Incorrect position in first update from MultiDragGestureRecognizer

[tgucio]

Steps to Reproduce

1. Execute flutter run on the code sample to run on a phone
2. Try a few drag gestures (with and without holding the finger down for a while first)

Expected results: The first drag update carries a different offset from the one supplied to the onStart callback

Actual results: The first drag update always has the same offset as in onStart callback despite delta being correct. Additionally, the DragUpdateDetails.localPosition field isn't provided meaning it will default to .globalPosition so it can't be relied on.

I/flutter (30743): _handleDragStart Offset(157.4, 413.1)
I/flutter (30743): _handleDragUpdate localPosition=Offset(157.4, 413.1) globalPosition=Offset(157.4, 413.1) delta=Offset(14.3, -13.7) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(173.7, 398.0) globalPosition=Offset(173.7, 398.0) delta=Offset(2.0, -1.4) primaryDelta=null
I/flutter (30743): _handleDragEnd

I/flutter (30743): _handleTapDown Offset(128.6, 227.4)
I/flutter (30743): _handleDragStart Offset(134.3, 409.1)
I/flutter (30743): _handleDragUpdate localPosition=Offset(134.3, 409.1) globalPosition=Offset(134.3, 409.1) delta=Offset(11.1, -14.3) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(148.0, 392.6) globalPosition=Offset(148.0, 392.6) delta=Offset(2.6, -2.3) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(150.6, 390.3) globalPosition=Offset(150.6, 390.3) delta=Offset(2.6, -2.3) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(152.9, 388.3) globalPosition=Offset(152.9, 388.3) delta=Offset(2.3, -2.0) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(154.6, 386.9) globalPosition=Offset(154.6, 386.9) delta=Offset(1.7, -1.4) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(156.0, 385.7) globalPosition=Offset(156.0, 385.7) delta=Offset(1.4, -1.1) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(157.1, 384.6) globalPosition=Offset(157.1, 384.6) delta=Offset(1.1, -1.1) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(158.0, 383.7) globalPosition=Offset(158.0, 383.7) delta=Offset(0.9, -0.9) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(158.9, 383.1) globalPosition=Offset(158.9, 383.1) delta=Offset(0.9, -0.6) primaryDelta=null
I/flutter (30743): _handleDragUpdate localPosition=Offset(160.3, 382.3) globalPosition=Offset(160.3, 382.3) delta=Offset(1.4, -0.9) primaryDelta=null
I/flutter (30743): _handleDragEnd

Code sample

import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title});

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  void _handleTapDown(TapDownDetails details) {
    print('_handleTapDown ${details.localPosition}');
  }

  void _handleTapUp(TapUpDetails details) {
    print('_handleTapUp ${details.localPosition}');
  }
 
  Drag? _handleDragStart(Offset position) {
    print('_handleDragStart $position');
    return _DragHandler(_handleDragUpdate, _handleDragCancel, _handleDragEnd);
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    print('_handleDragUpdate localPosition=${details.localPosition} globalPosition=${details.globalPosition} '
        'delta=${details.delta} primaryDelta=${details.primaryDelta}');
  }

  void _handleDragCancel() {
    print('_handleDragCancel');
  }

  void _handleDragEnd(DragEndDetails details) {
    print('_handleDragEnd');
  }

  @override
  Widget build(BuildContext context) {
    final Map<Type, GestureRecognizerFactory> gestures = <Type, GestureRecognizerFactory>{
      TapGestureRecognizer: GestureRecognizerFactoryWithHandlers<TapGestureRecognizer>(
        () => TapGestureRecognizer(debugOwner: this),
        (TapGestureRecognizer instance) {
          instance
            ..onTapDown = _handleTapDown
            ..onTapUp = _handleTapUp;
        },
      ),
      ImmediateMultiDragGestureRecognizer: GestureRecognizerFactoryWithHandlers<ImmediateMultiDragGestureRecognizer>(
        () => ImmediateMultiDragGestureRecognizer(debugOwner: this),
        (ImmediateMultiDragGestureRecognizer instance) {
          instance.onStart = _handleDragStart;
        },
      ),
    };

    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      body: Center(
        child: Container(
          width: 400.0,
          height: 400.0,
          color: Colors.red,
          child: RawGestureDetector(
            gestures: gestures,
          ),
        ),
      ),
    );
  }
}

class _DragHandler extends Drag {
  _DragHandler(this.onUpdate, this.onCancel, this.onEnd);

  final GestureDragUpdateCallback onUpdate;
  final GestureDragCancelCallback onCancel;
  final GestureDragEndCallback onEnd;

  @override
  void update(DragUpdateDetails details) {
    onUpdate(details);
  }

  @override
  void cancel() {
    onCancel();
  }

  @override
  void end(DragEndDetails details) {
    onEnd(details);
  }
}

Logs

[darshankawar]

