Pointer event resampler (#41118) #60558

dreveman · 2020-06-29T21:47:28Z

Pointer event resampler (#41118)

This introduces the PointerEventResampler class that contains
the core logic needed to resample pointer events. This can be used
to get smoother event processing at the cos of some added latency.
Devices with low frequency sensors or when the frequency is not a
multiple of the display frequency (e.g., 120Hz input and 90Hz
display) benefit from this.

The caller is responsible for deciding what sampling offset to use
and what frequency to sample at. If touch events arrive at 65 Hz,
then it's typically good to be using a sampling offset that is
current time - 22 ms for smooth touch event delivery.

This change is also using the PointerEventResampler class to
add resampling support to GestureBinding. Resampling is disabled
by default.

This can be used to fix: #41118

packages/flutter/lib/src/gestures/pointer_data_resampler.dart

dreveman · 2020-07-01T13:36:50Z

PTAL

goderbauer · 2020-07-01T19:01:10Z

nit: Can you please add a short description to the PR describing what this does and what it is for? Also, are there any issues associated with this? If so, please link them from the PR description.

dreveman · 2020-07-01T23:57:39Z

nit: Can you please add a short description to the PR describing what this does and what it is for? Also, are there any issues associated with this? If so, please link them from the PR description.

Done. Sorry for the lack of explanation earlier. We're planning to have this land together with a potential blog post that explains it in more detail but please let me know what more documentation would be useful in code comments and PR description as reading the post should not be necessary to understand this.

packages/flutter/lib/src/gestures/pointer_data_resampler.dart

packages/flutter/test/gestures/pointer_data_resampler_test.dart

packages/flutter/lib/src/gestures/pointer_data_resampler.dart

CareF · 2020-07-10T12:30:57Z

I don't mean any review comment or suggestion for modification, but I want to point out the thing I wrote in #60796 (comment) .
This resampling happens in the PointerData level, while our unit test framework flutter_test injects the driving input in the PointerEvent level. So if this is integrated into the framework in the future in someway (like become a on/off flag in the framework), rather than the current use case as replacing the binding in the app, the instance of PointerDataResampler in the framework wouldn't be able to be tested using WidgetTester for integration test because anything from PointerDataResampler would look like device input rather than test input and will be ignored.

goderbauer · 2020-07-22T21:37:17Z

(PR triage) @dreveman @liyuqian Are there still plans for this PR? It has been in-active for some weeks now...

liyuqian

The algorithm and the unit tests in this PR looks good. Here are a few more things to consider in my opinion in order to make this out of draft and ready to merge:

Use this in GestureBinding and expose a boolean flag to turn this resampling on or off. In the beginning, we'll make it default to off and only enable it in testing. Once we're confident with those tests, turn it to on by default. The use of resampling probably would happen inside GestureBinding._handlePointerDataPacket where PointerData is converted to PointerEvent and added to the queue.
Either in GestureBinding or PointerDataResampler , document why someone would want to resample events, and why someone would not want to resample events.
Decide whether we want to resample at PointerData level or PointerEvent level. According to previous discussion, as long as we're only exposing this resampling as a global flag instead of a widget-level flag, this shouldn't matter much. The small advantage of PointerEvent is a closer integration with flutter_test package, and it's possible for us to enable widget-level resampling in the future. The small advantage of PointerData is that it's closer to the platform implementation (e.g., Fuchsia), it has a simpler class inheritance structure (no subclass such as PointerAddEvent, PointerRemoveEvent, ...), and it may allow us to test lower level part of the system that's inaccessible to PointerEvent tests.

goderbauer · 2020-08-05T21:47:28Z

(PR triage) @dreveman @liyuqian Are there still plans for this PR? It has been in-active for some weeks now...

liyuqian · 2020-08-05T22:19:45Z

@dreveman told us this Monday that he's working on an updated resampler for PointerEvent instead of PointerData. The new work should be available soon.

