Flutter：如何更改另一个 dart 文件中变量的状态？

Question

我目前正在开发一个应用程序；我想更改在另一个 dart 文件中声明的 String 的值，然后将更改后的状态传递给有状态小部件。

IE;

1. 我创建了一个名为“Body.dart”的文件，我在其中声明了一个名为“scale”的字符串，其值最初为“Empty”。

2. 稍后当按下另一个 dart 文件“scale_button”中的按钮时，我想在我的 Body.dart 文件中分配字符串 scale = “Hello”。这样有状态的小部件也会在屏幕上显示相同的内容。

Answer 1

在这种情况下，您可以使用provider（或任何其他状态管理）包。在yaml文件中添加provider: ^4.3.2+4

class HomeApp extends StatefulWidget {
  @override
  _HomeAppState createState() => _HomeAppState();
}

class _HomeAppState extends State<HomeApp> {
  StringProvider _stringProvider;

  @override
  void initState() {
    super.initState();
    _stringProvider = Provider.of<StringProvider>(context, listen: false);
  }

  void updateString() {
    _stringProvider.setString('hai');
  }

  @override
  Widget build(BuildContext context) {
    StringProvider _stringProvider = Provider.of<StringProvider>(context);
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Container(
              child: Text(
                _stringProvider.str,
                style: TextStyle(
                  fontSize: 22,
                ),
              ),
            ),
            RaisedButton(
              onPressed: updateString,
              child: Text('Click'),
            ),
          ],
        ),
      ),
    );
  }
}

// class for storing data(StringProvider.dart)
import 'package:flutter/material.dart';

class StringProvider extends ChangeNotifier { // create a common file for data
  String _str = 'hello';

  String get str => _str;

  void setString(String st) {
    _str = st;
    notifyListeners();
  }
}

Answer 2

当您创建一个新的 Flutter 项目时，计数器的示例代码会向您展示如何执行此操作。查看下一个代码中的 cmets：

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      // here is passing a String to MyHomePage.
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  // And here you can see how to make the widget wait for a variable 
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

Sample Counter App 完整代码

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(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

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

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), 
    );
  }
}