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iOS] Migrate VSyncClient to a pure Obj-C implementation (#186166) #186935

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Merged
cbracken merged 2 commits into from
May 23, 2026
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iOS] Migrate VSyncClient to a pure Obj-C implementation (#186166) #186935

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iOS] Migrate VSyncClient to a pure Obj-C implementation (#186166)
cbracken May 22, 2026
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Address review feedback
cbracken May 22, 2026
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5 changes: 3 additions & 2 deletions engine/src/flutter/shell/platform/darwin/ios/BUILD.gn
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Expand Up @@ -274,8 +274,8 @@ if (enable_ios_unittests) {
# considerably slower to start than the tests in
# "//flutter/shell/platform/darwin/common:framework_common_swift_unittests".
# If your tests do not have to be run on a simulator (no UIKit dependencies
# for example), consider adding them to the ":framework_common_swift_unittests"
# target.
# for example), consider adding them to the
# ":framework_common_swift_unittests" target.
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@cbracken cbracken May 22, 2026

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Not an actual change but turns out we were past 80 cols which upset the format presubmit.

shared_library("ios_test_flutter") {
testonly = true
visibility = [ "*" ]
Expand Down Expand Up @@ -322,6 +322,7 @@ if (enable_ios_unittests) {
"framework/Source/SemanticsObjectTest.mm",
"framework/Source/SemanticsObjectTestMocks.h",
"framework/Source/UIViewController_FlutterScreenAndSceneIfLoadedTest.mm",
"framework/Source/VsyncWaiterIOSTest.mm",
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@cbracken cbracken May 22, 2026 •
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Yes, the filename is inconsistent with the implementation file, but the test is an Obj-C XCTest and the class under test is a C++ class, hence the difference in naming.

"framework/Source/accessibility_bridge_test.mm",
"framework/Source/availability_version_check_test.mm",
"ios_context_noop_unittests.mm",
Expand Down
2 changes: 1 addition & 1 deletion engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardInsetManager.mm
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Expand Up @@ -273,7 +273,7 @@ - (void)startKeyBoardAnimation:(NSTimeInterval)duration {
// Set animation begin value and DisplayLink tracking values.
CGFloat currentInset = delegate.physicalViewInsetBottom;
self.keyboardAnimationView.frame = CGRectMake(0, currentInset, 0, 0);
self.keyboardAnimationStartTime = fml::TimePoint::Now().ToEpochDelta().ToSecondsF();
self.keyboardAnimationStartTime = CACurrentMediaTime();
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@cbracken cbracken May 22, 2026

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Note that this was adjusted since FlutterVSyncClient is now pure Obj-C and sticks to CoreAnimation APIs (along with the rest of the embedder now).

self.originalViewInsetBottom = currentInset;

// Invalidate old vsync client if old animation is not completed.
Expand Down
36 changes: 10 additions & 26 deletions engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm
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Expand Up @@ -3,21 +3,20 @@
// found in the LICENSE file.

#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.h"
#include "flutter/shell/common/vsync_waiter.h"

#import <UIKit/UIKit.h>

#import "flutter/shell/platform/darwin/common/framework/Headers/FlutterMacros.h"
#import "flutter/shell/platform/darwin/common/framework/Source/FlutterTracing.h"
#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterFMLTaskRunner+FML.h"
#import "flutter/shell/platform/darwin/ios/framework/Source/FlutterFMLTaskRunner.h"

FLUTTER_ASSERT_ARC

NSString* const kCADisableMinimumFrameDurationOnPhoneKey = @"CADisableMinimumFrameDurationOnPhone";
static const double kDefaultRefreshRate = 60.0;

@implementation FlutterVSyncClient {
flutter::VsyncWaiter::Callback _callback;
void (^_callback)(CFTimeInterval startTime, CFTimeInterval targetTime);
CADisplayLink* _displayLink;
BOOL _isVariableRefreshRateEnabled;
}
Expand All @@ -27,20 +26,14 @@ - (instancetype)initWithTaskRunner:(FlutterFMLTaskRunner*)taskRunner
maxRefreshRate:(double)maxRefreshRate
callback:(void (^)(CFTimeInterval startTime,
CFTimeInterval targetTime))callback {
FML_DCHECK(callback);
FML_DCHECK(taskRunner);
fml::RefPtr<fml::TaskRunner> task_runner = taskRunner.taskRunner;
FML_DCHECK(task_runner);
NSAssert(callback, @"callback must not be nil");
NSAssert(taskRunner, @"taskRunner must not be nil");

if (self = [super init]) {
_refreshRate = maxRefreshRate;
_isVariableRefreshRateEnabled = isVariableRefreshRateEnabled;
_allowPauseAfterVsync = YES;
_callback = [callback](std::unique_ptr<flutter::FrameTimingsRecorder> recorder) {
double start_time_seconds = recorder->GetVsyncStartTime().ToEpochDelta().ToSecondsF();
double target_time_seconds = recorder->GetVsyncTargetTime().ToEpochDelta().ToSecondsF();
callback(start_time_seconds, target_time_seconds);
};
_callback = callback;
_displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(onDisplayLink:)];
_displayLink.paused = YES;

