默认 'List' 构造函数不可用

Question

为什么flutter不能使用默认的List构造函数？

每次我使用这个列表构造函数时它都会抛出一个错误（错误：错误：无法使用默认的 List 构造函数。请尝试使用 List.filled。）

错误行：List _items = new List();

import 'package:flutter/material.dart';
void main() {
 runApp(new MaterialApp(
   home: new MyApp(),
 ));
}

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

class MyItem {
 bool isExpanded;
 final String header;
 final Widget body;
  MyItem(this.isExpanded, this.header, this.body);
}

class _State extends State<MyApp> {
 List<MyItem> _items = new List<MyItem>();

@override
void initState() {
for (int i =0; i< 10; i++){
  _items.add(new MyItem(
    false,
    'Item ${i}',
    new Container(
      padding: new EdgeInsets.all(10.0),
      child: new Text('Hello World'),
    )
  ));
 }
}

ExpansionPanel _createitem(MyItem item) {
return new ExpansionPanel(
    headerBuilder: (BuildContext context, bool isExpanded) {
      return new Container(
        padding: new EdgeInsets.all(5.0),
        child: new Text('Header ${item.header}'),
      );
    },
    body: item.body,
    isExpanded: item.isExpanded
);
}

@override
Widget build(BuildContext context) {
return new Scaffold(
  appBar: new AppBar(
    title: new Text('Expansion Panel'),
  ),
  body: new Container(
      padding: new EdgeInsets.all(32.0),
      child: new Center(
        child: new Row(
          children: <Widget>[
            new ExpansionPanelList(
              expansionCallback: (int index, bool isExpanded) {
                setState(() {
                  _items[index].isExpanded = !_items[index].isExpanded;
                });
              },
              children: _items.map(_createitem).toList(),
            )
          ],
        ),
      )
  ),
);
}


}

请帮助我更改代码 ( List _items = new List(); ) 和 ( List _items = (); ) 它消除了错误，但没有在屏幕上显示任何内容

Answer 1

问题是您使用的是行而不是列。

试试这个代码

import 'package:flutter/material.dart';
void main() {
  runApp(new MaterialApp(
    home: new MyApp(),
  ));
}

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

class MyItem {
  bool isExpanded;
  final String header;
  final Widget body;
  MyItem(this.isExpanded, this.header, this.body);
}

class _State extends State<MyApp> {
  List<MyItem> _items = [];

  @override
  void initState() {
    for (int i =0; i< 10; i++){
      _items.add(new MyItem(
          false,
          'Item $i',
          new Container(
            padding: new EdgeInsets.all(10.0),
            child: new Text('Hello World'),
          )
      ));
    }
  }

  ExpansionPanel _createitem(MyItem item) {
    return new ExpansionPanel(
        headerBuilder: (BuildContext context, bool isExpanded) {
          return new Container(
            padding: new EdgeInsets.all(5.0),
            child: new Text('Header ${item.header}'),
          );
        },
        body: item.body,
        isExpanded: item.isExpanded
    );
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Expansion Panel'),
      ),
      body: SingleChildScrollView(
        child: Container(
 
            child: new Center(
              child: new Column(
                children: <Widget>[
                  new ExpansionPanelList(
                    expansionCallback: (int index, bool isExpanded) {
                      setState(() {
                        _items[index].isExpanded = !_items[index].isExpanded;
                      });
                    },
                    children: _items.map(_createitem).toList(),
                  )
                ],
              ),
            )
        ),
      )
    );
  }


}

默认 'List' 构造函数不可用

The default 'List' constructor isn't available

dart

android-studio

flutter

flutter-dependencies