Variable Refresh Rate Display (flutter/engine#30223)

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -765,6 +765,10 @@ FILE: ../../../flutter/shell/common/switches.h
 FILE: ../../../flutter/shell/common/switches_unittests.cc
 FILE: ../../../flutter/shell/common/thread_host.cc
 FILE: ../../../flutter/shell/common/thread_host.h
+FILE: ../../../flutter/shell/common/variable_refresh_rate_display.cc
+FILE: ../../../flutter/shell/common/variable_refresh_rate_display.h
+FILE: ../../../flutter/shell/common/variable_refresh_rate_display_unittests.cc
+FILE: ../../../flutter/shell/common/variable_refresh_rate_reporter.h
 FILE: ../../../flutter/shell/common/vsync_waiter.cc
 FILE: ../../../flutter/shell/common/vsync_waiter.h
 FILE: ../../../flutter/shell/common/vsync_waiter_fallback.cc

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

@@ -96,6 +96,9 @@ source_set("common") {
     "switches.h",
     "thread_host.cc",
     "thread_host.h",
+    "variable_refresh_rate_display.cc",
+    "variable_refresh_rate_display.h",
+    "variable_refresh_rate_reporter.h",
     "vsync_waiter.cc",
     "vsync_waiter.h",
     "vsync_waiter_fallback.cc",
@@ -266,6 +269,7 @@ if (enable_unittests) {
       "shell_unittests.cc",
       "skp_shader_warmup_unittests.cc",
       "switches_unittests.cc",
+      "variable_refresh_rate_display_unittests.cc",
     ]
 
     deps = [

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

@@ -206,6 +206,10 @@ void Animator::Render(std::unique_ptr<flutter::LayerTree> layer_tree) {
                            std::move(frame_timings_recorder_));
 }
 
+const VsyncWaiter& Animator::GetVsyncWaiter() const {
+  return *waiter_.get();
+}
+
 bool Animator::CanReuseLastLayerTree() {
   return !regenerate_layer_tree_;
 }

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

@@ -54,6 +54,8 @@ class Animator final {
 
   void Render(std::unique_ptr<flutter::LayerTree> layer_tree);
 
+  const VsyncWaiter& GetVsyncWaiter() const;
+
   //--------------------------------------------------------------------------
   /// @brief    Schedule a secondary callback to be executed right after the
   ///           main `VsyncWaiter::AsyncWaitForVsync` callback (which is added

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

@@ -8,6 +8,7 @@
 #include <optional>
 
 #include "flutter/fml/macros.h"
+#include "flutter/shell/common/variable_refresh_rate_reporter.h"
 
 namespace flutter {
 

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

@@ -600,4 +600,8 @@ void Engine::LoadDartDeferredLibraryError(intptr_t loading_unit_id,
   }
 }
 
+const VsyncWaiter& Engine::GetVsyncWaiter() const {
+  return animator_->GetVsyncWaiter();
+}
+
 }  // namespace flutter

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

@@ -903,6 +903,8 @@ class Engine final : public RuntimeDelegate, PointerDataDispatcher::Delegate {
     return runtime_controller_.get();
   }
 
+  const VsyncWaiter& GetVsyncWaiter() const;
+
  private:
   // |RuntimeDelegate|
   std::string DefaultRouteName() override;

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

@@ -1872,4 +1872,8 @@ Shell::GetPlatformMessageHandler() const {
   return platform_message_handler_;
 }
 
+const VsyncWaiter& Shell::GetVsyncWaiter() const {
+  return engine_->GetVsyncWaiter();
+}
+
 }  // namespace flutter

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

@@ -401,6 +401,8 @@ class Shell final : public PlatformView::Delegate,
   const std::shared_ptr<PlatformMessageHandler>& GetPlatformMessageHandler()
       const;
 
+  const VsyncWaiter& GetVsyncWaiter() const;
+
  private:
   using ServiceProtocolHandler =
       std::function<bool(const ServiceProtocol::Handler::ServiceProtocolMap&,