Expand All @@ -49,12 +42,12 @@ - (instancetype)initWithTaskRunner:(FlutterFMLTaskRunner*)taskRunner
// Capture a weak reference to self to ensure we don't add the display link
// to the run loop if the client has already been deallocated.
__weak FlutterVSyncClient* weakSelf = self;
task_runner->PostTask([weakSelf]() {
[taskRunner postTask:^{
FlutterVSyncClient* strongSelf = weakSelf;
if (strongSelf) {
[strongSelf.displayLink addToRunLoop:NSRunLoop.currentRunLoop forMode:NSRunLoopCommonModes];
}
});
}];
}

return self;
Expand Down Expand Up @@ -96,8 +89,6 @@ - (void)onDisplayLink:(CADisplayLink*)link {
if (timestamp == 0.0) {
timestamp = CACurrentMediaTime();
}
CFTimeInterval delay = CACurrentMediaTime() - timestamp;
fml::TimePoint frame_start_time = fml::TimePoint::Now() - fml::TimeDelta::FromSecondsF(delay);

// targetTimestamp is the anticipated presentation time of the next screen refresh. If
// targetTimestamp is zero or less than/equal to timestamp (which also occurs on paused/unpaused
Expand All @@ -108,13 +99,8 @@ - (void)onDisplayLink:(CADisplayLink*)link {
targetTimestamp = timestamp + (1.0 / effectiveRefreshRate);
}
CFTimeInterval duration = targetTimestamp - timestamp;
fml::TimePoint frame_target_time = frame_start_time + fml::TimeDelta::FromSecondsF(duration);

[FlutterTracing tracePlatformVsyncWithStartTime:frame_start_time.ToEpochDelta().ToSecondsF()
targetTime:frame_target_time.ToEpochDelta().ToSecondsF()];

std::unique_ptr<flutter::FrameTimingsRecorder> recorder =
std::make_unique<flutter::FrameTimingsRecorder>();
[FlutterTracing tracePlatformVsyncWithStartTime:timestamp targetTime:targetTimestamp];

// In steady-state, duration reflects the hardware refresh interval (e.g., ~0.01667s for 60Hz).
// We dynamically recalculate the refresh rate from the frame duration to adjust to ProMotion
Expand All @@ -129,12 +115,10 @@ - (void)onDisplayLink:(CADisplayLink*)link {
}
}

recorder->RecordVsync(frame_start_time, frame_target_time);

if (_allowPauseAfterVsync) {
link.paused = YES;
}
_callback(std::move(recorder));
_callback(timestamp, targetTimestamp);
}

- (void)dealloc {
Expand Down Expand Up @@ -168,7 +152,7 @@ + (double)displayRefreshRate {
CADisplayLink* displayLink = [CADisplayLink displayLinkWithTarget:[[[self class] alloc] init]
selector:@selector(onDisplayLink:)];
displayLink.paused = YES;
auto preferredFPS = displayLink.preferredFramesPerSecond;
NSInteger preferredFPS = displayLink.preferredFramesPerSecond;

// From Docs:
// The default value for preferredFramesPerSecond is 0. When this value is 0, the preferred
Expand Down
45 changes: 45 additions & 0 deletions engine/src/flutter/shell/platform/darwin/ios/framework/Source/VsyncWaiterIOSTest.mm
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@@ -0,0 +1,45 @@
// 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.

#import <XCTest/XCTest.h>

#import "flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.h"

@interface VsyncWaiterIOSTest : XCTestCase
@end

@implementation VsyncWaiterIOSTest

- (void)testSnapDurationWithValidDuration {
// 60Hz: 1/60 = 0.016666...
CFTimeInterval duration = 0.016667;
CFTimeInterval snapped = flutter::VsyncWaiterIOS::SnapDuration(duration, 60.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 60.0, 0.0001);

// 120Hz: 1/120 = 0.008333...
duration = 0.008334;
snapped = flutter::VsyncWaiterIOS::SnapDuration(duration, 120.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 120.0, 0.0001);
}

- (void)testSnapDurationWithInvalidDuration {
// Zero duration should fallback to max_refresh_rate.
CFTimeInterval snapped = flutter::VsyncWaiterIOS::SnapDuration(0.0, 120.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 120.0, 0.0001);

