[animator] Pass target frametime for Window.onBeginFrame (flutter/engine#14318)

This gives us a time closer to when the frame will be rendered on
screen.

Fixes: https://github.com/flutter/flutter/issues/10850

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -533,6 +533,7 @@ FILE: ../../../flutter/runtime/test_font_data.cc
 FILE: ../../../flutter/runtime/test_font_data.h
 FILE: ../../../flutter/shell/common/animator.cc
 FILE: ../../../flutter/shell/common/animator.h
+FILE: ../../../flutter/shell/common/animator_unittests.cc
 FILE: ../../../flutter/shell/common/canvas_spy.cc
 FILE: ../../../flutter/shell/common/canvas_spy.h
 FILE: ../../../flutter/shell/common/canvas_spy_unittests.cc

engine/src/flutter/fml/platform/linux/timerfd.cc

@@ -34,7 +34,13 @@ namespace fml {
 #endif
 
 bool TimerRearm(int fd, fml::TimePoint time_point) {
-  const uint64_t nano_secs = time_point.ToEpochDelta().ToNanoseconds();
+  uint64_t nano_secs = time_point.ToEpochDelta().ToNanoseconds();
+
+  // "0" will disarm the timer, desired behavior is to immediately
+  // trigger the timer.
+  if (nano_secs < 1) {
+    nano_secs = 1;
+  }
 
   struct itimerspec spec = {};
   spec.it_value.tv_sec = (time_t)(nano_secs / NSEC_PER_SEC);

engine/src/flutter/shell/common/BUILD.gn

@@ -159,6 +159,7 @@ if (current_toolchain == host_toolchain) {
 
   shell_host_executable("shell_unittests") {
     sources = [
+      "animator_unittests.cc",
       "canvas_spy_unittests.cc",
       "input_events_unittests.cc",
       "persistent_cache_unittests.cc",

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

@@ -137,7 +137,7 @@ void Animator::BeginFrame(fml::TimePoint frame_start_time,
   {
     TRACE_EVENT2("flutter", "Framework Workload", "mode", "basic", "frame",
                  FrameParity());
-    delegate_.OnAnimatorBeginFrame(last_begin_frame_time_);
+    delegate_.OnAnimatorBeginFrame(frame_target_time);
   }
 
   if (!frame_scheduled_) {

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

@@ -0,0 +1,95 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#define FML_USED_ON_EMBEDDER
+
+#include <functional>
+#include <future>
+#include <memory>
+
+#include "flutter/shell/common/animator.h"
+#include "flutter/shell/common/shell_test.h"
+#include "flutter/testing/testing.h"
+#include "gtest/gtest.h"
+
+namespace flutter {
+namespace testing {
+
+TEST_F(ShellTest, VSyncTargetTime) {
+  // Add native callbacks to listen for window.onBeginFrame
+  int64_t target_time;
+  fml::AutoResetWaitableEvent on_target_time_latch;
+  auto nativeOnBeginFrame = [&on_target_time_latch,
+                             &target_time](Dart_NativeArguments args) {
+    Dart_Handle exception = nullptr;
+    target_time =
+        tonic::DartConverter<int64_t>::FromArguments(args, 0, exception);
+    on_target_time_latch.Signal();
+  };
+  AddNativeCallback("NativeOnBeginFrame",
+                    CREATE_NATIVE_ENTRY(nativeOnBeginFrame));
+
+  // Create all te prerequisites for a shell.
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+  auto settings = CreateSettingsForFixture();
+
+  std::unique_ptr<Shell> shell;
+
+  TaskRunners task_runners = GetTaskRunnersForFixture();
+  // this is not used as we are not using simulated events.
+  const auto vsync_clock = std::make_shared<ShellTestVsyncClock>();
+  CreateVsyncWaiter create_vsync_waiter = [&]() {
+    return static_cast<std::unique_ptr<VsyncWaiter>>(
+        std::make_unique<ConstantFiringVsyncWaiter>(task_runners));
+  };
+
+  // create a shell with a constant firing vsync waiter.
+  fml::AutoResetWaitableEvent shell_creation;
+
+  auto platform_task = std::async(std::launch::async, [&]() {
+    shell = Shell::Create(
+        task_runners, settings,
+        [vsync_clock, &create_vsync_waiter](Shell& shell) {
+          return std::make_unique<ShellTestPlatformView>(
+              shell, shell.GetTaskRunners(), vsync_clock,
+              std::move(create_vsync_waiter));
+        },
+        [](Shell& shell) {
+          return std::make_unique<Rasterizer>(shell, shell.GetTaskRunners());
+        });
+    ASSERT_TRUE(DartVMRef::IsInstanceRunning());
+
+    auto configuration = RunConfiguration::InferFromSettings(settings);
+    ASSERT_TRUE(configuration.IsValid());
+    configuration.SetEntrypoint("onBeginFrameMain");
+
+    RunEngine(shell.get(), std::move(configuration));
+    shell_creation.Signal();
+  });
+
+  shell_creation.Wait();
+
+  // schedule a frame to trigger window.onBeginFrame
+  fml::TaskRunner::RunNowOrPostTask(shell->GetTaskRunners().GetUITaskRunner(),
+                                    [engine = shell->GetEngine()]() {
+                                      if (engine) {
+                                        // this implies we can re-use the last
+                                        // frame to trigger begin frame rather
+                                        // than re-generating the layer tree.
+                                        engine->ScheduleFrame(true);
+                                      }
+                                    });
+
+  on_target_time_latch.Wait();
+  ASSERT_EQ(ConstantFiringVsyncWaiter::frame_target_time.ToEpochDelta()
+                .ToMicroseconds(),
+            target_time);
+
+  // teardown.
+  DestroyShell(std::move(shell), std::move(task_runners));
+  ASSERT_FALSE(DartVMRef::IsInstanceRunning());
+}
+
+}  // namespace testing
+}  // namespace flutter

