_ReorderableBuilderState._handleDragEnd thows null check error

[meomap]

Hello, thank you for your great package.

We've got this exception thrown at the production app.

flutter_reorderable_grid_view: ^3.1.2

Sentry stacktrace:

_CastError: Null check operator used on a null value
  split_config.arm64_v8a.apk0x727de7b968 _ReorderableBuilderState._handleDragEnd (reorderable_builder.dart:318)
  split_config.arm64_v8a.apk0x727de7b558 _ReorderableBuilderState._handleDragEnd (reorderable_builder.dart:315)
  split_config.arm64_v8a.apk0x727de7ca24 _ReorderableScrollingListenerState.build.<T> (reorderable_scrolling_listener.dart:82)
  split_config.arm64_v8a.apk0x727dc6e674 RenderPointerListener.handleEvent (proxy_box.dart:2916)
  split_config.arm64_v8a.apk0x727e15f3a0 GestureBinding.dispatchEvent (binding.dart:425)
  split_config.arm64_v8a.apk0x727db0df7c RendererBinding.dispatchEvent (binding.dart:329)
  split_config.arm64_v8a.apk0x727db0dec0 GestureBinding._handlePointerEventImmediately (binding.dart:380)
  split_config.arm64_v8a.apk0x727db0dac4 GestureBinding.handlePointerEvent (binding.dart:344)
  split_config.arm64_v8a.apk0x727dbd560c GestureBinding._handlePointerDataPacket (binding.dart:285)
  split_config.arm64_v8a.apk0x727dbd5510 GestureBinding._handlePointerDataPacket (binding.dart:280)
  split_config.arm64_v8a.apk0x727e1143a0 _rootRunUnary (zone.dart:1442)
  split_config.arm64_v8a.apk0x727da35d4c _rootRunUnary (zone.dart:1432)
  split_config.arm64_v8a.apk0x727e00f5f8 _CustomZone.runUnary (zone.dart:1335)
  split_config.arm64_v8a.apk0x727e23269c _CustomZone.runUnaryGuarded (zone.dart:1244)
  split_config.arm64_v8a.apk0x727da46af0 _invoke1 (hooks.dart:170)
  split_config.arm64_v8a.apk0x727da469f0 PlatformDispatcher._dispatchPointerDataPacket (platform_dispatcher.dart:331)
  split_config.arm64_v8a.apk0x727da46968 _dispatchPointerDataPacket (hooks.dart:94)

[karvulf]

Hi @meomap
Thank you for opening the issue.
I will try to fix that problem

[karvulf]

I am wondering why that problem is happening. Could you share the code snippet of the part where you use ReorderableBuilder? Are you changing some values of ReorderableBuilder while dragging? @meomap

[meomap]

Sure, we use ReorderableBuilder to render a grid view of photo collection where user can add new photo, remove existing one and dragging to change order.

Here is code snippet:

import 'package:flutter/material.dart';
import 'package:equatable/equatable.dart';
import 'package:cached_network_image/cached_network_image.dart';
import 'package:flutter_reorderable_grid_view/entities/order_update_entity.dart';
import 'package:flutter_reorderable_grid_view/widgets/widgets.dart';

class PhotoCollectionModal extends StatefulWidget {
  const PhotoCollectionModal({Key? key}) : super(key: key);

  @override
  State<PhotoCollectionModal> createState() => _PhotoCollectionModalState();
}

class _PhotoCollectionModalState extends State<PhotoCollectionModal> {
  bool draggingItem = false;

  final _gridViewKey = GlobalKey();
  final _scrollController = ScrollController();
  final List<Photo> items =
      List<Photo>.generate(10, (index) => _dummyPhoto(index));

