Support querying display refresh rate in engine

runtime/service_protocol.cc

@@ -30,6 +30,8 @@ const fml::StringView ServiceProtocol::kFlushUIThreadTasksExtensionName =
     "_flutter.flushUIThreadTasks";
 const fml::StringView ServiceProtocol::kSetAssetBundlePathExtensionName =
     "_flutter.setAssetBundlePath";
+const fml::StringView ServiceProtocol::kGetDisplayRefreshRateExtensionName =
+    "_flutter.getDisplayRefreshRate";
 
 static constexpr fml::StringView kViewIdPrefx = "_flutterView/";
 static constexpr fml::StringView kListViewsExtensionName = "_flutter.listViews";
@@ -45,6 +47,7 @@ ServiceProtocol::ServiceProtocol()
           kRunInViewExtensionName,
           kFlushUIThreadTasksExtensionName,
           kSetAssetBundlePathExtensionName,
+          kGetDisplayRefreshRateExtensionName,
       }),
       handlers_mutex_(fml::SharedMutex::Create()) {}
 

runtime/service_protocol.h

@@ -27,6 +27,7 @@ class ServiceProtocol {
   static const fml::StringView kRunInViewExtensionName;
   static const fml::StringView kFlushUIThreadTasksExtensionName;
   static const fml::StringView kSetAssetBundlePathExtensionName;
+  static const fml::StringView kGetDisplayRefreshRateExtensionName;
 
   class Handler {
    public:

shell/common/animator.cc

@@ -39,6 +39,10 @@ Animator::Animator(Delegate& delegate,
 
 Animator::~Animator() = default;
 
+float Animator::GetDisplayRefreshRate() const {
+  return waiter_->GetDisplayRefreshRate();
+}
+
 void Animator::Stop() {
   paused_ = true;
 }

shell/common/animator.h

@@ -36,6 +36,8 @@ class Animator final {
 
   ~Animator();
 
+  float GetDisplayRefreshRate() const;
+
   void RequestFrame(bool regenerate_layer_tree = true);
 
   void Render(std::unique_ptr<flow::LayerTree> layer_tree);

shell/common/engine.cc

@@ -69,6 +69,10 @@ Engine::Engine(Delegate& delegate,
 
 Engine::~Engine() = default;
 
+float Engine::GetDisplayRefreshRate() const {
+  return animator_->GetDisplayRefreshRate();
+}
+
 fml::WeakPtr<Engine> Engine::GetWeakPtr() const {
   return weak_factory_.GetWeakPtr();
 }

shell/common/engine.h

@@ -66,6 +66,8 @@ class Engine final : public blink::RuntimeDelegate {
 
   ~Engine() override;
 
+  float GetDisplayRefreshRate() const;
+
   fml::WeakPtr<Engine> GetWeakPtr() const;
 
   FML_WARN_UNUSED_RESULT

shell/common/shell.cc

@@ -293,6 +293,11 @@ Shell::Shell(blink::TaskRunners task_runners, blink::Settings settings)
           task_runners_.GetUITaskRunner(),
           std::bind(&Shell::OnServiceProtocolSetAssetBundlePath, this,
                     std::placeholders::_1, std::placeholders::_2)};
+  service_protocol_handlers_
+      [blink::ServiceProtocol::kGetDisplayRefreshRateExtensionName.ToString()] =
+          {task_runners_.GetUITaskRunner(),
+           std::bind(&Shell::OnServiceProtocolGetDisplayRefreshRate, this,
+                     std::placeholders::_1, std::placeholders::_2)};
 }
 
 Shell::~Shell() {
@@ -939,6 +944,16 @@ bool Shell::OnServiceProtocolFlushUIThreadTasks(
   return true;
 }
 
+bool Shell::OnServiceProtocolGetDisplayRefreshRate(
+    const blink::ServiceProtocol::Handler::ServiceProtocolMap& params,
+    rapidjson::Document& response) {
+  FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
+  response.SetObject();
+  response.AddMember("fps", engine_->GetDisplayRefreshRate(),
+                     response.GetAllocator());
+  return true;
+}
+
 // Service protocol handler
 bool Shell::OnServiceProtocolSetAssetBundlePath(
     const blink::ServiceProtocol::Handler::ServiceProtocolMap& params,

shell/common/shell.h

@@ -225,6 +225,11 @@ class Shell final : public PlatformView::Delegate,
       const blink::ServiceProtocol::Handler::ServiceProtocolMap& params,
       rapidjson::Document& response);
 