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

@@ -435,10 +435,10 @@ TEST_F(ShellTest, FrameRasterizedCallbackIsCalled) {
   auto configuration = RunConfiguration::InferFromSettings(settings);
   configuration.SetEntrypoint("onBeginFrameMain");
 
-  int64_t begin_frame;
-  auto nativeOnBeginFrame = [&begin_frame](Dart_NativeArguments args) {
+  int64_t frame_target_time;
+  auto nativeOnBeginFrame = [&frame_target_time](Dart_NativeArguments args) {
     Dart_Handle exception = nullptr;
-    begin_frame =
+    frame_target_time =
         tonic::DartConverter<int64_t>::FromArguments(args, 0, exception);
   };
   AddNativeCallback("NativeOnBeginFrame",
@@ -455,10 +455,11 @@ TEST_F(ShellTest, FrameRasterizedCallbackIsCalled) {
   std::vector<FrameTiming> timings = {timing};
   CheckFrameTimings(timings, start, finish);
 
-  // Check that onBeginFrame has the same timestamp as FrameTiming's build start
+  // Check that onBeginFrame, which is the frame_target_time, is after
+  // FrameTiming's build start
   int64_t build_start =
       timing.Get(FrameTiming::kBuildStart).ToEpochDelta().ToMicroseconds();
-  ASSERT_EQ(build_start, begin_frame);
+  ASSERT_GT(frame_target_time, build_start);
   DestroyShell(std::move(shell));
 }
 

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

@@ -57,5 +57,13 @@ void ShellTestVsyncWaiter::AwaitVSync() {
   });
 }
 
+void ConstantFiringVsyncWaiter::AwaitVSync() {
+  FML_DCHECK(task_runners_.GetUITaskRunner()->RunsTasksOnCurrentThread());
+  auto async_wait = std::async([this]() {
+    task_runners_.GetPlatformTaskRunner()->PostTask(
+        [this]() { FireCallback(frame_begin_time, frame_target_time); });
+  });
+}
+
 }  // namespace testing
 }  // namespace flutter

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

@@ -41,6 +41,21 @@ class ShellTestVsyncWaiter : public VsyncWaiter {
   std::shared_ptr<ShellTestVsyncClock> clock_;
 };
 
+class ConstantFiringVsyncWaiter : public VsyncWaiter {
+ public:
+  // both of these are set in the past so as to fire immediately.
+  static constexpr fml::TimePoint frame_begin_time =
+      fml::TimePoint::FromEpochDelta(fml::TimeDelta::FromSeconds(0));
+  static constexpr fml::TimePoint frame_target_time =
+      fml::TimePoint::FromEpochDelta(fml::TimeDelta::FromSeconds(100));
+
+  ConstantFiringVsyncWaiter(TaskRunners task_runners)
+      : VsyncWaiter(std::move(task_runners)) {}
+
+ protected:
+  void AwaitVSync() override;
+};
+
 }  // namespace testing
 }  // namespace flutter