  @override
  Widget build(BuildContext context) {
    final colorScheme = Theme.of(context).colorScheme;
    const double space = 16;
    const cols = 3;
    final generatedChildren = List<Widget>.generate(
        items.length,
        (index) => Container(
              key: ValueKey(items[index]),
              decoration: BoxDecoration(
                color: colorScheme.surface,
                borderRadius: BorderRadius.circular(4),
                border: Border.all(color: colorScheme.outline),
              ),
              child: Stack(
                children: [
                  Center(
                    child: CachedNetworkImage(
                      imageUrl: items[index].networkUrl,
                      fit: BoxFit.scaleDown,
                    ),
                  ),
                  Align(
                      alignment: Alignment.topRight,
                      child: Padding(
                        padding: const EdgeInsets.all(4),
                        child: GestureDetector(
                            onTap: () => handleRemove(index),
                            child:
                                Icon(Icons.remove_circle, color: colorScheme.error)),
                      ))
                ],
              ),
            ));

    return SafeArea(
      child: Scaffold(
          appBar: AppBar(title: const Text('Collection')),
          floatingActionButton: draggingItem
              ? null
              : FloatingActionButton(
                  child: const Icon(Icons.add), onPressed: handleAdd),
          body: Padding(
            padding: const EdgeInsets.all(space),
            child: ReorderableBuilder(
              children: generatedChildren,
              onReorder: handleReorder,
              enableLongPress: false,
              dragChildBoxDecoration: BoxDecoration(
                  color: Theme.of(context)
                      .colorScheme
                      .secondaryContainer
                      .withOpacity(0.54)),
              onDragStarted: () {
                setState(() {
                  draggingItem = true;
                });
              },
              onDragEnd: () {
                setState(() {
                  draggingItem = false;
                });
              },
              scrollController: _scrollController,
              builder: (children) {
                return GridView.builder(
                  key: _gridViewKey,
                  controller: _scrollController,
                  itemCount: children.length,
                  itemBuilder: (context, index) {
                    return children[index];
                  },
                  gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
                    crossAxisCount: cols,
                    mainAxisSpacing: space,
                    crossAxisSpacing: space,
                  ),
                );
              },
            ),
          )),
    );
  }

  void handleAdd() {
    setState(() {
      items.add(_dummyPhoto(items.length));
    });
  }

  void handleRemove(int index) {
    setState(() {
      items.removeAt(index);
    });
  }

  void handleReorder(List<OrderUpdateEntity> updates) {
    for (final entry in updates) {
      if (entry.oldIndex >= items.length || entry.newIndex >= items.length) {
        // Skip deleted one
        continue;
      }
      final child = items.removeAt(entry.oldIndex);
      items.insert(entry.newIndex, child);
    }
    if (!mounted) return;
    setState(() {});
  }
}

Photo _dummyPhoto(int index) => Photo(
    'https://picsum.photos/${100 + index}/${100 + index}/',
    DateTime.now().microsecondsSinceEpoch + index);

class Photo extends Equatable {
  final int id;
  final String networkUrl;

  const Photo(this.networkUrl, this.id);

  @override
  List<Object?> get props => [networkUrl, id];
}

[meomap]

In void handleReorder(List<OrderUpdateEntity> updates), we noticed that when user removed an item then immediately drag one at the end of list, entry.oldIndex sometimes equals current items.length. It happens occasionally and is reproducible in dev environment.

[karvulf]

I realized that this bug that you describe seems to happen when using GridView.builder. I also recognized that the animation of removing an item is not working.
So if you don't have a special reason to use GridView.builder, you could also use GridView as workaround. I will try to fix this problem in the next days. @meomap

return  GridView(
  key: _gridViewKey,
  controller: _scrollController,
  children: children,
  gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
    crossAxisCount: cols,
    mainAxisSpacing: space,
    crossAxisSpacing: space,
  ),

[karvulf]

I think I found a solution, so you could also wait for the update in the next days 👍 @meomap

[meomap]

@karvulf It's ok, I think it's just a minor glitch and we don't have much users affected by it yet. I can use GridView as well. No need to hurry :)

Thank you so much for your support 👌

[karvulf]

The bug should be fixed and released in a few minutes.
You can use version 3.1.3 for that. Also the animation for adding or removing your images should work now :)
I am closing this issue, if there are some other new issues with this update, feel free to open a new one @meomap

[meomap]

That's great! @karvulf Thanks so much 👌