Fix iOS build to account for Shell rasterizer registration updates. (#4295)

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

@@ -7,6 +7,7 @@
 #include "flutter/common/settings.h"
 #include "flutter/common/threads.h"
 #include "flutter/flow/layers/layer_tree.h"
+#include "flutter/shell/common/platform_view.h"
 #include "flutter/shell/common/rasterizer.h"
 #include "flutter/shell/common/shell.h"
 #include "lib/fxl/synchronization/waitable_event.h"
@@ -88,17 +89,24 @@ void SnapshotRasterizer(fxl::WeakPtr<shell::Rasterizer> rasterizer,
   SkCGDrawBitmap(context, bitmap, 0, 0);
 }
 
+static fxl::WeakPtr<shell::Rasterizer> GetRandomRasterizer() {
+  fxl::WeakPtr<shell::Rasterizer> rasterizer;
+  shell::Shell::Shared().IteratePlatformViews([&rasterizer](shell::PlatformView* view) -> bool {
+    rasterizer = view->rasterizer().GetWeakRasterizerPtr();
+    // We just grab the first rasterizer so there is no need to iterate
+    // further.
+    return false;
+  });
+  return rasterizer;
+}
+
 void SnapshotContents(CGContextRef context, bool is_opaque) {
   // TODO(chinmaygarde): Currently, there is no way to get the rasterizer for
   // a particular platform view from the shell. But, for now, we only have one
   // platform view. So use that. Once we support multiple platform views, the
   // shell will need to provide a way to get the rasterizer for a specific
   // platform view.
-  std::vector<fxl::WeakPtr<shell::Rasterizer>> registered_rasterizers;
-  shell::Shell::Shared().GetRasterizers(&registered_rasterizers);
-  for (auto& rasterizer : registered_rasterizers) {
-    SnapshotRasterizer(rasterizer, context, is_opaque);
-  }
+  SnapshotRasterizer(GetRandomRasterizer(), context, is_opaque);
 }
 
 void SnapshotContentsSync(CGContextRef context, UIView* view) {

shell/platform/darwin/ios/platform_view_ios.mm

@@ -31,7 +31,7 @@ void PlatformViewIOS::Attach() {
 
 void PlatformViewIOS::Attach(fxl::Closure firstFrameCallback) {
   CreateEngine();
-  PostAddToShellTask();
+
   if (firstFrameCallback) {
     firstFrameCallback_ = firstFrameCallback;
     rasterizer_->AddNextFrameCallback([weakSelf = GetWeakPtr()] {