On iOS report the preferred frames per second to tools via service protocol. (flutter/engine#11006)

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

@@ -26,6 +26,9 @@ class VsyncWaiterIOS final : public VsyncWaiter {
   // |VsyncWaiter|
   void AwaitVSync() override;
 
+  // |VsyncWaiter|
+  float GetDisplayRefreshRate() const override;
+
   FML_DISALLOW_COPY_AND_ASSIGN(VsyncWaiterIOS);
 };
 

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

@@ -8,6 +8,7 @@
 
 #include <Foundation/Foundation.h>
 #include <QuartzCore/CADisplayLink.h>
+#include <UIKit/UIKit.h>
 #include <mach/mach_time.h>
 
 #include "flutter/common/task_runners.h"
@@ -23,6 +24,17 @@
 
 - (void)invalidate;
 
+//------------------------------------------------------------------------------
+/// @brief      The display refresh rate used for reporting purposes. The engine does not care
+///             about this for frame scheduling. It is only used by tools for instrumentation. The
+///             engine uses the duration field of the link per frame for frame scheduling.
+///
+/// @attention  Do not use the this call in frame scheduling. It is only meant for reporting.
+///
+/// @return     The refresh rate in frames per second.
+///
+- (float)displayRefreshRate;
+
 @end
 
 namespace flutter {
@@ -45,6 +57,11 @@ void VsyncWaiterIOS::AwaitVSync() {
   [client_.get() await];
 }
 
+// |VsyncWaiter|
+float VsyncWaiterIOS::GetDisplayRefreshRate() const {
+  return [client_.get() displayRefreshRate];
+}
+
 }  // namespace flutter
 
 @implementation VSyncClient {
@@ -73,6 +90,25 @@ void VsyncWaiterIOS::AwaitVSync() {
   return self;
 }
 
+- (float)displayRefreshRate {
+  if (@available(iOS 10.3, *)) {
+    auto preferredFPS = display_link_.get().preferredFramesPerSecond;  // iOS 10.0
+
+    // From Docs:
+    // The default value for preferredFramesPerSecond is 0. When this value is 0, the preferred
+    // frame rate is equal to the maximum refresh rate of the display, as indicated by the
+    // maximumFramesPerSecond property.
+
+    if (preferredFPS != 0) {
+      return preferredFPS;
+    }
+
+    return [UIScreen mainScreen].maximumFramesPerSecond;  // iOS 10.3
+  } else {
+    return 60.0;
+  }
+}
+
 - (void)await {
   display_link_.get().paused = NO;
 }