+  // Service protocol handler
+  bool OnServiceProtocolGetDisplayRefreshRate(
+      const blink::ServiceProtocol::Handler::ServiceProtocolMap& params,
+      rapidjson::Document& response);
+
   FML_DISALLOW_COPY_AND_ASSIGN(Shell);
 };
 

shell/common/vsync_waiter.h

@@ -14,6 +14,8 @@
 
 namespace shell {
 
+constexpr float kUnknownRefreshRateFPS = 0.0;
+
 class VsyncWaiter : public std::enable_shared_from_this<VsyncWaiter> {
  public:
   using Callback = std::function<void(fml::TimePoint frame_start_time,
@@ -26,6 +28,10 @@ class VsyncWaiter : public std::enable_shared_from_this<VsyncWaiter> {
   void FireCallback(fml::TimePoint frame_start_time,
                     fml::TimePoint frame_target_time);
 
+  // Get the display's maximum refresh rate in the unit of frame per second.
+  // Return 0.0 if the refresh rate is unkonwn.
+  virtual float GetDisplayRefreshRate() const { return 0.0; }
+
  protected:
   const blink::TaskRunners task_runners_;
   std::mutex callback_mutex_;

shell/platform/android/io/flutter/view/FlutterView.java

@@ -806,6 +806,7 @@ private void updateViewportMetrics() {
         WindowManager wm = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
         float fps = wm.getDefaultDisplay().getRefreshRate();
         VsyncWaiter.refreshPeriodNanos = (long) (1000000000.0 / fps);
+        VsyncWaiter.refreshRateFPS = fps;
     }
 
     // Called by native to update the semantics/accessibility tree.

shell/platform/android/io/flutter/view/VsyncWaiter.java

@@ -10,6 +10,10 @@ public class VsyncWaiter {
     // This estimate will be updated by FlutterView when it is attached to a Display.
     public static long refreshPeriodNanos = 1000000000 / 60;
 
+    // This should also be updated by FlutterView when it is attached to a Display.
+    // The initial value of 0.0 indicates unkonwn refresh rate.
+    public static float refreshRateFPS = 0.0f;
+
     public static void asyncWaitForVsync(final long cookie) {
         Choreographer.getInstance().postFrameCallback(new Choreographer.FrameCallback() {
             @Override

shell/platform/android/vsync_waiter_android.cc

@@ -81,6 +81,22 @@ bool VsyncWaiterAndroid::Register(JNIEnv* env) {
   return env->RegisterNatives(clazz, methods, arraysize(methods)) == 0;
 }
 
+float VsyncWaiterAndroid::GetDisplayRefreshRate() const {
+  JNIEnv* env = fml::jni::AttachCurrentThread();
+  if (g_vsync_waiter_class == nullptr) {
+    return kUnknownRefreshRateFPS;
+  }
+  jclass clazz = g_vsync_waiter_class->obj();
+  if (clazz == nullptr) {
+    return kUnknownRefreshRateFPS;
+  }
+  jfieldID fid = env->GetStaticFieldID(clazz, "refreshRateFPS", "F");
+  // We can safely read this 32-bit float from Java in any thread because
+  // 32-bits read and write are guaranteed to be atomic:
+  // https://stackoverflow.com/questions/11459543/should-getters-and-setters-be-synchronized/11459616#11459616
+  return env->GetStaticFloatField(clazz, fid);
+}
+
 static void ConsumePendingCallback(jlong java_baton,
                                    fml::TimePoint frame_start_time,
                                    fml::TimePoint frame_target_time) {

shell/platform/android/vsync_waiter_android.h

@@ -21,6 +21,8 @@ class VsyncWaiterAndroid final : public VsyncWaiter {
 
   ~VsyncWaiterAndroid() override;
 
+  float GetDisplayRefreshRate() const override;
+
  private:
   // |shell::VsyncWaiter|
   void AwaitVSync() override;