_identifier,
-                           SkISize p_surface_size)
-        : view_identifier(p_view_identifier), surface_size(p_surface_size) {}
+    explicit RenderTargetDescriptor(const SkISize& p_surface_size)
+        : surface_size(p_surface_size) {}
 
     struct Hash {
       constexpr std::size_t operator()(
           const RenderTargetDescriptor& desc) const {
         return fml::HashCombine(desc.surface_size.width(),
-                                desc.surface_size.height(),
-                                ViewIdentifier::Hash{}(desc.view_identifier));
+                                desc.surface_size.height());
       }
     };
 
     struct Equal {
       bool operator()(const RenderTargetDescriptor& lhs,
                       const RenderTargetDescriptor& rhs) const {
-        return lhs.surface_size == rhs.surface_size &&
-               ViewIdentifier::Equal{}(lhs.view_identifier,
-                                       rhs.view_identifier);
+        return lhs.surface_size == rhs.surface_size;
       }
     };
   };
@@ -107,9 +102,10 @@ class EmbedderExternalView {
 
   SkISize GetRenderSurfaceSize() const;
 
-  bool Render(const EmbedderRenderTarget& render_target);
+  bool Render(const EmbedderRenderTarget& render_target,
+              bool clear_surface = true);
 
-  std::list<SkRect> GetEngineRenderedContentsRegion(const SkRect& query) const;
+  const DlRegion& GetDlRegion() const;
 
  private:
   // End the recording of the slice.