Flutter - 错误：参数类型'String/*1*/'不能分配给参数类型'String/*2*/'

Posted 2023-03-27

【中文标题】Flutter - 错误：参数类型\'String/*1*/\'不能分配给参数类型\'String/*2*/\'

【英文标题】：Flutter - Error: The argument type 'String/*1*/' can't be assigned to the parameter type 'String/*2*/'Flutter - 错误：参数类型'String/*1*/'不能分配给参数类型'String/*2*/'

【发布时间】：2020-02-20 04:23:04

【问题描述】：

String selectedValue;

Widget _dropDownMenu<String>(String title, List<String> list, List<String> strList, Function(String) onChange) 

    return Container(
      decoration: containerDecoration(border: Border.all(color: Colors.grey)),
      width: MediaQuery.of(context).size.width - 20,
      child: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          SizedBox(width: 10),
          Container(
            width: MediaQuery.of(context).size.width - 60,
            child: DropdownButtonHideUnderline(
              child: DropdownButton<String>(
                value: selectedValue,
                hint: Text(title),
                items: List.generate(list.length, (index) 
                  return DropdownMenuItem<String>(
                    value: list[index],
                    child: Text(strList[index], key: UniqueKey()),
                  );
                ),
                onChanged: (value) 
                  setState(() 
                    selectedValue = value;
                    onChange(value);
                  );
                
              )
            ),
          ),
          SizedBox(width: 10)
        ] 
      )
    );
  

这段代码给我这条信息

lib/src/ui/Filter_page.dart:136:24：错误：参数类型 'String/1/' 不能分配给参数类型 'String/2/'。 - 'String/1/' 来自 'dart:core'。 - 'String/2/' 来自 'package:fundaqah_flutter/src/ui/Filter_page.dart' （'lib/src/ui/Filter_page.dart'）。尝试更改类型 参数，或将参数转换为 'String/2/'。 值：选定值，

和.toString() 不工作

【问题讨论】：

编辑：基于docs 中的value arg onChanged 明确键入为字符串。

String ，我添加了它也没有工作

添加了什么？类型？

fundaqah_flutter 的作者创建了一个名为String（在Filter_page.dart 中）的新类，它与“正常”String 发生冲突。更改类名或导入带有前缀的包。

抱歉打扰，您是否将类型添加到 onChanged 参数中？如果是这样，您可以更新您的代码 sn-p 并可能显示您的导入。理查兹的评论很有趣……

【参考方案1】：

请删除<String>，错误将消失

 _dropDownMenu<String>(String title

到

Widget _dropDownMenu(String title

为了测试，我更新了一些Container宽度，你可以看到完整的测试代码

完整的测试代码

import 'package:flutter/material.dart';

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(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  


class MyHomePage extends StatefulWidget 
  MyHomePage(Key key, this.title) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();


class _MyHomePageState extends State<MyHomePage> 
  int _counter = 0;

  void _incrementCounter() 
    setState(() 
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    );
  

  String selectedValue;

  Widget _dropDownMenu(
      String title,
      List<String> list,
      List<String> strList,
      Function(String) onChange) 
    return Container(
        //decoration: containerDecoration(border: Border.all(color: Colors.grey)),
        //height: 100,
        //width: MediaQuery.of(context).size.width - 20,
        child:
            Row(mainAxisAlignment: MainAxisAlignment.center, children: <Widget>[
          SizedBox(width: 10),
          Container(
            //width: MediaQuery.of(context).size.width - 60,
            child: DropdownButtonHideUnderline(
                child: DropdownButton<String>(
                    value: selectedValue,
                    hint: Text(title),
                    items: List.generate(list.length, (index) 
                      return DropdownMenuItem<String>(
                        value: list[index],
                        child: Text(strList[index], key: UniqueKey()),
                      );
                    ),
                    onChanged: (String value) 
                      setState(() 
                        selectedValue = value;
                        onChange(value);
                      );
                    )),
          ),
          SizedBox(width: 10)
        ]));
  


  @override
  Widget build(BuildContext context) 
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            _dropDownMenu(
                title: "",
                list: ["a", "b"],
                strList: ["a", "b"],
                onChange: (value) => print("CLICKED")
            ),
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );