Allow long-press gestures to continue even if buttons change. #127877

Hixie · 2023-05-30T20:43:12Z

Previously, if you changed buttons during a long-press gesture, if it was before the gesture was accepted we would discard it, and if it was after the gesture was accepted we would silently end it without firing any of the relevant events.

This silent cancelation behavior is terrible because it means there's no way for consumers to know what state they're in, so you end up with widgets that thing they're still being long-pressed even though nothing is happening.

We could change the behavior in three ways, as far as I can tell:

we could send a cancel event when you change buttons. This would introduce a new kind of transition (start->cancel) which I don't think we currently require people to support. This would therefore not fix existing code and would make future code more complicated to handle a really obscure user action that it seems unlikely anyone cares about.
we could send an end event when you change buttons. This would mean the action commits, even though the user is still holding the mouse button down. This seems slightly better than the previous option but still not ideal as it means nudging the mouse button commits you even though you're still holding the button down.
we could ignore button changes after the long-press has been accepted.

I implemented the last one in this PR.

Renzo-Olivares

I agree this is a very obscure user interaction, option 1 did sound reasonable to me and it looks like we do something similar by handling down (accepted) -> cancel in TapGestureRecognizer.

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

Lines 257 to 268 in f92f441

    
           @override 
        
           void resolve(GestureDisposition disposition) { 
        
             if (_wonArenaForPrimaryPointer && disposition == GestureDisposition.rejected) { 
        
               // This can happen if the gesture has been canceled. For example, when 
        
               // the pointer has exceeded the touch slop, the buttons have been changed, 
        
               // or if the recognizer is disposed. 
        
               assert(_sentTapDown); 
        
               _checkCancel(null, 'spontaneous'); 
        
               _reset(); 
        
             } 
        
             super.resolve(disposition); 
        
           }

But I agree with the point that it wouldn't fix current users. I wonder if in the future we want to provide the buttons in the gesture details object so a user could potentially handle this use-case by checking if the initial down buttons are the same as the end buttons.

Renzo-Olivares · 2023-06-02T20:00:57Z

packages/flutter/test/gestures/long_press_test.dart

From testing on macOS, after this first move4, when releasing the primary button, another move event is dispatched with the buttons set as 2.

Renzo-Olivares · 2023-06-02T20:02:16Z

packages/flutter/test/gestures/long_press_test.dart

This might be outside of the scope of this PR, but when testing on macOS and web with a local example app the buttons of this final PointerUpEvent is actually 0. Is this expected?

Hixie · 2023-06-06T18:03:33Z

Can you elaborate on what you're seeing on macOS?

In general I would hope that we normalize our inputs so the same physical inputs turn into the same events at the Flutter level but that may be optimistic.

I'm happy to add tests for other sequences, to make sure they all make sense. Is that what you had in mind?

Renzo-Olivares · 2023-06-06T18:45:58Z

Using this example app with the change in this PR.

import 'package:flutter/material.dart';

void main() {
  runApp(const MainApp());
}

class MainApp extends StatelessWidget {
  const MainApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: GestureDetector(
            onLongPressStart: (LongPressStartDetails details) {
              debugPrint('start');
            },
            onLongPressMoveUpdate: (LongPressMoveUpdateDetails details) {
              debugPrint('move');
            },
            onLongPressEnd: (LongPressEndDetails details) {
              debugPrint('end');
            },
            child: Container(
              color: Colors.green,
              height: 200,
              width: 200,
              child: const Text('Hello World!'),
            ),
          ),
        ),
      ),
    );
  }
}

and adding some logging in PrimaryPointerGestureRecognizer.addAllowedPointer and PrimaryPointerGestureRecognizer.handleEvent.

For the following click sequence.

long press left click.
sometime after long press also press the right click.
release the left click.
release the right click.

This is what I see in the logs for macOS native/and macOS chrome.

flutter: PrimaryPointerGestureRecognizer addAllowedPointer down _TransformedPointerDownEvent#b28e4(position: Offset(378.2, 287.3))
flutter: PrimaryPointerGestureRecognizer handleEvent _TransformedPointerDownEvent#a57f1(position: Offset(378.2, 287.3)) buttons 1
flutter: start
flutter: PrimaryPointerGestureRecognizer handleEvent _TransformedPointerMoveEvent#06cd2(position: Offset(378.2, 287.3)) buttons 3
flutter: move
flutter: PrimaryPointerGestureRecognizer handleEvent _TransformedPointerMoveEvent#fd781(position: Offset(378.2, 287.3)) buttons 2
flutter: move
flutter: PrimaryPointerGestureRecognizer handleEvent _TransformedPointerUpEvent#fad41(position: Offset(378.2, 287.3)) buttons 0
flutter: end

I was more or less wondering if we should test that extra move event when the left click is released. And if the pointer up event ending with buttons: 0 is expected. Looking at your test it looks like it should be buttons: 2.

