ExternalViewEmbedder can CancelFrame after pre-roll (flutter#9660)

* ExternalViewEmbedder can CancelFrame after pre-roll

- Resets the state so next pre-roll can be successful.
- Commit any pending `CATransaction` so we don't create
  nested transactions.

* Update flow/embedded_views.h

flow/embedded_views.h

@@ -187,6 +187,10 @@ class ExternalViewEmbedder {
   // have mutated for last layer tree.
   virtual bool HasPendingViewOperations() = 0;
 
+  // Call this in-lieu of |SubmitFrame| to clear pre-roll state and
+  // sets the stage for the next pre-roll.
+  virtual void CancelFrame() = 0;
+
   virtual void BeginFrame(SkISize frame_size) = 0;
 
   virtual void PrerollCompositeEmbeddedView(

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

@@ -160,6 +160,10 @@
   frame_size_ = frame_size;
 }
 
+void FlutterPlatformViewsController::CancelFrame() {
+  composition_order_.clear();
+}
+
 bool FlutterPlatformViewsController::HasPendingViewOperations() {
   if (!views_to_recomposite_.empty()) {
     return true;

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

@@ -81,6 +81,8 @@ class FlutterPlatformViewsController {
 
   bool HasPendingViewOperations();
 
+  void CancelFrame();
+
   void PrerollCompositeEmbeddedView(int view_id,
                                     std::unique_ptr<flutter::EmbeddedViewParams> params);
 

shell/platform/darwin/ios/ios_surface_gl.h

@@ -51,6 +51,9 @@ class IOSSurfaceGL final : public IOSSurface,
   // |GPUSurfaceGLDelegate|
   flutter::ExternalViewEmbedder* GetExternalViewEmbedder() override;
 
+  // |flutter::ExternalViewEmbedder|
+  void CancelFrame() override;
+
   // |flutter::ExternalViewEmbedder|
   bool HasPendingViewOperations() override;
 

shell/platform/darwin/ios/ios_surface_gl.mm

@@ -82,6 +82,15 @@
   }
 }
 
+void IOSSurfaceGL::CancelFrame() {
+  FlutterPlatformViewsController* platform_views_controller = GetPlatformViewsController();
+  FML_CHECK(platform_views_controller != nullptr);
+  platform_views_controller->CancelFrame();
+  // Committing the current transaction as |BeginFrame| will create a nested
+  // CATransaction otherwise.
+  [CATransaction commit];
+}
+
 bool IOSSurfaceGL::HasPendingViewOperations() {
   FlutterPlatformViewsController* platform_views_controller = GetPlatformViewsController();
   FML_CHECK(platform_views_controller != nullptr);

shell/platform/darwin/ios/ios_surface_software.h

@@ -45,6 +45,9 @@ class IOSSurfaceSoftware final : public IOSSurface,
   // |GPUSurfaceSoftwareDelegate|
   flutter::ExternalViewEmbedder* GetExternalViewEmbedder() override;
 
+  // |flutter::ExternalViewEmbedder|
+  void CancelFrame() override;
+
   // |flutter::ExternalViewEmbedder|
   bool HasPendingViewOperations() override;
 

shell/platform/darwin/ios/ios_surface_software.mm

@@ -135,6 +135,12 @@
   }
 }
 
+void IOSSurfaceSoftware::CancelFrame() {
+  FlutterPlatformViewsController* platform_views_controller = GetPlatformViewsController();
+  FML_CHECK(platform_views_controller != nullptr);
+  platform_views_controller->CancelFrame();
+}
+
 bool IOSSurfaceSoftware::HasPendingViewOperations() {
   FlutterPlatformViewsController* platform_views_controller = GetPlatformViewsController();
   FML_CHECK(platform_views_controller != nullptr);