packages/flutter/lib/src/gestures/resampler.dart

packages/flutter/lib/src/gestures/binding.dart

liyuqian

LGTM

CareF · 2020-08-21T16:17:59Z

I tried this flag on https://github.com/flutter/flutter/tree/master/dev/benchmarks/complex_layout
with changing the main() in https://github.com/flutter/flutter/blob/master/dev/benchmarks/complex_layout/lib/main.dart to

import 'package:flutter/gestures.dart';

void main() {
  runApp(ComplexLayoutApp());
  GestureBinding.instance.resamplingEnabled = true;
}

It failed and this is what I got:

E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136): 
I/chatty  (15136): uid=10643(com.example.complex_layout) 1.ui identical 84 lines
E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136):

Did I misunderstood how it should be used?

dreveman · 2020-08-21T16:54:36Z

I tried this flag on https://github.com/flutter/flutter/tree/master/dev/benchmarks/complex_layout
with changing the main() in https://github.com/flutter/flutter/blob/master/dev/benchmarks/complex_layout/lib/main.dart to

import 'package:flutter/gestures.dart';

void main() {
  runApp(ComplexLayoutApp());
  GestureBinding.instance.resamplingEnabled = true;
}

It failed and this is what I got:

E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136): 
I/chatty  (15136): uid=10643(com.example.complex_layout) 1.ui identical 84 lines
E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136):

Did I misunderstood how it should be used?

What platform is this? Looks like either currentSystemFrameTimeStamp is not set currently or event timestamps are not set correctly.

CareF · 2020-08-21T16:57:22Z

What platform is this? Looks like either currentSystemFrameTimeStamp is not set currently or event timestamps are not set correctly.

I tested it on a Pixel 4, with flutter run (debug mode). If I do similar thing on a Fuchsia device should it work?

CareF · 2020-08-21T17:02:30Z

FYI, if I add

        print(event.timeStamp);
        print(scheduler?.currentSystemFrameTimeStamp);

before that assert, I get:

I/flutter (15686): 413:20:24.378000
I/flutter (15686): 413:20:23.399630

which looks kind of reasonable numbers but for the time difference. If I wait longer time to touch the screen after the app starts, I get larger time difference. It seems that currentSystemFrameTimeStamp is not for the coming frame but the last frame.

CareF · 2020-08-21T17:18:15Z

If I also add print(scheduler?.currentFrameTimeStamp);, I get

[ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/scheduler/binding.dart': Failed assertion: line 932 pos 12: '_currentFrameTimeStamp != null': is not true.

I guess the timestamp is not yet initialized to the value we expect.

According to https://api.flutter.dev/flutter/scheduler/SchedulerBinding/currentFrameTimeStamp.html I believe that's because we are not using the timestamp within a frame scheduling process. And currentSystemFrameTimeStamp is NOT reset to null which, if that value is going to be documented something other than "this is a more or less arbitrary value", means there's a bug for this.

Comment out this assert will avoid the error message but also blocked all scroll gesture. Tap still works.

dreveman · 2020-08-21T18:09:47Z

currentSystemFrameTimeStamp

This resampling logic depends on currentSystemFrameTimeStamp being a valid time stamp for the expected presentation of the next frame. Hence, always in the future. That's what it is on Fuchsia. We can adjust this logic to expect something else on a different platform but ideally we'd instead make currentSystemFrameTimeStamp the same across all platforms.

dreveman · 2020-08-22T21:08:06Z

I tried this flag on https://github.com/flutter/flutter/tree/master/dev/benchmarks/complex_layout
with changing the main() in https://github.com/flutter/flutter/blob/master/dev/benchmarks/complex_layout/lib/main.dart to

import 'package:flutter/gestures.dart';

void main() {
  runApp(ComplexLayoutApp());
  GestureBinding.instance.resamplingEnabled = true;
}

It failed and this is what I got:

