Do not involve external_view_embedder in submit frame process if threads are not merged.

Author: Chris Yang

shell/common/rasterizer.cc

@@ -190,8 +190,7 @@ void Rasterizer::Draw(fml::RefPtr<Pipeline<flutter::LayerTree>> pipeline,
     consume_result = PipelineConsumeResult::MoreAvailable;
   }
 
-  // Merging the thread as we know the next `Draw` should be run on the platform
-  // thread.
+  // EndFrame should perform cleanups for the external_view_embedder.
   if (surface_ != nullptr && surface_->GetExternalViewEmbedder() != nullptr) {
     surface_->GetExternalViewEmbedder()->EndFrame(should_resubmit_frame,
                                                   raster_thread_merger_);
@@ -469,7 +468,8 @@ RasterStatus Rasterizer::DrawToSurface(flutter::LayerTree& layer_tree) {
         raster_status == RasterStatus::kSkipAndRetry) {
       return raster_status;
     }
-    if (external_view_embedder != nullptr) {
+    if (external_view_embedder != nullptr &&
+        (!raster_thread_merger_ || raster_thread_merger_->IsMerged())) {
       FML_DCHECK(!frame->IsSubmitted());
       external_view_embedder->SubmitFrame(surface_->GetContext(),
                                           std::move(frame));

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

@@ -469,12 +469,8 @@
 bool FlutterPlatformViewsController::SubmitFrame(GrDirectContext* gr_context,
                                                  std::shared_ptr<IOSContext> ios_context,
                                                  std::unique_ptr<SurfaceFrame> frame) {
-  FML_DCHECK(flutter_view_);
-
   // Any UIKit related code has to run on main thread.
-  // When on a non-main thread, we only allow the rest of the method to run if there is no
-  // Pending UIView operations.
-  FML_DCHECK([[NSThread currentThread] isMainThread] || !HasPlatformViewThisOrNextFrame());
+  FML_DCHECK(flutter_view_ && [[NSThread currentThread] isMainThread]);
 
   DisposeViews();
 
@@ -558,8 +554,6 @@
   BringLayersIntoView(platform_view_layers);
   // Mark all layers as available, so they can be used in the next frame.
   layer_pool_->RecycleLayers();
-  // Reset the composition order, so next frame starts empty.
-  composition_order_.clear();
 
   did_submit &= frame->Submit();
 
@@ -599,7 +593,10 @@
 
 void FlutterPlatformViewsController::EndFrame(
     bool should_resubmit_frame,
-    fml::RefPtr<fml::RasterThreadMerger> raster_thread_merger) {}
+    fml::RefPtr<fml::RasterThreadMerger> raster_thread_merger) {
+  // Reset the composition order, so next frame starts empty.
+  composition_order_.clear();
+}
 
 std::shared_ptr<FlutterPlatformViewLayer> FlutterPlatformViewsController::GetLayer(
     GrDirectContext* gr_context,