Do not call Animator::Delegate::OnAnimatorNotifyIdle until at least one frame has been rendered. (flutter/engine#29015)

engine/src/flutter/shell/common/animator.cc

@@ -147,7 +147,7 @@ void Animator::BeginFrame(
     delegate_.OnAnimatorBeginFrame(frame_target_time, frame_number);
   }
 
-  if (!frame_scheduled_) {
+  if (!frame_scheduled_ && has_rendered_) {
     // Under certain workloads (such as our parent view resizing us, which is
     // communicated to us by repeat viewport metrics events), we won't
     // actually have a frame scheduled yet, despite the fact that we *will* be
@@ -177,6 +177,7 @@ void Animator::BeginFrame(
 }
 
 void Animator::Render(std::unique_ptr<flutter::LayerTree> layer_tree) {
+  has_rendered_ = true;
   if (dimension_change_pending_ &&
       layer_tree->frame_size() != last_layer_tree_size_) {
     dimension_change_pending_ = false;
@@ -268,9 +269,10 @@ void Animator::AwaitVSync() {
           }
         }
       });
-
-  delegate_.OnAnimatorNotifyIdle(
-      dart_frame_deadline_.ToEpochDelta().ToMicroseconds());
+  if (has_rendered_) {
+    delegate_.OnAnimatorNotifyIdle(
+        dart_frame_deadline_.ToEpochDelta().ToMicroseconds());
+  }
 }
 
 void Animator::ScheduleSecondaryVsyncCallback(uintptr_t id,

engine/src/flutter/shell/common/animator.h

@@ -117,6 +117,7 @@ class Animator final {
   bool dimension_change_pending_ = false;
   SkISize last_layer_tree_size_ = {0, 0};
   std::deque<uint64_t> trace_flow_ids_;
+  bool has_rendered_ = false;
 
   fml::WeakPtrFactory<Animator> weak_factory_;
 

engine/src/flutter/shell/common/animator_unittests.cc

@@ -18,6 +18,25 @@
 namespace flutter {
 namespace testing {
 
+class FakeAnimatorDelegate : public Animator::Delegate {
+ public:
+  void OnAnimatorBeginFrame(fml::TimePoint frame_target_time,
+                            uint64_t frame_number) override {}
+
+  void OnAnimatorNotifyIdle(int64_t deadline) override {
+    notify_idle_called_ = true;
+  }
+
+  void OnAnimatorDraw(
+      std::shared_ptr<Pipeline<flutter::LayerTree>> pipeline,
+      std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder) override {}
+
+  void OnAnimatorDrawLastLayerTree(
+      std::unique_ptr<FrameTimingsRecorder> frame_timings_recorder) override {}
+
+  bool notify_idle_called_ = false;
+};
+
 TEST_F(ShellTest, VSyncTargetTime) {
   // Add native callbacks to listen for window.onBeginFrame
   int64_t target_time;
@@ -103,7 +122,8 @@ TEST_F(ShellTest, AnimatorStartsPaused) {
   auto settings = CreateSettingsForFixture();
   TaskRunners task_runners = GetTaskRunnersForFixture();
 
-  auto shell = CreateShell(std::move(settings), task_runners);
+  auto shell = CreateShell(std::move(settings), task_runners,
+                           /* simulate_vsync=*/true);
   ASSERT_TRUE(DartVMRef::IsInstanceRunning());
 
   auto configuration = RunConfiguration::InferFromSettings(settings);
@@ -120,5 +140,88 @@ TEST_F(ShellTest, AnimatorStartsPaused) {
   ASSERT_FALSE(DartVMRef::IsInstanceRunning());
 }
 
+TEST_F(ShellTest, AnimatorDoesNotNotifyIdleBeforeRender) {
+  FakeAnimatorDelegate delegate;
+  TaskRunners task_runners = {
+      "test",
+      CreateNewThread(),  // platform
+      CreateNewThread(),  // raster
+      CreateNewThread(),  // ui
+      CreateNewThread()   // io
+  };
+
+  auto clock = std::make_shared<ShellTestVsyncClock>();
+  fml::AutoResetWaitableEvent latch;
+  std::shared_ptr<Animator> animator;
+
+  auto flush_vsync_task = [&] {
+    fml::AutoResetWaitableEvent ui_latch;
+    task_runners.GetUITaskRunner()->PostTask([&] { ui_latch.Signal(); });
+    do {
+      clock->SimulateVSync();
+    } while (ui_latch.WaitWithTimeout(fml::TimeDelta::FromMilliseconds(1)));
+    latch.Signal();
+  };
+
+  // Create the animator on the UI task runner.
+  task_runners.GetUITaskRunner()->PostTask([&] {
+    auto vsync_waiter = static_cast<std::unique_ptr<VsyncWaiter>>(
+        std::make_unique<ShellTestVsyncWaiter>(task_runners, clock));
+    animator = std::make_unique<Animator>(delegate, task_runners,
+                                          std::move(vsync_waiter));
+    latch.Signal();
+  });
+  latch.Wait();
+
+  // Validate it has not notified idle and start it. This will request a frame.
+  task_runners.GetUITaskRunner()->PostTask([&] {
+    ASSERT_FALSE(delegate.notify_idle_called_);
+    animator->Start();
+    // Immediately request a frame saying it can reuse the last layer tree to
+    // avoid more calls to BeginFrame by the animator.
+    animator->RequestFrame(false);
+    task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
+  });
+  latch.Wait();
+  ASSERT_FALSE(delegate.notify_idle_called_);
+
+  // Validate it has not notified idle and try to render.
+  task_runners.GetUITaskRunner()->PostDelayedTask(
+      [&] {
+        ASSERT_FALSE(delegate.notify_idle_called_);
+        auto layer_tree =
+            std::make_unique<LayerTree>(SkISize::Make(600, 800), 1.0);
+        animator->Render(std::move(layer_tree));
+        task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
+      },
+      // See kNotifyIdleTaskWaitTime in animator.cc.
+      fml::TimeDelta::FromMilliseconds(60));
+  latch.Wait();
+
+  // Still hasn't notified idle because there has been no frame request.
+  task_runners.GetUITaskRunner()->PostTask([&] {
+    ASSERT_FALSE(delegate.notify_idle_called_);
+    // False to avoid getting cals to BeginFrame that will request more frames
+    // before we are ready.
+    animator->RequestFrame(false);
+    task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
+  });
+  latch.Wait();
+
+  // Now it should notify idle. Make sure it is destroyed on the UI thread.
+  ASSERT_TRUE(delegate.notify_idle_called_);
+
+  // Stop and do one more flush so we can safely clean up on the UI thread.
+  animator->Stop();
+  task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
+  latch.Wait();
+
+  task_runners.GetUITaskRunner()->PostTask([&] {
+    animator.reset();
+    latch.Signal();
+  });
+  latch.Wait();
+}
+
 }  // namespace testing
 }  // namespace flutter