E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136): 
I/chatty  (15136): uid=10643(com.example.complex_layout) 1.ui identical 84 lines
E/flutter (15136): [ERROR:flutter/lib/ui/ui_dart_state.cc(171)] Unhandled Exception: 'package:flutter/src/gestures/binding.dart': Failed assertion: line 56 pos 16: 'event.timeStamp <= (scheduler?.currentSystemFrameTimeStamp ?? Duration.zero)': is not true.
E/flutter (15136): #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:46:39)
E/flutter (15136): #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:36:5)
E/flutter (15136): #2      _Resampler.addOrDispatchAll (package:flutter/src/gestures/binding.dart:56:16)
E/flutter (15136): #3      GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:207:18)
E/flutter (15136): #4      GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:190:7)
E/flutter (15136): #5      _rootRunUnary (dart:async/zone.dart:1206:13)
E/flutter (15136): #6      _CustomZone.runUnary (dart:async/zone.dart:1100:19)
E/flutter (15136): #7      _CustomZone.runUnaryGuarded (dart:async/zone.dart:1005:7)
E/flutter (15136): #8      _invoke1 (dart:ui/hooks.dart:265:10)
E/flutter (15136): #9      _dispatchPointerDataPacket (dart:ui/hooks.dart:174:5)
E/flutter (15136):

Did I misunderstood how it should be used?

This should work fine now. I found a bug in the copyWith logic that caused down/move/up events to not be produced correctly. Added some tests for this.

Latest version of this change also contains a debug flag that is used to determine if resampling works correctly on a device. I've verified that it works correctly on a Pixel 3 device.

CareF

LGTM. I wrote a unit test (or in this context, more close to an integration test?) for the resampling performing on widgets, by tracking the events from Listener, which should be a natural unit test for the sampling working with widgets. I'll file another PR for that after this lands and hope you can review that.

This introduces the PointerEventResampler class that contains the core logic needed to resample pointer events. This can be used to get smoother event processing at the cos of some added latency. Devices with low frequency sensors or when the frequency is not a multiple of the display frequency (e.g., 120Hz input and 90Hz display) benefit from this. The caller is responsible for deciding what sampling offset to use and what frequency to sample at. If touch events arrive at 65 Hz, then it's typically good to be using a sampling offset that is current time - 22 ms for smooth touch event delivery. This change is also using the PointerEventResampler class to add resampling support to GestureBinding. Resampling is disabled by default. This can be used to fix: flutter#41118

CareF · 2020-08-24T14:46:06Z

packages/flutter/lib/src/gestures/binding.dart

+
+        final PointerEventResampler resampler = _resamplers.putIfAbsent(
+          event.device,
+          () => PointerEventResampler(),


Is it true that when this constructor is called (i.e., event.device key is absent in _resamplers), event can only be PointerAddedEvent or PointerDownEvent?

No, that would only be true if resampling was enabled before the first pointer event. If you enable resampling after first receiving a down event then we'll need to create the resampler here when processing a move event.

CareF · 2020-09-28T21:12:30Z

packages/flutter/lib/src/gestures/binding.dart

+
+    if (resamplingEnabled) {
+      _resampler.addOrDispatchAll(_pendingPointerEvents);
+      _resampler.sample(samplingOffset);


Does this mean that, when the pending event queue in the resampler is not empty, but there are no more future inputs, these pending events will stay in the queue and will not be processed, since _flushPointerEventQueue is not called?
I realize this when I'm trying to move resampler to handlePointerEvent.
cc @liyuqian

No, the _Resampler will schedule _handleSampleTimeChanged after frame callback which will call _flushPointerEventQueue. See: https://github.com/flutter/flutter/pull/60558/files/b7802f69f91e1b5466d6b5b58e98193160964ca5#diff-6305361e0c7677f0eebfb05dfcbe0336

dreveman requested review from CareF and liyuqian June 29, 2020 21:47

fluttergithubbot added the framework flutter/packages/flutter repository. See also f: labels. label Jun 29, 2020

googlebot added the cla: yes label Jun 29, 2020

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CareF mentioned this pull request Jul 10, 2020

Add support in WidgetTester for an array of inputs #60796

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CareF mentioned this pull request Jul 13, 2020

Navigator can still get spurious writes during navigation #4770

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A benchmark test case for measuring scroll smoothness #61998

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Resample input events due to irregular events delivery or VSYNC and input frequency mismatch #41118

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Add resampler unit test with widget #64424

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Pointer event resampler (flutter#41118) (flutter#60558)

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move resampler to handlePointerEvent and fix complex_layout_android__scroll_smoothness with PointerEvent #66745

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Pointer event resampler (#41118) #60558

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Pointer event resampler (#41118) #60558

Pointer event resampler (#41118) #60558

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