[Bug report] onDragEnd gesture -> Null check operator used on a null value #601

EgorK0rshun · 2023-08-16T11:29:54Z

Version

8.0.2

Platforms

dart, Android, iOS

Device Model

All devices, Galaxy A12, Galaxy A32, Galaxy A13, Redmi 8A, Redmi 9C, Redmi Note 8

flutter info

flutter doctor -v
[✓] Flutter (Channel stable, 3.10.6, on macOS 13.0.1 22A400 darwin-arm64, locale ru-BY)
    • Flutter version 3.10.6 on channel stable at /Users/k/develop/flutter
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    • Framework revision f468f3366c (5 weeks ago), 2023-07-12 15:19:05 -0700
    • Engine revision cdbeda788a
    • Dart version 3.0.6
    • DevTools version 2.23.1

[✓] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
    • Android SDK at /Users/k/Library/Android/sdk
    • Platform android-33, build-tools 33.0.1
    • Java binary at: /Applications/Android Studio.app/Contents/jre/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 14.2)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 14C18
    • CocoaPods version 1.11.3

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[✓] Android Studio (version 2021.3)
    • Android Studio at /Applications/Android Studio.app/Contents
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      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
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      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)

[✓] Android Studio (version 2021.3)
    • Android Studio at /Users/k/Library/Application Support/JetBrains/Toolbox/apps/AndroidStudio/ch-0/213.7172.25.2113.9123335/Android Studio.app/Contents
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    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 11.0.13+0-b1751.21-8125866)

[✓] VS Code (version 1.81.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.56.0

[✓] Connected device (3 available)
    • SM A137F (mobile) • RF8T80AXGPY • android-arm    • Android 13 (API 33)
    • macOS (desktop)   • macos       • darwin-arm64   • macOS 13.0.1 22A400 darwin-arm64
    • Chrome (web)      • chrome      • web-javascript • Google Chrome 115.0.5790.170

[✓] Network resources
    • All expected network resources are available.

• No issues found!

How to reproduce?

Use ExtendedImageGesturePageView.builder and try to execute onDrag gesture. The exception will be throw for onDragEnd callback

Logs

UnhandledFlutterException: Null check operator used on a null value
       at extended_image.src.gesture.page_view.gesture_page_view.dart.ExtendedImageGesturePageViewState.onDragEnd(gesture_page_view.dart:426)
       at extended_image.src.gesture_detector.drag.dart.ExtendedDragGestureRecognizer._checkEnd.<fn>(drag.dart:623)
       at flutter.src.gestures.recognizer.dart.GestureRecognizer.invokeCallback(recognizer.dart:253)
       at extended_image.src.gesture_detector.drag.dart.ExtendedDragGestureRecognizer._checkEnd(drag.dart:623)
       at extended_image.src.gesture_detector.drag.dart.ExtendedDragGestureRecognizer.didStopTrackingLastPointer(drag.dart:526)
       at flutter.src.gestures.recognizer.dart.OneSequenceGestureRecognizer.stopTrackingPointer(recognizer.dart:446)
       at extended_image.src.gesture_detector.drag.dart.ExtendedDragGestureRecognizer._giveUpPointer(drag.dart:535)
       at extended_image.src.gesture_detector.drag.dart.ExtendedDragGestureRecognizer.handleEvent(drag.dart:449)
       at flutter.src.gestures.pointer_router.dart.PointerRouter._dispatch(pointer_router.dart:98)
       at flutter.src.gestures.pointer_router.dart.PointerRouter._dispatchEventToRoutes.<fn>(pointer_router.dart:143)
       at collection-patch.compact_hash.dart._LinkedHashMapMixin.forEach(compact_hash.dart:625)
       at flutter.src.gestures.pointer_router.dart.PointerRouter._dispatchEventToRoutes(pointer_router.dart:141)
       at flutter.src.gestures.pointer_router.dart.PointerRouter.route(pointer_router.dart:127)
       at flutter.src.gestures.binding.dart.GestureBinding.handleEvent(binding.dart:460)
       at flutter.src.gestures.binding.dart.GestureBinding.dispatchEvent(binding.dart:440)
       at flutter.src.rendering.binding.dart.RendererBinding.dispatchEvent(binding.dart:336)
       at flutter.src.gestures.binding.dart.GestureBinding._handlePointerEventImmediately(binding.dart:395)
       at flutter.src.gestures.binding.dart.GestureBinding.handlePointerEvent(binding.dart:357)
       at flutter.src.gestures.binding.dart.GestureBinding._flushPointerEventQueue(binding.dart:314)
       at flutter.src.gestures.binding.dart.GestureBinding._handlePointerDataPacket(binding.dart:295)
       at async.zone.dart._rootRunUnary(zone.dart:1414)
       at async.zone.dart._CustomZone.runUnary(zone.dart:1307)
       at async.zone.dart._CustomZone.runUnaryGuarded(zone.dart:1216)
       at ui.hooks.dart._invoke1(hooks.dart:166)
       at ui.platform_dispatcher.dart.PlatformDispatcher._dispatchPointerDataPacket(platform_dispatcher.dart:361)
       at ui.hooks.dart._dispatchPointerDataPacket(hooks.dart:91)