Thanks for the report @tgucio
Running your code sample on latest master using iOS simulator gives me the expected result, ie, the _handleDragStart and first _handleDragUpdate offset is same:

flutter: _handleTapDown Offset(224.3, 171.2)
flutter: _handleDragStart Offset(238.3, 485.7)
flutter: _handleDragUpdate localPosition=Offset(238.3, 485.7) globalPosition=Offset(238.3, 485.7) delta=Offset(-4.3, -22.3) primaryDelta=null
flutter: _handleDragUpdate localPosition=Offset(231.0, 451.3) globalPosition=Offset(231.0, 451.3) delta=Offset(-3.0, -12.0) primaryDelta=null

Channel master, 3.7.0-10.0.pre.15

[tgucio]

Hi @darshankawar the two offsets should be different (note the non-zero delta in the first _handleDragUpdate).
Also, localPosition == globalPosition which should not be the case unless the widget is located at Offset(0, 0).

[darshankawar]

Thanks for the update.

stable, master flutter doctor -v

[✓] Flutter (Channel stable, 3.3.10, on macOS 12.2.1 21D62 darwin-x64, locale
    en-GB)
    • Flutter version 3.3.10 on channel stable at
      /Users/dhs/documents/fluttersdk/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 135454af32 (15 hours ago), 2022-12-15 07:36:55 -0800
    • Engine revision 3316dd8728
    • Dart version 2.18.6
    • DevTools version 2.15.0

[!] Xcode - develop for iOS and macOS (Xcode 12.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    ! Flutter recommends a minimum Xcode version of 13.
      Download the latest version or update via the Mac App Store.
    • CocoaPods version 1.11.2

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[✓] VS Code (version 1.62.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.21.0

[✓] Connected device (5 available)
    • SM G975F (mobile)       • RZ8M802WY0X • android-arm64   • Android 11 (API 30)
    • Darshan's iphone (mobile)  • 21150b119064aecc249dfcfe05e259197461ce23 •
      ios            • iOS 14.4.1 18D61
    • iPhone 12 Pro Max (mobile) • A5473606-0213-4FD8-BA16-553433949729     •
      ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-3 (simulator)
    • macOS (desktop)            • macos                                    •
      darwin-x64     • Mac OS X 10.15.4 19E2269 darwin-x64
    • Chrome (web)               • chrome                                   •
      web-javascript • Google Chrome 98.0.4758.80

[✓] HTTP Host Availability
    • All required HTTP hosts are available

! Doctor found issues in 1 category.

[!] Flutter (Channel master, 3.7.0-10.0.pre.15, on macOS 12.2.1 21D62
    darwin-x64, locale en-GB)
    • Flutter version 3.7.0-10.0.pre.15 on channel master at
      /Users/dhs/documents/fluttersdk/flutter
    ! Warning: `flutter` on your path resolves to
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      your current Flutter SDK checkout at
      /Users/dhs/documents/fluttersdk/flutter. Consider adding
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      current Flutter SDK checkout at /Users/dhs/documents/fluttersdk/flutter.
      Consider adding /Users/dhs/documents/fluttersdk/flutter/bin to the front
      of your path.
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 76bb8ead5a (62 minutes ago), 2022-12-15 19:38:53 -0800
    • Engine revision 29196519c1
    • Dart version 3.0.0 (build 3.0.0-21.0.dev)
    • DevTools version 2.20.0
    • If those were intentional, you can disregard the above warnings; however
      it is recommended to use "git" directly to perform update checks and
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[!] Xcode - develop for iOS and macOS (Xcode 12.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    ! Flutter recommends a minimum Xcode version of 13.
      Download the latest version or update via the Mac App Store.
    • CocoaPods version 1.11.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] VS Code (version 1.62.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.21.0

[✓] Connected device (5 available)
    • SM G975F (mobile)       • RZ8M802WY0X • android-arm64   • Android 11 (API 30)
    • Darshan's iphone (mobile)  • 21150b119064aecc249dfcfe05e259197461ce23 •
      ios            • iOS 14.4.1 18D61
    • iPhone 12 Pro Max (mobile) • A5473606-0213-4FD8-BA16-553433949729     •
      ios            • com.apple.CoreSimulator.SimRuntime.iOS-14-3 (simulator)
    • macOS (desktop)            • macos                                    •
      darwin-x64     • Mac OS X 10.15.4 19E2269 darwin-x64
    • Chrome (web)               • chrome                                   •
      web-javascript • Google Chrome 98.0.4758.80

[✓] HTTP Host Availability
    • All required HTTP hosts are available

! Doctor found issues in 1 category.

[goderbauer]

For the DragGestureRecognizer you should be able to control this via the dragStartBehavior. Looks like we don't have this as an option for the MultiDragGestureRecognizer, though.