// Negative duration should fallback to max_refresh_rate.
snapped = flutter::VsyncWaiterIOS::SnapDuration(-0.1, 80.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 80.0, 0.0001);
}

- (void)testSnapDurationWithZeroMaxRefreshRateFallback {
// If duration is invalid AND max_refresh_rate is 0, fallback to 60Hz.
CFTimeInterval snapped = flutter::VsyncWaiterIOS::SnapDuration(0.0, 0.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 60.0, 0.0001);

snapped = flutter::VsyncWaiterIOS::SnapDuration(-1.0, -10.0);
XCTAssertEqualWithAccuracy(snapped, 1.0 / 60.0, 0.0001);
}

@end
13 changes: 13 additions & 0 deletions engine/src/flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.h
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Expand Up @@ -22,6 +22,19 @@ class VsyncWaiterIOS final : public VsyncWaiter,
// |VariableRefreshRateReporter|
double GetRefreshRate() const override;

// @brief Snaps the duration to the nearest whole Hz value and provides safe
// fallbacks. This ensures we don't introduce frame timing issues due
// to floating point error. e.g.
// 59.998, 60.004, 59.995, ... --> 60.000
//
// Additionally, guards against divide-by-zero and non-positive
// durations, which can occur on paused/unpaused transitions.
//
// Visible for testing.
static CFTimeInterval SnapDuration(CFTimeInterval duration,
double max_refresh_rate);

private:
// |VsyncWaiter|
// Made public for testing.
void AwaitVSync() override;
Expand Down
26 changes: 24 additions & 2 deletions engine/src/flutter/shell/platform/darwin/ios/framework/Source/vsync_waiter_ios.mm
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Expand Up @@ -18,8 +18,21 @@
VsyncWaiterIOS::VsyncWaiterIOS(const flutter::TaskRunners& task_runners)
: VsyncWaiter(task_runners) {
auto vsyncCallback = ^(CFTimeInterval startTime, CFTimeInterval targetTime) {
fml::TimePoint start_time = fml::TimePoint() + fml::TimeDelta::FromSecondsF(startTime);
fml::TimePoint target_time = fml::TimePoint() + fml::TimeDelta::FromSecondsF(targetTime);
// Compute delay using the same CACurrentMediaTime() clock.
CFTimeInterval delay = CACurrentMediaTime() - startTime;
if (delay < 0.0) {
delay = 0.0;
}

// Align the start time to the C++ steady_clock used by fml::TimePoint.
fml::TimePoint start_time = fml::TimePoint::Now() - fml::TimeDelta::FromSecondsF(delay);

// Snap to the nearest whole Hz value to avoid floating point errors.
CFTimeInterval duration =
VsyncWaiterIOS::SnapDuration(targetTime - startTime, max_refresh_rate_);

// Align target time to the C++ steady_clock used by fml::TimePoint.
fml::TimePoint target_time = start_time + fml::TimeDelta::FromSecondsF(duration);
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@cbracken cbracken May 22, 2026

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The core engine deals in fml::TimePoints which are based on std::chrono::steady_clock which is backed by mach_continuous_time, whereas CoreAnimation uses mach_absolute_time.

Consistent with embedders on the embedder API, we adjust the epoch for start and target time here.

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@hellohuanlin hellohuanlin May 22, 2026

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The core engine deals in fml::TimePoints which are based on std::chrono::steady_clock which is backed by mach_continuous_time, whereas CoreAnimation uses mach_absolute_time.

This comment is helpful. can you add this info to the comment?

FireCallback(start_time, target_time, true);
};
FlutterFMLTaskRunner* uiTaskRunner =
Expand Down Expand Up @@ -52,4 +65,13 @@
return client_.refreshRate;
}

CFTimeInterval VsyncWaiterIOS::SnapDuration(CFTimeInterval duration, double max_refresh_rate) {
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@hellohuanlin hellohuanlin May 22, 2026

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what's "snap duration"?

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@cbracken cbracken May 22, 2026

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See the comment on the method: it snaps the duration to a round number of Hz. Due to floating point error or tiny inconsistencies in timing the duration may be off by a few microseconds etc, so we compute the inverse (frequency in Hz) snap to the closes integer Hz (60, 120, etc. whatever the nearest int value of Hz is), then adjust the time to keep the frame timing from drifting)

We were doing this before but decided it'd be better to put it in a named method with a doc comment.

if (duration > 0.0) {
double roundedRefreshRate = round(1.0 / duration);
return 1.0 / roundedRefreshRate;
}
double fallbackRefreshRate = max_refresh_rate > 0.0 ? max_refresh_rate : 60.0;
return 1.0 / fallbackRefreshRate;
}

} // namespace flutter
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