Hixie · 2023-06-14T22:07:32Z

I can add some tests that check that sequence and others, sure.

Previously, if you changed buttons during a long-press gesture, if it was before the gesture was accepted we would discard it, and if it was after the gesture was accepted we would silently end it without firing any of the relevant events. This silent cancelation behavior is terrible because it means there's no way for consumers to know what state they're in, so you end up with widgets that thing they're still being long-pressed even though nothing is happening. We could change the behavior in three ways, as far as I can tell: - we could send a cancel event when you change buttons. This would introduce a new kind of transition (start->cancel) which I don't think we currently require people to support. This would therefore not fix existing code and would make future code more complicated to handle a really obscure user action that it seems unlikely anyone cares about. - we could send an end event when you change buttons. This would mean the action commits, even though the user is still holding the mouse button down. This seems slightly better than the previous option but still not ideal as it means nudging the mouse button commits you even though you're still holding the button down. - we could ignore button changes after the long-press has been accepted. I implemented the last one in this PR.

Hixie · 2023-06-15T04:59:22Z

@Renzo-Olivares PTAL

Renzo-Olivares

LGTM

Renzo-Olivares · 2023-06-21T22:10:21Z

I think the broken google testing may have been unrelated since it was having some issues around the time when this PR was approved.

…flutter/flutter#127877)

flutter/flutter@35085c3...bc49cd1 2023-07-06 ian@hixie.ch Allow long-press gestures to continue even if buttons change. (flutter/flutter#127877) 2023-07-06 goderbauer@google.com Enable unreachable_from_main lint - it is stable now!!1 (flutter/flutter#129854) 2023-07-06 ian@hixie.ch Update labeler to new label names (flutter/flutter#130040) 2023-07-05 47639380+Snonky@users.noreply.github.com MergeableMaterial: Fix adding a slice and separating it (flutter/flutter#128804) 2023-07-05 ian@hixie.ch Update infrastructure issue template for new priority scheme (flutter/flutter#129741) 2023-07-05 737941+loic-sharma@users.noreply.github.com Fix typo in canvas example (flutter/flutter#129879) 2023-07-05 hans.muller@gmail.com Reland Fix AnimatedList & AnimatedGrid doesn't apply MediaQuery padding #129556 (flutter/flutter#129860) 2023-07-05 ian@hixie.ch Change from "created via performance template" to "from: performance template" (flutter/flutter#130035) 2023-07-05 109253501+pdblasi-google@users.noreply.github.com Removes deprecated APIs from v2.6 in `binding.dart` and `widget_tester.dart` (flutter/flutter#129663) 2023-07-05 helinx@google.com Add new hot reload case string (flutter/flutter#130008) 2023-07-05 engine-flutter-autoroll@skia.org Manual roll Flutter Engine from 987b621eac4e to bd2e42b203e1 (32 revisions) (flutter/flutter#130023) 2023-07-05 69246223+moylanm@users.noreply.github.com Add simple unit tests for annotations.dart file (flutter/flutter#128902) 2023-07-05 gipcjs@gmail.com fix a bug when android uses CupertinoPageTransitionsBuilder... (flutter/flutter#114303) 2023-07-05 piotr.fleury@gmail.com Add .env file support for option `--dart-define-from-file` (flutter/flutter#128668) If this roll has caused a breakage, revert this CL and stop the roller using the controls here: https://autoroll.skia.org/r/flutter-packages Please CC bmparr@google.com,rmistry@google.com,stuartmorgan@google.com on the revert to ensure that a human is aware of the problem. To file a bug in Packages: https://github.com/flutter/flutter/issues/new/choose To report a problem with the AutoRoller itself, please file a bug: https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug Documentation for the AutoRoller is here: https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

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Renzo-Olivares reviewed Jun 2, 2023

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Hixie force-pushed the longpress branch from fcbd13b to 47d6cac Compare June 14, 2023 22:41

Renzo-Olivares approved these changes Jun 15, 2023

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Merge branch 'master' into longpress

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	@override
	void resolve(GestureDisposition disposition) {
	if (_wonArenaForPrimaryPointer && disposition == GestureDisposition.rejected) {
	// This can happen if the gesture has been canceled. For example, when
	// the pointer has exceeded the touch slop, the buttons have been changed,
	// or if the recognizer is disposed.
	assert(_sentTapDown);
	_checkCancel(null, 'spontaneous');
	_reset();
	}
	super.resolve(disposition);
	}

Allow long-press gestures to continue even if buttons change. #127877

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