flutter - Stateful Widget 子项未更新

Question

有两个有状态的小部件。MyHomePage 的状态包含计数器。MyHomePage 将内容包装在第二个有状态小部件 SubPage 中。SubPage 有一个子部件，其中包含来自 MyHomePage 的数据。

为了澄清这个问题，子页面内的第一个文本小部件不会在计数器更改时更新。

SubPage 之外的文本小部件按预期递增。

如果我们想要更新内部有状态小部件的内容，我们必须做什么？

我们必须在那里使用有状态的小部件。在实际应用中，这个小部件有一个真实的用例。

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

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> {
  int _counter = 0;

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

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: Column(
        children: <Widget>[
          SubPage(
            child: new Center(
              child: new Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  new Text(
                    'You have pushed the button this many times:',
                  ),
                  new Text(
                    '$_counter',
                    style: Theme.of(context).textTheme.display1,
                  ),
                ],
              ),
            ),
          ),
          new Text(
            '$_counter',
            style: Theme.of(context).textTheme.display1,
          ),
        ],
      ),

      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class SubPage extends StatefulWidget {
  SubPage({Key key, this.child}) : super(key: key);

  final Widget child;

  @override
  SubPageState createState() => new SubPageState(child);
}

class SubPageState extends State<SubPage> {
  final Widget child;

  SubPageState(this.child);

  @override
  Widget build(BuildContext context) {
    print("subpage build");
    return this.child;
  }
}

Answer 1

您不必将 child 设置为状态字段。实际上这是导致此错误的原因。这里是工作代码

class SubPage extends StatefulWidget {
  SubPage({Key key, this.child}) : super(key: key);

  final Widget child;

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

class SubPageState extends State<SubPage> {
  @override
  Widget build(BuildContext context) {
    return widget.child;
  }
}