Engine

  /// Requests that, at the next appropriate opportunity, the [onBeginFrame]
  /// and [onDrawFrame] callbacks be invoked.
  ///
  /// See also:
  ///
  ///  * [SchedulerBinding], the Flutter framework class which manages the
  ///    scheduling of frames.
  void scheduleFrame() native 'PlatformConfiguration_scheduleFrame';

void PlatformConfiguration::RegisterNatives(
    tonic::DartLibraryNatives* natives) {
  natives->Register({
      {"PlatformConfiguration_defaultRouteName", DefaultRouteName, 1, true},
      {"PlatformConfiguration_scheduleFrame", ScheduleFrame, 1, true},
      {"PlatformConfiguration_sendPlatformMessage", _SendPlatformMessage, 4,
       true},
      {"PlatformConfiguration_respondToPlatformMessage",
       _RespondToPlatformMessage, 3, true},
      {"PlatformConfiguration_render", Render, 2, true},
      {"PlatformConfiguration_updateSemantics", UpdateSemantics, 2, true},
      {"PlatformConfiguration_setIsolateDebugName", SetIsolateDebugName, 2,
       true},
      {"PlatformConfiguration_reportUnhandledException",
       ReportUnhandledException, 2, true},
      {"PlatformConfiguration_setNeedsReportTimings", SetNeedsReportTimings, 2,
       true},
      {"PlatformConfiguration_getPersistentIsolateData",
       GetPersistentIsolateData, 1, true},
      {"PlatformConfiguration_computePlatformResolvedLocale",
       _ComputePlatformResolvedLocale, 2, true},
  });
}

void ScheduleFrame(Dart_NativeArguments args) {
  UIDartState::ThrowIfUIOperationsProhibited();
  UIDartState::Current()->platform_configuration()->client()->ScheduleFrame();
}

//------------------------------------------------------------------------------
/// Represents an instance of a running root isolate with window bindings. In
/// normal operation, a single instance of this object is owned by the engine
/// per shell. This object may only be created, used, and collected on the UI
/// task runner. Window state queried by the root isolate is stored by this
/// object. In cold-restart scenarios, the engine may collect this before
/// installing a new runtime controller in its place. The Clone method may be
/// used by the engine to copy the currently accumulated window state so it can
/// be referenced by the new runtime controller.
///
class RuntimeController : public PlatformConfigurationClient {

  // |PlatformConfigurationClient|
  void RuntimeController::ScheduleFrame() {
    client_.ScheduleFrame();
  }
}

class Engine final : public RuntimeDelegate,
                     public HintFreedDelegate,
                     PointerDataDispatcher::Delegate {
  private:
  Delegate& delegate_;
                       
                       
  void Engine::ScheduleFrame(bool regenerate_layer_tree) {
    animator_->RequestFrame(regenerate_layer_tree);
  }
}

class Shell final : public PlatformView::Delegate,
                    public Animator::Delegate,
                    public Engine::Delegate,
                    public Rasterizer::Delegate,
                    public ServiceProtocol::Handler {

void Animator::RequestFrame(bool regenerate_layer_tree) {

  // The AwaitVSync is going to call us back at the next VSync. However, we want
  // to be reasonably certain that the UI thread is not in the middle of a
  // particularly expensive callout. We post the AwaitVSync to run right after
  // an idle. This does NOT provide a guarantee that the UI thread has not
  // started an expensive operation right after posting this message however.
  // To support that, we need edge triggered wakes on VSync.

  task_runners_.GetUITaskRunner()->PostTask([self = weak_factory_.GetWeakPtr(),
                                             frame_number = frame_number_]() {
    if (!self) {
      return;
    }
    TRACE_EVENT_ASYNC_BEGIN0("flutter", "Frame Request Pending", frame_number);
    self->AwaitVSync();
  });
  frame_scheduled_ = true;
}

void Animator::AwaitVSync() {
  waiter_->AsyncWaitForVsync(
      [self = weak_factory_.GetWeakPtr()](fml::TimePoint vsync_start_time,
                                          fml::TimePoint frame_target_time) {
        if (self) {
          if (self->CanReuseLastLayerTree()) {
            self->DrawLastLayerTree();
          } else {
            self->BeginFrame(vsync_start_time, frame_target_time);
          }
        }
      });

  delegate_.OnAnimatorNotifyIdle(dart_frame_deadline_);
}

// Public method invoked by the animator.
void VsyncWaiter::AsyncWaitForVsync(const Callback& callback) {
  if (!callback) {
    return;
  }

  TRACE_EVENT0("flutter", "AsyncWaitForVsync");

  {
    std::scoped_lock lock(callback_mutex_);
    if (callback_) {
      // The animator may request a frame more than once within a frame
      // interval. Multiple calls to request frame must result in a single
      // callback per frame interval.
      TRACE_EVENT_INSTANT0("flutter", "MultipleCallsToVsyncInFrameInterval");
      return;
    }
    callback_ = std::move(callback);
    if (secondary_callback_) {
      // Return directly as `AwaitVSync` is already called by
      // `ScheduleSecondaryCallback`.
      return;
    }
  }
  AwaitVSync();
}

// |VsyncWaiter|
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(),     //调用FlutterJNI.asyncWaitForVsync
                              g_async_wait_for_vsync_method_,  //
                              java_baton                       //
    );
  });
}

  // TODO(mattcarroll): add javadocs
  // Called by native.
  private static void asyncWaitForVsync(final long cookie) {
    if (asyncWaitForVsyncDelegate != null) {
      asyncWaitForVsyncDelegate.asyncWaitForVsync(cookie);
    } else {
      throw new IllegalStateException(
          "An AsyncWaitForVsyncDelegate must be registered with FlutterJNI before asyncWaitForVsync() is invoked.");
    }
  }

public class VsyncWaiter {
  
  private final FlutterJNI.AsyncWaitForVsyncDelegate asyncWaitForVsyncDelegate =
      new FlutterJNI.AsyncWaitForVsyncDelegate() {
        @Override
        public void asyncWaitForVsync(long cookie) {
          Choreographer.getInstance()
              .postFrameCallback(
                  new Choreographer.FrameCallback() {
                    @Override
                    public void doFrame(long frameTimeNanos) {
                      float fps = windowManager.getDefaultDisplay().getRefreshRate();
                      long refreshPeriodNanos = (long) (1000000000.0 / fps);
                      FlutterJNI.nativeOnVsync(
                          frameTimeNanos, frameTimeNanos + refreshPeriodNanos, cookie);
                    }
                  });
        }
      };

// static
void VsyncWaiterAndroid::OnNativeVsync(JNIEnv* env,
                                       jclass jcaller,
                                       jlong frameTimeNanos,
                                       jlong frameTargetTimeNanos,
                                       jlong java_baton) {
  TRACE_EVENT0("flutter", "VSYNC");

  auto frame_time = fml::TimePoint::FromEpochDelta(
      fml::TimeDelta::FromNanoseconds(frameTimeNanos));
  auto target_time = fml::TimePoint::FromEpochDelta(
      fml::TimeDelta::FromNanoseconds(frameTargetTimeNanos));

  ConsumePendingCallback(java_baton, frame_time, target_time);
}

void VsyncWaiterAndroid::ConsumePendingCallback(
    jlong java_baton,
    fml::TimePoint frame_start_time,
    fml::TimePoint frame_target_time) {
  auto* weak_this = reinterpret_cast<std::weak_ptr<VsyncWaiter>*>(java_baton);
  auto shared_this = weak_this->lock();
  delete weak_this;

  if (shared_this) {
    shared_this->FireCallback(frame_start_time, frame_target_time);
  }
}