Multiview pipeline Pt. 1: Skip illegal render calls (#49266)

This is one of a series of changes to reland #47239.

This PR changes `Animator` so that if `Render` is not called after a `BeginFrame`, this call is ignored.

Note that this is slightly different from #47239. Instead of saying that we should ultimately change this skip to an assertion, this PR aims to keep the skip as the final shape. This is because a while ago we (with @goderbauer and @loic-sharma) decided that `PlatformDispatcher` should contain as little logic as possible to allow testing, and instead serve as a minimal native function binding, which means that we should eventually move the code that validates calling convention to the engine. 

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: dkwingsmt

shell/common/animator.cc

@@ -143,15 +143,12 @@ void Animator::BeginFrame(
 
 void Animator::Render(std::unique_ptr<flutter::LayerTree> layer_tree,
                       float device_pixel_ratio) {
-  has_rendered_ = true;
-
-  if (!frame_timings_recorder_) {
-    // Framework can directly call render with a built scene.
-    frame_timings_recorder_ = std::make_unique<FrameTimingsRecorder>();
-    const fml::TimePoint placeholder_time = fml::TimePoint::Now();
-    frame_timings_recorder_->RecordVsync(placeholder_time, placeholder_time);
-    frame_timings_recorder_->RecordBuildStart(placeholder_time);
+  // Animator::Render should be called after BeginFrame, which is indicated by
+  // frame_timings_recorder_ being non-null. Otherwise, this call is ignored.
+  if (frame_timings_recorder_ == nullptr) {
+    return;
   }
+  has_rendered_ = true;
 
   TRACE_EVENT_WITH_FRAME_NUMBER(frame_timings_recorder_, "flutter",
                                 "Animator::Render", /*flow_id_count=*/0,

shell/common/animator_unittests.cc

@@ -158,20 +158,30 @@ TEST_F(ShellTest, AnimatorDoesNotNotifyIdleBeforeRender) {
   latch.Wait();
   ASSERT_FALSE(delegate.notify_idle_called_);
 
+  fml::AutoResetWaitableEvent render_latch;
   // 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>(LayerTree::Config(),
-                                                      SkISize::Make(600, 800));
-        animator->Render(std::move(layer_tree), 1.0);
+        EXPECT_CALL(delegate, OnAnimatorBeginFrame).WillOnce([&] {
+          auto layer_tree = std::make_unique<LayerTree>(
+              LayerTree::Config(), SkISize::Make(600, 800));
+          animator->Render(std::move(layer_tree), 1.0);
+          render_latch.Signal();
+        });
+        // Request a frame that builds a layer tree and renders a frame.
+        // When the frame is rendered, render_latch will be signaled.
+        animator->RequestFrame(true);
         task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
       },
       // See kNotifyIdleTaskWaitTime in animator.cc.
       fml::TimeDelta::FromMilliseconds(60));
   latch.Wait();
+  render_latch.Wait();
 
-  // Still hasn't notified idle because there has been no frame request.
+  // A frame has been rendered, and the next frame request will notify idle.
+  // But at the moment there isn't another frame request, therefore it still
+  // hasn't notified idle.
   task_runners.GetUITaskRunner()->PostTask([&] {
     ASSERT_FALSE(delegate.notify_idle_called_);
     // False to avoid getting cals to BeginFrame that will request more frames
@@ -239,16 +249,16 @@ TEST_F(ShellTest, AnimatorDoesNotNotifyDelegateIfPipelineIsNotEmpty) {
 
   for (int i = 0; i < 2; i++) {
     task_runners.GetUITaskRunner()->PostTask([&] {
+      EXPECT_CALL(delegate, OnAnimatorBeginFrame).WillOnce([&] {
+        auto layer_tree = std::make_unique<LayerTree>(LayerTree::Config(),
+                                                      SkISize::Make(600, 800));
+        animator->Render(std::move(layer_tree), 1.0);
+        begin_frame_latch.Signal();
+      });
       animator->RequestFrame();
       task_runners.GetPlatformTaskRunner()->PostTask(flush_vsync_task);
     });
     begin_frame_latch.Wait();
-
-    PostTaskSync(task_runners.GetUITaskRunner(), [&] {
-      auto layer_tree = std::make_unique<LayerTree>(LayerTree::Config(),
-                                                    SkISize::Make(600, 800));
-      animator->Render(std::move(layer_tree), 1.0);
-    });
   }
 
   PostTaskSync(task_runners.GetUITaskRunner(), [&] { animator.reset(); });