Example code (optional)

child: Stack(
              children: <Widget>[
                ValueListenableBuilder<Map<String, ImageDownloadingState>>(
                  valueListenable: context.read<Model>().imageDownloadingStatesValueNotifier,
                  builder: (_, Map<String, ImageDownloadingState> imageDownloadingStates, __) {
                    return ExtendedImageGesturePageView.builder(
                      controller: _extendedPageController,
                      physics: _ImageScrollPhysics(),
                      itemCount: _imagePaths.length,
                      onPageChanged: (int index) {
                   _onPageChanged();
                        _startPrecachingImages(index);
                      },
                      canScrollPage: (GestureDetails? details) {
                        if (details?.totalScale == 1.0) {
                          return true;
                        }
                        return false;
                      },
                      itemBuilder: (BuildContext context, int index) {
                        final Widget image = AnimatedScale(
                          key: Key(_imagePaths[index]),
                          duration: _deleteAnimationDuration,
                          scale: _someConditionalForImage[index] ? 0 : 1,
                          child: _someConditional
                              ? const FractionallySizedBox(
                                  widthFactor: 0.75,
                                  child: ImagePreviewProgress(
                                    size: double.infinity,
                                  ),
                                )
                              : ExtendedImage(
                                  key: Key(_imagePaths[index] + _shouldBeFullscreen.toString()),
                                  image: ExtendedFileImageProvider(File(_imagePaths[index])),
                                  fit: BoxFit.contain,
                                  mode: ExtendedImageMode.gesture,
                                  initGestureConfigHandler: (ExtendedImageState state) {
                                    return GestureConfig(
                                      animationMinScale: 1.0,
                                      minScale: 1,
                                      inPageView: true,
                                      cacheGesture: false,
                                    );
                                  },
                                  loadStateChanged: (ExtendedImageState state) {
                                    switch (state.extendedImageLoadState) {
                                      case LoadState.loading:
                                        return null;
                                      case LoadState.completed:
                                        _imageLoaded = true;
                                        return null;
                                      case LoadState.failed:
                                        _imageLoaded = false;
                                        // remove memory cached
                                        state.imageProvider.evict();
                                        return const FractionallySizedBox(
                                          widthFactor: 0.75,
                                          child: ImagePreviewError(
                                            size: double.infinity,
                                          ),
                                        );
                                    }
                                  },
                                ),
                        );
                        ...
}
                       
    void _startPrecachingImages(int currentImageIndex) {
    final List<String> filesToPrecache = <String>[];
    if (currentImageIndex < _imagePaths.length - 1) {
      filesToPrecache.add(_imagePaths[currentImageIndex + 1]);
    }
    if (currentImageIndex != 0) {
      filesToPrecache.add(_imagePaths[currentImageIndex - 1]);
    }
    for (String path in filesToPrecache) {
      precacheImage(ExtendedFileImageProvider(File(path)), context);
    }
  }

Contact

https://github.com/EgorK0rshun,
korshun.dev@gmail.com

The text was updated successfully, but these errors were encountered:

zmtzawqlp · 2023-08-16T11:59:22Z

could you provide a simple runnable demo to reproduce your issue?