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

@@ -0,0 +1,25 @@
+// 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.
+
+#include "flutter/shell/common/variable_refresh_rate_display.h"
+#include "flutter/fml/logging.h"
+
+namespace flutter {
+
+VariableRefreshRateDisplay::VariableRefreshRateDisplay(
+    DisplayId display_id,
+    const VariableRefreshRateReporter& refresh_rate_reporter)
+    : Display(display_id, refresh_rate_reporter.GetRefreshRate()),
+      refresh_rate_reporter_(refresh_rate_reporter) {}
+
+VariableRefreshRateDisplay::VariableRefreshRateDisplay(
+    const VariableRefreshRateReporter& refresh_rate_reporter)
+    : Display(refresh_rate_reporter.GetRefreshRate()),
+      refresh_rate_reporter_(refresh_rate_reporter) {}
+
+double VariableRefreshRateDisplay::GetRefreshRate() const {
+  return refresh_rate_reporter_.GetRefreshRate();
+}
+
+}  // namespace flutter

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

@@ -0,0 +1,37 @@
+// 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.
+
+#ifndef FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_DISPLAY_H_
+#define FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_DISPLAY_H_
+
+#include <optional>
+
+#include "display.h"
+#include "flutter/fml/macros.h"
+#include "variable_refresh_rate_reporter.h"
+
+namespace flutter {
+
+/// A Display where the refresh rate can change over time.
+class VariableRefreshRateDisplay : public Display {
+ public:
+  explicit VariableRefreshRateDisplay(
+      DisplayId display_id,
+      const VariableRefreshRateReporter& refresh_rate_reporter);
+  explicit VariableRefreshRateDisplay(
+      const VariableRefreshRateReporter& refresh_rate_reporter);
+  ~VariableRefreshRateDisplay() = default;
+
+  // |Display|
+  double GetRefreshRate() const override;
+
+ private:
+  const VariableRefreshRateReporter& refresh_rate_reporter_;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(VariableRefreshRateDisplay);
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_DISPLAY_H_

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

@@ -0,0 +1,29 @@
+// 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.
+
+#include "variable_refresh_rate_display.h"
+#include "vsync_waiters_test.h"
+
+#include "gtest/gtest.h"
+
+namespace flutter {
+namespace testing {
+
+TEST(VariableRefreshRateDisplayTest, ReportCorrectInitialRefreshRate) {
+  auto refresh_rate_reporter = std::make_unique<TestRefreshRateReporter>(60);
+  auto display =
+      flutter::VariableRefreshRateDisplay(*refresh_rate_reporter.get());
+  ASSERT_EQ(display.GetRefreshRate(), 60);
+}
+
+TEST(VariableRefreshRateDisplayTest, ReportCorrectRefreshRateWhenUpdated) {
+  auto refresh_rate_reporter = std::make_unique<TestRefreshRateReporter>(60);
+  auto display =
+      flutter::VariableRefreshRateDisplay(*refresh_rate_reporter.get());
+  refresh_rate_reporter->UpdateRefreshRate(30);
+  ASSERT_EQ(display.GetRefreshRate(), 30);
+}
+
+}  // namespace testing
+}  // namespace flutter

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

@@ -0,0 +1,28 @@
+// 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.
+
+#ifndef FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_REPORTER_H_
+#define FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_REPORTER_H_
+
+#include <functional>
+#include <memory>
+#include <mutex>
+#include <unordered_map>
+
+namespace flutter {
+
+/// Abstract class that reprents a platform specific mechanism to report current
+/// refresh rates.
+class VariableRefreshRateReporter {
+ public:
+  VariableRefreshRateReporter() = default;
+
+  virtual double GetRefreshRate() const = 0;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(VariableRefreshRateReporter);
+};
+
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_COMMON_VARIABLE_REFRESH_RATE_REPORTER_H_

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

@@ -64,5 +64,16 @@ void ConstantFiringVsyncWaiter::AwaitVSync() {
   });
 }
 
+TestRefreshRateReporter::TestRefreshRateReporter(double refresh_rate)
+    : refresh_rate_(refresh_rate) {}
+
+void TestRefreshRateReporter::UpdateRefreshRate(double refresh_rate) {
+  refresh_rate_ = refresh_rate;
+}
+
+double TestRefreshRateReporter::GetRefreshRate() const {
+  return refresh_rate_;
+}
+
 }  // namespace testing
 }  // namespace flutter

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

@@ -56,6 +56,18 @@ class ConstantFiringVsyncWaiter : public VsyncWaiter {
   void AwaitVSync() override;
 };
 
+class TestRefreshRateReporter final : public VariableRefreshRateReporter {
+ public:
+  explicit TestRefreshRateReporter(double refresh_rate);
+  void UpdateRefreshRate(double refresh_rate);
+
+  // |RefreshRateReporter|
+  double GetRefreshRate() const override;
+
+ private:
+  double refresh_rate_;
+};
+
 }  // namespace testing
 }  // namespace flutter
 

