“List<Object>”类型不是“List<String>”类型的子类型答案

【问题标题】：Type 'List<Object>' is not a subtype of type 'List<String>'“List<Object>”类型不是“List<String>”类型的子类型
【发布时间】：2021-03-23 13:16:32
【问题描述】：

代码如图：

static const _modes = [
    ['KZTimer', 'SimpleKZ', 'Vanilla'],
    [128, 102, 64],
  ];

出错的部分是二维数组被调用到 ToggleButton() 类：

child: ToggleButton(_modes[0]),

ToggleButton 转到此构建方法，其中 _modes[0] 变为此处声明的 list：

final List<String> list;

Widget build(BuildContext context) {
  return new Container(
    child: ToggleButtons(
      isSelected: _selections,
      onPressed:(int i){}

      children: <Widget>[
        ...(widget.list).map((str) {
          return Padding(
            padding: EdgeInsets.symmetric(horizontal: 12),
            child: Text(
              str,
              style: TextStyle(fontSize: 18),
            ),
          );
        }).toList(),

      ],
    ),
  );
}

模拟器变成红色：'List <Object>' 类型不是'List <String>' 类型的子类型

【问题讨论】：

标签： flutter dart

【解决方案1】：

Update the final List<String> list; to either final List<dynamic> list; or final List list;
Please refer below code


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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: Settings());
  }
}

class Settings extends StatelessWidget {
  final String currentPage = 'Settings';
  static const modes = [
    ['KZTimer', 'SimpleKZ', 'Vanilla'],
    [128, 102, 64],
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SingleChildScrollView(
        child: Container(
          child: Padding(
            padding: EdgeInsets.all(8),
            child: Center(
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  buildHeader(
                    title: 'Mode',
                    child: ToggleButton(list: modes[0]),
                  ),
                  SizedBox(height: 32),
                  buildHeader(
                    title: 'Tick rate',
                    child: ToggleButton(list: modes[1]),
                  ),
                  SizedBox(height: 32),
                  buildHeader(
                    title: 'Mode',
                    child: ToggleButton(list: modes[0]),
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

Widget buildHeader({@required String title, @required Widget child}) {
  return Column(
    children: [
      Text(
        title,
        style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
      ),
      SizedBox(height: 16),
      child,
    ],
  );
}

class ToggleButton extends StatefulWidget {
  //Instead of this final List<String> list; update this to either final List<dynamic> list; or final List list;
  //final List<dynamic> list;
  final List list;

  const ToggleButton({Key key, this.list}) : super(key: key);

  @override
  State createState() => new _ToggleButtonState();
}

class _ToggleButtonState extends State<ToggleButton> {
  List<bool> _selections = [false, false, false];

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.blue.shade200,
      child: ToggleButtons(
        isSelected: _selections,
        fillColor: Colors.lightBlue,
        color: Colors.black,
        selectedColor: Colors.white,
        renderBorder: false,
        children: [
          ...(widget.list as List)?.map((answer) {
            return Padding(
              padding: EdgeInsets.symmetric(horizontal: 12),
              child: Text(
                answer.toString() ?? '',
                style: TextStyle(fontSize: 18),
              ),
            );
          })?.toList(),
        ],
        onPressed: (int index) {
          setState(() {
            for (int i = 0; i < _selections.length; i++) {
              if (index == i) {
                _selections[i] = true;
              } else {
                _selections[i] = false;
              }
            }
          });
        },
      ),
    );
  }
}

【讨论】：

【解决方案2】：

尝试将类型<Widget> 提供给map：

      children: <Widget>[
        ...(widget.list).map<Widget>((str) {
          return Padding(
            padding: EdgeInsets.symmetric(horizontal: 12),
            child: Text(
              str,
              style: TextStyle(fontSize: 18),
            ),
          );
        }).toList(),

【讨论】：

对不起，我弄错了，异常是在我调用类的地方捕获的

【解决方案3】：

错误可能发生在文本小部件中。因为 str 不是 String 类型，它是 Object 类型。

 children: <Widget>[
        ...(widget.list).map<Widget>((str) {
          return Padding(
            padding: EdgeInsets.symmetric(horizontal: 12),
            child: Text(
              str,
              style: TextStyle(fontSize: 18),
            ),
          );
        }).toList(),

【讨论】：

对不起，我弄错了，异常是在我调用类的地方捕获的