EgorK0rshun · 2023-08-16T12:08:31Z

@zmtzawqlp , unfortunately no. This is reproduced by real users. i only have stats. And in code of library
onDragUpdate have ambiguous implementation:

  `void onDragUpdate(DragUpdateDetails details) {
    // _drag might be null if the drag activity ended and called _disposeDrag.
    assert(_hold == null || _drag == null);
    //final Offset delta = details.delta;
    if (!widget.canScrollPage(extendedImageGestureState?.gestureDetails)) {
      return;
    }

    _drag?.update(details);

//     return;

//     if (extendedImageGestureState != null) {
//       final GestureDetails? gestureDetails =
//           extendedImageGestureState!.gestureDetails;
//       if (gestureDetails != null) {
//         final int currentPage = pageController.page!.round();
// //        bool pageChanging = false;
// //
// //        if (widget.scrollDirection == Axis.horizontal) {
// //          if (delta.dx != 0.0) {
// //            if (delta.dx < 0) {
// //              pageChanging = pageController.page > currentPage;
// //            } else {
// //              pageChanging = pageController.page < currentPage;
// //            }
// //          }
// //        } else {
// //          if (delta.dy != 0.0) {
// //            if (delta.dy < 0) {
// //              pageChanging = pageController.page < currentPage;
// //            } else {
// //              pageChanging = pageController.page > currentPage;
// //            }
// //          }
// //        }

//         if ((gestureDetails.movePage(delta, widget.scrollDirection) ||
//                 (currentPage != pageController.page)) &&
//             widget.canMovePage(gestureDetails)) {
//           _drag?.update(details);
//         } else {
//           if (currentPage == pageController.page) {
//             extendedImageGestureState!.gestureDetails = GestureDetails(
//                 offset: gestureDetails.offset! +
//                     delta *
//                         extendedImageGestureState!.imageGestureConfig!.speed,
//                 totalScale: gestureDetails.totalScale,
//                 gestureDetails: gestureDetails);
//           }
//         }
//       } else {
//         _drag?.update(details);
//       }
//     } else {
//       _drag?.update(details);
//     }
  }

but ok. Its work. And here used ? operator ( _drag?.update(details);).

In the method onDragEnd you use bang operator:

  // _drag might be null if the drag activity ended and called _disposeDrag.
  assert(_hold == null || _drag == null);
  if (!widget.canScrollPage(extendedImageGestureState?.gestureDetails)) {
    _drag!.end(DragEndDetails(primaryVelocity: 0.0));
    return;
  }
  _drag!.end(details);
  assert(_drag == null);
  // return;
  // DragEndDetails temp = details;
  // if (extendedImageGestureState != null) {
  //   final GestureDetails? gestureDetails =
  //       extendedImageGestureState!.gestureDetails;
  //   final int currentPage = pageController.page!.round();
  //   final bool movePage = pageController.page != currentPage;

  //   if (!widget.canMovePage(gestureDetails)) {
  //     //stop
  //     temp = DragEndDetails(primaryVelocity: 0.0);
  //   }

  //   /// stop when zoom in, so that it will not move to next/previous page
  //   if (!movePage &&
  //       gestureDetails != null &&
  //       gestureDetails.totalScale! > 1.0 &&
  //       (gestureDetails.computeHorizontalBoundary ||
  //           gestureDetails.computeVerticalBoundary)) {
  //     //stop
  //     temp = DragEndDetails(primaryVelocity: 0.0);

  //     // get magnitude from gesture velocity
  //     final double magnitude = details.velocity.pixelsPerSecond.distance;

  //     // do a significant magnitude
  //     if (magnitude.greaterThanOrEqualTo(minMagnitude)) {
  //       Offset direction = details.velocity.pixelsPerSecond /
  //           magnitude *
  //           (extendedImageGestureState!.imageGestureConfig!.inertialSpeed);

  //       if (widget.scrollDirection == Axis.horizontal) {
  //         direction = Offset(direction.dx, 0.0);
  //       } else {
  //         direction = Offset(0.0, direction.dy);
  //       }

  //       _gestureAnimation.animationOffset(
  //           gestureDetails.offset, gestureDetails.offset! + direction);
  //     }
  //   }
  // }

  // _drag!.end(temp);

  // assert(_drag == null);
}

maybe error here _drag!.end(details);

zmtzawqlp · 2023-08-17T06:36:32Z

_drag should not be null at that moment , it's better to reproduce it so that i can bug it.

