有人可以启动一个快速的 flutter 项目并将 main.dart 替换为以下内容，看看我做错了什么吗？我正在尝试在 ListView 中进行拖放操作。

我什至不确定这是正确的方法，所以如果不正确，请告诉我。

我现在得到的错误是:

另一个异常被抛出:'package:flutter/src/rendering/box.dart': Failed assertion: line 1446 pos 12: 'hasSize': is not true.

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final title = 'Basic List';

    var tile1 = new Material(child:
       new ListTile(
          leading: new Icon(Icons.photo),
          title: new Text('Row 1'),
          trailing: new Icon(Icons.reorder),

    ));


    var tile2 = new Material(
        child:
          new ListTile(
            leading: new Icon(Icons.photo),
            title: new Text('Row 2'),
            trailing: new Icon(Icons.reorder),
    ));


    return new MaterialApp(
      title: title,
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text(title),
        ),
        body:
        new GestureDetector(
          onVerticalDragStart: startDrag,
          onVerticalDragEnd: endDrag,
          child: new ListView(
            shrinkWrap: true,
            children: [
                  new Flex (

                    children: <Widget>[
                      new Flexible(
                        child: new Draggable(child: tile1, feedback: 
tile1),
                      ),
                      new Flexible(
                        child: new Draggable(child: tile2, feedback: 
tile2),
                      ),
                    ],
                    direction: Axis.vertical,
                    mainAxisAlignment: MainAxisAlignment.start,
                    mainAxisSize: MainAxisSize.min,
                  ),
            ],
          ),
        ),
      ),
    );
  }

  void startDrag(DragStartDetails event) {}

  void endDrag(DragEndDetails event) {}
}

谢谢

最佳答案

在@Darky 的帮助下解决了 hasSize 问题，这是完成的可排序 ListView 示例:

https://github.com/marchampson/FluterSortableListView

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => new MyAppState();
}

class MyAppState extends State<MyApp> {

  List<String> rows = new List<String>()
    ..add('Row 1')
    ..add('Row 2')
    ..add('Row 3')
    ..add('Row 4');

  void _handleAccept(int data, int index) {
    setState(() {
      String imageToMove = rows[data];
      rows.removeAt(data);
      rows.insert(index, imageToMove);
    });
  }

  @override
  Widget build(BuildContext context) {

    final title = 'Sortable ListView';

    return new MaterialApp(
      title: title,
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text(title),
        ),
        body:
        new LayoutBuilder(builder: (context, constraint) {
          return new ListView.builder(
            itemCount: rows.length,
            addRepaintBoundaries: true,
            itemBuilder: (context, index) {
              return new LongPressDraggable(
                key: new ObjectKey(index),
                data: index,
                child: new DragTarget<int>(
                  onAccept: (int data) {
                    _handleAccept(data, index);
                  },
                  builder: (BuildContext context, List<int> data, List<dynamic> rejects) {
                    return new Card(
                        child: new Column(
                          children: <Widget>[
                            new ListTile(
                                leading: new Icon(Icons.photo),
                                title: new Text(rows[index])
                            ),
                          ],
                        )
                    );
                  },
                  onLeave: (int data) {
                    // Debug
                    print('$data is Leaving row $index');
                  },
                  onWillAccept: (int data) {
                    // Debug
                    print('$index will accept row $data');

                    return true;
                  },
                ),
                onDragStarted: () {
                  Scaffold.of(context).showSnackBar(new SnackBar (
                    content: new Text("Drag the row onto another row to change places"),
                  ));

                },
                onDragCompleted: () {
                  print("Finished");
                },
                feedback: new SizedBox(
                    width: constraint.maxWidth,
                     child: new Card (
                      child: new Column(
                        children: <Widget>[
                          new ListTile(
                            leading: new Icon(Icons.photo),
                            title: new Text(rows[index]),
                            trailing: new Icon(Icons.reorder),
                          ),
                        ],
                      ),
                      elevation: 18.0,
                    )
                ),
                childWhenDragging: new Container(),
              );
            },
          );
        }),
      ),
    );
  }
}

关于dart - Flutter 拖放 ListView hasSize 不是真的，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/49369147/