如何将数据从子窗口小部件传递到其父窗口小部件

问题描述：

我有下面的自定义窗口小部件，它们执行 Switch 并读取其状态（是/否）

I've the below custom widget that make a Switch and reads its status (true/false)

然后将其添加到我的主应用程序小部件（父级）中，如何使父级知道开关的值！

Then I add this one to my main app widget (parent), how can I make the parent knows the value of the switch!

import 'package:flutter/material.dart';

class Switchy extends StatefulWidget{
  Switchy({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new _SwitchyState();
  }

class _SwitchyState extends State<Switchy> {
  var myvalue = true;

  void onchange(bool value) {
    setState(() {
      this.myvalue = value;      // I need the parent to receive this one!
      print('value is: $value');
    });
  }

  @override
  Widget build(BuildContext context) {
    return             
      new Card(
      child: new Container(
        child: new Row(
          mainAxisAlignment: MainAxisAlignment.end,
          children: <Widget>[
            new Text("Enable/Disable the app in the background",
              textAlign: TextAlign.left,
              textDirection: TextDirection.ltr,),
            new Switch(value: myvalue, onChanged: (bool value) => onchange(value)),
          ],
        ),
      ),
    );
  }
}

在 main.dart中（父）文件，我从以下开始：

In the main.dart (parent) file, I started with this:

import 'widgets.dart';
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.deepOrange,
      ),
      home: new MyHomePage(title: 'My App settup'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  Widget e = new Switchy();
  //...

}

答

第一种可能性是向您的孩子传递回调，第二种可能性是对有状态窗口小部件使用 of 模式。见下文。

The first possibility is to pass a callback into your child, and the second is to use the of pattern for your stateful widget. See below.

import 'package:flutter/material.dart';

class MyStatefulWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new MyStatefulWidgetState();

  static MyStatefulWidgetState of(BuildContext context) {
    final MyStatefulWidgetState navigator =
        context.ancestorStateOfType(const TypeMatcher<MyStatefulWidgetState>());

    assert(() {
      if (navigator == null) {
        throw new FlutterError(
            'MyStatefulWidgetState operation requested with a context that does '
            'not include a MyStatefulWidget.');
      }
      return true;
    }());

    return navigator;
  }
}

class MyStatefulWidgetState extends State<MyStatefulWidget> {
  String _string = "Not set yet";

  set string(String value) => setState(() => _string = value);

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        new Text(_string),
        new MyChildClass(callback: (val) => setState(() => _string = val))
      ],
    );
  }
}

typedef void StringCallback(String val);

class MyChildClass extends StatelessWidget {
  final StringCallback callback;

  MyChildClass({this.callback});

  @override
  Widget build(BuildContext context) {
    return new Column(
      children: <Widget>[
        new FlatButton(
          onPressed: () {
            callback("String from method 1");
          },
          child: new Text("Method 1"),
        ),
        new FlatButton(
          onPressed: () {
            MyStatefulWidget.of(context).string = "String from method 2";
          },
          child: new Text("Method 2"),
        )
      ],
    );
  }
}

void main() => runApp(
      new MaterialApp(
        builder: (context, child) => new SafeArea(child: new Material(color: Colors.white, child: child)),
        home: new MyStatefulWidget(),
      ),
    );

还有一种替代方法是使用InheritedWidget而不是StatefulWidget；如果您希望您的子窗口小部件在父窗口小部件的数据发生更改并且父级不是直接父级的情况下重建时特别有用。请参见继承的窗口小部件文档

There is also the alternative of using an InheritedWidget instead of a StatefulWidget; this is particularly useful if you want your child widgets to rebuild if the parent widget's data changes and the parent isn't a direct parent. See the inherited widget documentation

如何将数据从子窗口小部件传递到其父窗口小部件

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