iOS: only add explicit transactions when there are platform views (only on main threads)

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

@@ -279,6 +279,13 @@
     CancelFrame();
     return PostPrerollResult::kSkipAndRetryFrame;
   }
+  // If the post preroll action is successful, we will display platform views in the current frame.
+  // In order to sync the rendering of the platform views (quartz) with skia's rendering,
+  // We need to begin an explicit CATransaction. This transaction needs to be submitted
+  // after the current frame is submitted.
+  FML_DCHECK([[NSThread currentThread] isMainThread]);
+  [CATransaction begin];
+  catransaction_added_ = true;
   raster_thread_merger->ExtendLeaseTo(kDefaultMergedLeaseDuration);
   return PostPrerollResult::kSuccess;
 }
@@ -548,6 +555,14 @@
 
   did_submit &= frame->Submit();
 
+  // The frame is submitted with embedded platform views.
+  // There should be a |[CATransaction begin]| call in this frame prior to all the drawing.
+  // Now we need to commit the transaction.
+  if (catransaction_added_) {
+    FML_DCHECK([[NSThread currentThread] isMainThread]);
+    [CATransaction commit];
+    catransaction_added_ = false;
+  }
   return did_submit;
 }
 

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

@@ -245,6 +245,8 @@ class FlutterPlatformViewsController {
 
   std::unique_ptr<fml::WeakPtrFactory<FlutterPlatformViewsController>> weak_factory_;
 
+  bool catransaction_added_ = false;
+
   void OnCreate(FlutterMethodCall* call, FlutterResult& result);
   void OnDispose(FlutterMethodCall* call, FlutterResult& result);
   void OnAcceptGesture(FlutterMethodCall* call, FlutterResult& result);

shell/platform/darwin/ios/ios_surface.mm

@@ -71,9 +71,6 @@
   TRACE_EVENT0("flutter", "IOSSurface::CancelFrame");
   FML_CHECK(platform_views_controller_ != nullptr);
   platform_views_controller_->CancelFrame();
-  // Committing the current transaction as |BeginFrame| will create a nested
-  // CATransaction otherwise.
-  [CATransaction commit];
 }
 
 // |ExternalViewEmbedder|
@@ -84,7 +81,6 @@
   TRACE_EVENT0("flutter", "IOSSurface::BeginFrame");
   FML_CHECK(platform_views_controller_ != nullptr);
   platform_views_controller_->SetFrameSize(frame_size);
-  [CATransaction begin];
 }
 
 // |ExternalViewEmbedder|
@@ -102,10 +98,6 @@
   TRACE_EVENT0("flutter", "IOSSurface::PostPrerollAction");
   FML_CHECK(platform_views_controller_ != nullptr);
   PostPrerollResult result = platform_views_controller_->PostPrerollAction(raster_thread_merger);
-  if (result == PostPrerollResult::kSkipAndRetryFrame) {
-    // Commit the current transaction if the frame is dropped.
-    [CATransaction commit];
-  }
   return result;
 }
 
@@ -126,13 +118,8 @@
 void IOSSurface::SubmitFrame(GrDirectContext* context, std::unique_ptr<SurfaceFrame> frame) {
   TRACE_EVENT0("flutter", "IOSSurface::SubmitFrame");
   FML_CHECK(platform_views_controller_ != nullptr);
-  bool submitted =
-      platform_views_controller_->SubmitFrame(std::move(context), ios_context_, std::move(frame));
-
-  if (submitted) {
-    TRACE_EVENT0("flutter", "IOSSurface::DidSubmitFrame");
-    [CATransaction commit];
-  }
+  platform_views_controller_->SubmitFrame(std::move(context), ios_context_, std::move(frame));
+  TRACE_EVENT0("flutter", "IOSSurface::DidSubmitFrame");
 }
 
 // |ExternalViewEmbedder|