如何将数据从子小部件传递到其父小部件 [英] How to pass data from child widget to its parent

问题描述

我有下面的自定义小部件，可以制作 Switch 并读取其状态(真/假)

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

然后我将这个添加到我的主应用程序小部件(父级)中，如何让父级知道开关的值！

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(父)文件中，我从这个开始:

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();
  //...

}

推荐答案

第一种可能性是将回调传递给您的孩子，第二种可能性是对您的有状态小部件使用 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();

  // note: updated as context.ancestorStateOfType is now deprecated
  static MyStatefulWidgetState of(BuildContext context) =>
    context.findAncestorStateOfType<MyStatefulWidgetState>();
}

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(),
  ),
);

还有一种选择是使用 InheritedWidget 而不是 StatefulWidget；如果您希望您的子小部件在父小部件的数据更改并且父小部件不是直接父小部件时重建，这将特别有用.请参阅继承的小部件文档

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

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