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterEngine.mm

@@ -17,6 +17,7 @@
 #include "flutter/shell/common/shell.h"
 #include "flutter/shell/common/switches.h"
 #include "flutter/shell/common/thread_host.h"
+#include "flutter/shell/common/variable_refresh_rate_display.h"
 #import "flutter/shell/platform/darwin/common/command_line.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterBinaryMessengerRelay.h"
 #import "flutter/shell/platform/darwin/ios/framework/Source/FlutterDartProject_Internal.h"
@@ -699,9 +700,10 @@ static void SetEntryPoint(flutter::Settings* settings, NSString* entrypoint, NSS
 }
 
 - (void)initializeDisplays {
-  double refresh_rate = [DisplayLinkManager displayRefreshRate];
+  const flutter::VsyncWaiterIOS& vsync_waiter_ios =
+      static_cast<const flutter::VsyncWaiterIOS&>(_shell->GetVsyncWaiter());
   std::vector<std::unique_ptr<flutter::Display>> displays;
-  displays.push_back(std::make_unique<flutter::Display>(refresh_rate));
+  displays.push_back(std::make_unique<flutter::VariableRefreshRateDisplay>(vsync_waiter_ios));
   _shell->OnDisplayUpdates(flutter::DisplayUpdateType::kStartup, std::move(displays));
 }
 

engine/src/flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.h

@@ -8,6 +8,7 @@
 #include "flutter/fml/macros.h"
 #include "flutter/fml/memory/weak_ptr.h"
 #include "flutter/fml/platform/darwin/scoped_nsobject.h"
+#include "flutter/shell/common/variable_refresh_rate_reporter.h"
 #include "flutter/shell/common/vsync_waiter.h"
 
 @interface DisplayLinkManager : NSObject
@@ -34,16 +35,21 @@
 
 - (void)invalidate;
 
+- (double)getRefreshRate;
+
 @end
 
 namespace flutter {
 
-class VsyncWaiterIOS final : public VsyncWaiter {
+class VsyncWaiterIOS final : public VsyncWaiter, public VariableRefreshRateReporter {
  public:
   explicit VsyncWaiterIOS(flutter::TaskRunners task_runners);
 
   ~VsyncWaiterIOS() override;
 
+  // |VariableRefreshRateReporter|
+  double GetRefreshRate() const override;
+
  private:
   fml::scoped_nsobject<VSyncClient> client_;
 

engine/src/flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm

@@ -39,11 +39,17 @@ void VsyncWaiterIOS::AwaitVSync() {
   [client_.get() await];
 }
 
+// |VariableRefreshRateReporter|
+double VsyncWaiterIOS::GetRefreshRate() const {
+  return [client_.get() getRefreshRate];
+}
+
 }  // namespace flutter
 
 @implementation VSyncClient {
   flutter::VsyncWaiter::Callback callback_;
   fml::scoped_nsobject<CADisplayLink> display_link_;
+  double current_refresh_rate_;
 }
 
 - (instancetype)initWithTaskRunner:(fml::RefPtr<fml::TaskRunner>)task_runner
@@ -51,6 +57,7 @@ void VsyncWaiterIOS::AwaitVSync() {
   self = [super init];
 
   if (self) {
+    current_refresh_rate_ = [DisplayLinkManager displayRefreshRate];
     callback_ = std::move(callback);
     display_link_ = fml::scoped_nsobject<CADisplayLink> {
       [[CADisplayLink displayLinkWithTarget:self selector:@selector(onDisplayLink:)] retain]
@@ -87,6 +94,9 @@ void VsyncWaiterIOS::AwaitVSync() {
 
   std::unique_ptr<flutter::FrameTimingsRecorder> recorder =
       std::make_unique<flutter::FrameTimingsRecorder>();
+
+  current_refresh_rate_ = round(1 / (frame_target_time - frame_start_time).ToSecondsF());
+
   recorder->RecordVsync(frame_start_time, frame_target_time);
   display_link_.get().paused = YES;
 
@@ -103,6 +113,10 @@ void VsyncWaiterIOS::AwaitVSync() {
   [super dealloc];
 }
 
+- (double)getRefreshRate {
+  return current_refresh_rate_;
+}
+
 @end
 
 @implementation DisplayLinkManager