[Impeller] mark the end of a frame boundary for system compositor interop.

flow/surface_frame.h

@@ -80,6 +80,13 @@ class SurfaceFrame {
     // Time at which this frame is scheduled to be presented. This is a hint
     // that can be passed to the platform to drop queued frames.
     std::optional<fml::TimePoint> presentation_time;
+
+    // Whether this surface frame represents the last in a group frames that
+    // were submitted as part of a platform compositor interop step, such as
+    // during iOS platform view compositing.
+    //
+    // Defaults to true, which is generally a safe value.
+    bool frame_boundary = true;
   };
 
   bool Submit();

shell/gpu/gpu_surface_gl_impeller.cc

@@ -127,14 +127,16 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceGLImpeller::AcquireFrame(
             impeller_dispatcher,
             SkIRect::MakeWH(cull_rect.width, cull_rect.height));
         auto picture = impeller_dispatcher.EndRecordingAsPicture();
+        const bool reset_host_buffer =
+            surface_frame.submit_info().frame_boundary;
 
         return renderer->Render(
             std::move(surface),
             fml::MakeCopyable(
-                [aiks_context, picture = std::move(picture)](
+                [aiks_context, picture = std::move(picture), reset_host_buffer](
                     impeller::RenderTarget& render_target) -> bool {
                   return aiks_context->Render(picture, render_target,
-                                              /*reset_host_buffer=*/true);
+                                              reset_host_buffer);
                 }));
       });
 

shell/gpu/gpu_surface_metal_impeller.mm

@@ -187,12 +187,13 @@
         impeller::DlDispatcher impeller_dispatcher(cull_rect);
         display_list->Dispatch(impeller_dispatcher, sk_cull_rect);
         auto picture = impeller_dispatcher.EndRecordingAsPicture();
+        const bool reset_host_buffer = surface_frame.submit_info().frame_boundary;
 
         return renderer->Render(
             std::move(surface),
-            fml::MakeCopyable([aiks_context, picture = std::move(picture)](
-                                  impeller::RenderTarget& render_target) -> bool {
-              return aiks_context->Render(picture, render_target, /*reset_host_buffer=*/true);
+            fml::MakeCopyable([aiks_context, picture = std::move(picture),
+                               reset_host_buffer](impeller::RenderTarget& render_target) -> bool {
+              return aiks_context->Render(picture, render_target, reset_host_buffer);
             }));
 #endif
       });
@@ -307,11 +308,12 @@
         display_list->Dispatch(impeller_dispatcher, sk_cull_rect);
         auto picture = impeller_dispatcher.EndRecordingAsPicture();
 
+        const bool reset_host_buffer = surface_frame.submit_info().frame_boundary;
         bool render_result = renderer->Render(
             std::move(surface),
-            fml::MakeCopyable([aiks_context, picture = std::move(picture)](
-                                  impeller::RenderTarget& render_target) -> bool {
-              return aiks_context->Render(picture, render_target, /*reset_host_buffer=*/true);
+            fml::MakeCopyable([aiks_context, picture = std::move(picture),
+                               reset_host_buffer](impeller::RenderTarget& render_target) -> bool {
+              return aiks_context->Render(picture, render_target, reset_host_buffer);
             }));
 #endif
         if (!render_result) {

shell/gpu/gpu_surface_metal_impeller_unittests.mm

@@ -97,5 +97,32 @@ GPUCAMetalLayerHandle GetCAMetalLayer(const SkISize& frame_info) const override
   ASSERT_TRUE(frame->Submit());
 }
 
+TEST(GPUSurfaceMetalImpeller, ResetHostBufferBasedOnFrameBoundary) {
+  auto delegate = std::make_shared<TestGPUSurfaceMetalDelegate>();
+  delegate->SetDevice();
+
+  auto context = CreateImpellerContext();
+  std::unique_ptr<Surface> surface =
+      std::make_unique<GPUSurfaceMetalImpeller>(delegate.get(), CreateImpellerContext());
+
+  ASSERT_TRUE(surface->IsValid());
+
+  auto& host_buffer = surface->GetAiksContext()->GetContentContext().GetTransientsBuffer();
+
+  EXPECT_EQ(host_buffer.GetStateForTest().current_frame, 0u);
+
+  auto frame = surface->AcquireFrame(SkISize::Make(100, 100));
+  frame->set_submit_info({.frame_boundary = false});
+
+  ASSERT_TRUE(frame->Submit());
+  EXPECT_EQ(host_buffer.GetStateForTest().current_frame, 0u);
+
+  frame = surface->AcquireFrame(SkISize::Make(100, 100));
+  frame->set_submit_info({.frame_boundary = true});
+
+  ASSERT_TRUE(frame->Submit());
+  EXPECT_EQ(host_buffer.GetStateForTest().current_frame, 1u);
+}
+
 }  // namespace testing
 }  // namespace flutter

shell/gpu/gpu_surface_vulkan_impeller.cc

@@ -117,9 +117,10 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceVulkanImpeller::AcquireFrame(
                   impeller_dispatcher,
                   SkIRect::MakeWH(cull_rect.width, cull_rect.height));
               auto picture = impeller_dispatcher.EndRecordingAsPicture();
-
+              const bool reset_host_buffer =
+                  surface_frame.submit_info().frame_boundary;
               return aiks_context->Render(picture, render_target,
-                                          /*reset_host_buffer=*/true);
+                                          reset_host_buffer);
 #endif
             }));
       });

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

@@ -864,6 +864,10 @@ static bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
   slice->render_into(overlay_canvas);
   overlay_canvas->RestoreToCount(restore_count);
 
+  // This flutter view is never the last in a frame, since we always submit the
+  // underlay view last.
+  frame->set_submit_info({.frame_boundary = false});
+
   layer->did_submit_last_frame = frame->Submit();
   return layer;
 }