Add API for embedder clients to process platform events (flutter/engine#4549)

engine/src/flutter/fml/message_loop.cc

@@ -71,4 +71,8 @@ void MessageLoop::RemoveTaskObserver(TaskObserver* observer) {
   loop_->RemoveTaskObserver(observer);
 }
 
+void MessageLoop::RunExpiredTasksNow() {
+  loop_->RunExpiredTasksNow();
+}
+
 }  // namespace fml

engine/src/flutter/fml/message_loop.h

@@ -30,6 +30,10 @@ class MessageLoop {
 
   fxl::RefPtr<fxl::TaskRunner> GetTaskRunner() const;
 
+  // Exposed for the embedder shell which allows clients to poll for events
+  // instead of dedicating a thread to the message loop.
+  void RunExpiredTasksNow();
+
   static void EnsureInitializedForCurrentThread();
 
   static bool IsInitializedForCurrentThread();

engine/src/flutter/fml/message_loop_impl.h

@@ -43,11 +43,13 @@ class MessageLoopImpl : public fxl::RefCountedThreadSafe<MessageLoopImpl> {
 
   void DoTerminate();
 
+  // Exposed for the embedder shell which allows clients to poll for events
+  // instead of dedicating a thread to the message loop.
+  void RunExpiredTasksNow();
+
  protected:
   MessageLoopImpl();
 
-  void RunExpiredTasksNow();
-
  private:
   struct DelayedTask {
     size_t order;

engine/src/flutter/shell/platform/embedder/embedder.cc

@@ -8,6 +8,7 @@
 
 #include <type_traits>
 #include "flutter/common/threads.h"
+#include "flutter/fml/message_loop.h"
 #include "flutter/shell/platform/embedder/platform_view_embedder.h"
 #include "lib/fxl/functional/make_copyable.h"
 
@@ -321,3 +322,8 @@ FlutterResult FlutterEngineSendPlatformMessageResponse(
 
   return kSuccess;
 }
+
+FlutterResult __FlutterEngineFlushPendingTasksNow() {
+  fml::MessageLoop::GetCurrent().RunExpiredTasksNow();
+  return kSuccess;
+}

engine/src/flutter/shell/platform/embedder/embedder.h

@@ -167,6 +167,12 @@ FlutterResult FlutterEngineSendPlatformMessageResponse(
     const uint8_t* data,
     size_t data_length);
 
+// This API is only meant to be used by platforms that need to flush tasks on a
+// message loop not controlled by the Flutter engine. This API will be
+// deprecated soon.
+FLUTTER_EXPORT
+FlutterResult __FlutterEngineFlushPendingTasksNow();
+
 #if defined(__cplusplus)
 }  // extern "C"
 #endif