Listen for Vsync callback on the UI thread directly

fml/platform/android/message_loop_android.cc

@@ -13,6 +13,31 @@ namespace fml {
 
 static constexpr int kClockType = CLOCK_MONOTONIC;
 
+static fml::jni::ScopedJavaGlobalRef<jclass>* g_looper_class = nullptr;
+static jmethodID g_looper_prepare_method_ = nullptr;
+static jmethodID g_looper_loop_method_ = nullptr;
+static jmethodID g_looper_my_looper_method_ = nullptr;
+static jmethodID g_looper_quit_method_ = nullptr;
+
+static void LooperPrepare() {
+  JNIEnv* env = fml::jni::AttachCurrentThread();
+  env->CallStaticVoidMethod(g_looper_class->obj(), g_looper_prepare_method_);
+}
+
+static void LooperLoop() {
+  JNIEnv* env = fml::jni::AttachCurrentThread();
+  env->CallStaticVoidMethod(g_looper_class->obj(), g_looper_loop_method_);
+}
+
+static void LooperQuit() {
+  JNIEnv* env = fml::jni::AttachCurrentThread();
+  auto my_looper = env->CallStaticObjectMethod(g_looper_class->obj(),
+                                               g_looper_my_looper_method_);
+  if (my_looper != nullptr) {
+    env->CallVoidMethod(my_looper, g_looper_quit_method_);
+  }
+}
+
 static ALooper* AcquireLooperForThread() {
   ALooper* looper = ALooper_forThread();
 
@@ -63,23 +88,13 @@ void MessageLoopAndroid::Run() {
   FML_DCHECK(looper_.get() == ALooper_forThread());
 
   running_ = true;
-
-  while (running_) {
-    int result = ::ALooper_pollOnce(-1,       // infinite timeout
-                                    nullptr,  // out fd,
-                                    nullptr,  // out events,
-                                    nullptr   // out data
-    );
-    if (result == ALOOPER_POLL_TIMEOUT || result == ALOOPER_POLL_ERROR) {
-      // This handles the case where the loop is terminated using ALooper APIs.
-      running_ = false;
-    }
-  }
+  LooperPrepare();
+  LooperLoop();
 }
 
 void MessageLoopAndroid::Terminate() {
   running_ = false;
-  ALooper_wake(looper_.get());
+  LooperQuit();
 }
 
 void MessageLoopAndroid::WakeUp(fml::TimePoint time_point) {
@@ -93,4 +108,33 @@ void MessageLoopAndroid::OnEventFired() {
   }
 }
 
+bool MessageLoopAndroid::Register(JNIEnv* env) {
+  jclass clazz = env->FindClass("android/os/Looper");
+
+  if (clazz == nullptr) {
+    return false;
+  }
+
+  g_looper_class = new fml::jni::ScopedJavaGlobalRef<jclass>(env, clazz);
+
+  FML_CHECK(!g_looper_class->is_null());
+
+  g_looper_prepare_method_ =
+      env->GetStaticMethodID(g_looper_class->obj(), "prepare", "()V");
+  FML_CHECK(g_looper_prepare_method_ != nullptr);
+
+  g_looper_loop_method_ =
+      env->GetStaticMethodID(g_looper_class->obj(), "loop", "()V");
+  FML_CHECK(g_looper_loop_method_ != nullptr);
+
+  g_looper_my_looper_method_ = env->GetStaticMethodID(
+      g_looper_class->obj(), "myLooper", "()Landroid/os/Looper;");
+  FML_CHECK(g_looper_my_looper_method_ != nullptr);
+
+  g_looper_quit_method_ =
+      env->GetMethodID(g_looper_class->obj(), "quit", "()V");
+
+  return true;
+}
+
 }  // namespace fml

fml/platform/android/message_loop_android.h

@@ -11,6 +11,7 @@
 
 #include "flutter/fml/macros.h"
 #include "flutter/fml/message_loop_impl.h"
+#include "flutter/fml/platform/android/jni_util.h"
 #include "flutter/fml/unique_fd.h"
 
 namespace fml {
@@ -26,6 +27,9 @@ struct UniqueLooperTraits {
 /// This implemenation wraps usage of Android's \p looper.
 /// \see https://developer.android.com/ndk/reference/group/looper
 class MessageLoopAndroid : public MessageLoopImpl {
+ public:
+  static bool Register(JNIEnv* env);
+
  private:
   fml::UniqueObject<ALooper*, UniqueLooperTraits> looper_;
   fml::UniqueFD timer_fd_;

shell/platform/android/io/flutter/embedding/engine/FlutterJNI.java

@@ -243,9 +243,10 @@ public void setRefreshRateFPS(float refreshRateFPS) {
 
   /**
    * The Android vsync waiter implementation in C++ needs to know when a vsync signal arrives, which
-   * is obtained via Java API. The delegate set here is called on the C++ side when the engine is
-   * ready to wait for the next vsync signal. The delegate is expected to add a postFrameCallback to
-   * the {@link android.view.Choreographer}, and call {@link nativeOnVsync} to notify the engine.
+   * is obtained via Java API. The delegate set here is called on the C++ side on the ui thread when
+   * the engine is ready to wait for the next vsync signal. The delegate is expected to add a
+   * postFrameCallback to the {@link android.view.Choreographer}, and call {@link nativeOnVsync} to
+   * notify the engine.
    *
    * @param delegate The delegate that will call the engine back on the next vsync signal.
    */
@@ -254,7 +255,7 @@ public void setAsyncWaitForVsyncDelegate(@Nullable AsyncWaitForVsyncDelegate del
   }
 
   // TODO(mattcarroll): add javadocs
-  // Called by native.
+  // Called by native on the ui thread.
   private static void asyncWaitForVsync(final long cookie) {
     if (asyncWaitForVsyncDelegate != null) {
       asyncWaitForVsyncDelegate.asyncWaitForVsync(cookie);

shell/platform/android/library_loader.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "flutter/fml/platform/android/jni_util.h"
+#include "flutter/fml/platform/android/message_loop_android.h"
 #include "flutter/shell/platform/android/android_image_generator.h"
 #include "flutter/shell/platform/android/flutter_main.h"
 #include "flutter/shell/platform/android/platform_view_android.h"
@@ -32,5 +33,9 @@ JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved) {
   result = flutter::AndroidImageGenerator::Register(env);
   FML_CHECK(result);
 
+  // Register MessageLoopAndroid.
+  result = fml::MessageLoopAndroid::Register(env);
+  FML_CHECK(result);
+
   return JNI_VERSION_1_4;
 }

shell/platform/android/vsync_waiter_android.cc

@@ -29,13 +29,11 @@ void VsyncWaiterAndroid::AwaitVSync() {
   auto* weak_this = new std::weak_ptr<VsyncWaiter>(shared_from_this());
   jlong java_baton = reinterpret_cast<jlong>(weak_this);
 
-  task_runners_.GetPlatformTaskRunner()->PostTask([java_baton]() {
-    JNIEnv* env = fml::jni::AttachCurrentThread();
-    env->CallStaticVoidMethod(g_vsync_waiter_class->obj(),     //
-                              g_async_wait_for_vsync_method_,  //
-                              java_baton                       //
-    );
-  });
+  JNIEnv* env = fml::jni::AttachCurrentThread();
+  env->CallStaticVoidMethod(g_vsync_waiter_class->obj(),     //
+                            g_async_wait_for_vsync_method_,  //
+                            java_baton                       //
+  );
 }
 
 // static