EgorK0rshun · 2023-08-17T14:30:01Z

no, drug = null, because u a set null when start onDragStart gesture:

 void _disposeDrag() {
    _drag = null;
  }

let me fork it, fix this string, look at the stats, and if it helps, I'll make a requisition for the extended_image

zmtzawqlp · 2023-08-18T03:24:45Z

no, drug = null, because u a set null when start onDragStart gesture:
 void _disposeDrag() {
    _drag = null;
  }
let me fork it, fix this string, look at the stats, and if it helps, I'll make a requisition for the extended_image

i mean that it should not be null at that moment(onDragEnd). _disposeDrag callback is called after onDragEnd

EgorK0rshun · 2023-08-18T09:38:53Z

@zmtzawqlp , shouldn't, but there is. analytics and stacktrace of the exception says that this place is null

zmtzawqlp · 2023-08-18T09:57:55Z

@zmtzawqlp , shouldn't, but there is. analytics and stacktrace of the exception says that this place is null

so we should better find out the case of this issue, not simplely use '?' instead of '!'.

EgorK0rshun · 2023-08-21T10:54:45Z

@zmtzawqlp , so it is, after all, for the reason that he wrote above: at the beginning of the gesture, _drag is dispositions. you can debug. according to the order of calls, _drag becomes null in the course of the gesture execution

EgorK0rshun · 2023-09-06T11:27:46Z

@zmtzawqlp , please review
#620

zmtzawqlp · 2023-09-06T11:37:38Z

@zmtzawqlp , please review #620

Hi guy, thank you very much for reporting this issue.
and I think we should find the real reason for this problem. I only received a report about this issue from you for now. I added an assert here because it should not be null. If it happens, we should find the real reason to solve it instead of avoiding it directly.

EgorK0rshun · 2023-09-06T12:22:50Z

@zmtzawqlp
I described the reason above. The changes in this request helped to get rid of a bug that several thousand users encountered. It's all about the _disposeDrag method. in all references to the drag in this class is used ? , only for onDragUpdate you use !
Flutter generally recommends avoiding bang operators.
You can try to look deeper if you are not sure about the correctness of my wording, but I would ask you not to delay, because maintaining a fork of a third-party library is quite expensive in terms of time and resources. In the current implementation, everything works correctly. Doesn't lead to any additional errors.

EgorK0rshun · 2023-09-06T12:27:35Z

here is a diagram showing that when switching to a new version of the application, in which the line was corrected, as in the attached request ( https://github.com//pull/620 ) , the error disappears. the smoothness of the graphics is ensured by the gradual rolling out of the new version.

svyatoslavb-digitalchemy · 2023-09-06T12:47:49Z

Hi, It is not recommended to use the bang operator to interact with gestures. When checking using null-aware access, the method will simply fail if the value is null.

zmtzawqlp · 2023-09-07T01:45:47Z

Those codes are from https://github.com/flutter/flutter/blob/e8ff2aed31692142fb3494e2ee8b5b11d3e28693/packages/flutter/lib/src/widgets/scrollable.dart#L831

it is already use '?' here, and I believe our modification won't introduce any other issues. Thanks again for your pull request and explanation. LGTM

EgorK0rshun mentioned this issue Sep 6, 2023

Fix issue with onDragEnd #620

Merged

zmtzawqlp closed this as completed in #620 Sep 7, 2023

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[Bug report] onDragEnd gesture -> Null check operator used on a null value #601

[Bug report] onDragEnd gesture -> Null check operator used on a null value #601

EgorK0rshun commented Aug 16, 2023 •

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zmtzawqlp commented Sep 7, 2023

[Bug report] onDragEnd gesture -> Null check operator used on a null value #601

[Bug report] onDragEnd gesture -> Null check operator used on a null value #601

Comments

EgorK0rshun commented Aug 16, 2023 • edited Loading

Version

Platforms

Device Model

flutter info

How to reproduce?

Logs

Example code (optional)

Contact

zmtzawqlp commented Aug 16, 2023

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