在颤动中设置父状态时，计时器不会重置

Question

这是一个显示倒计时的简单颤振小部件：-

class MyWidget extends StatefulWidget {
@override
_MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
@override
Widget build(BuildContext context) {
    return Column(
    mainAxisSize: MainAxisSize.min,
    children: <Widget>[
        MyTimer(
        count: 25,
        ),
        RaisedButton(
        onPressed: () {
            setState(() {});
        },
        )
    ],
    );
}
}

class MyTimer extends StatefulWidget {
final int count;
MyTimer({this.count});
@override
_MyTimerState createState() => _MyTimerState(count: this.count);
}

class _MyTimerState extends State<MyTimer> {
final int count;
Timer _timer;
int _start;
_MyTimerState({this.count});

@override
void initState() {
    _start = this.count;
    const oneSec = const Duration(seconds: 1);
    _timer = Timer.periodic(
    oneSec,
    (Timer timer) => setState(
        () {
        if (_start < 1) {
            _timer.cancel();
        } else {
            _start = _start - 1;
        }
        },
    ),
    );
}

@override
Widget build(BuildContext context) {
    return Text(this._start.toString());
}
}

当我按下父级上的button 时，它将setState() 并重建父级，这将导致MyTimer 小部件再次构建并因此将计时器再次重置为25。

这就是我的想法，但是按下按钮并在父级上设置状态不会重建MyTimer 小部件？ 这种行为的原因是什么？

编辑：您可以在DartPad 中通过使用颤振创建新的pad 来尝试此代码。

Answer 1

您可以在下面复制粘贴运行完整代码
因为initState只被调用一次，你可以在_MyTimerState中覆盖didUpdateWidget

代码sn-p

@override
  void didUpdateWidget(MyTimer oldWidget) {
    super.didUpdateWidget(oldWidget);
    _start = widget.count;   
  }

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:flutter/foundation.dart';
import 'dart:async';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisSize: MainAxisSize.min,
      children: <Widget>[
        MyTimer(
          count: 25,
        ),
        RaisedButton(
          onPressed: () {
            setState(() {});
          },
        )
      ],
    );
  }
}

class MyTimer extends StatefulWidget {
  final int count;
  MyTimer({this.count});
  @override
  _MyTimerState createState() => _MyTimerState(count: this.count);
}

class _MyTimerState extends State<MyTimer> {
  final int count;
  Timer _timer;
  int _start;
  _MyTimerState({this.count});

  @override
  void initState() {
    print("initSate");
    _start = this.count;
    const oneSec = const Duration(seconds: 1);
    _timer = Timer.periodic(
      oneSec,
      (Timer timer) => setState(
        () {
          if (_start < 1) {
            _timer.cancel();
          } else {
            _start = _start - 1;
          }
        },
      ),
    );
  }

  @override
  void didUpdateWidget(MyTimer oldWidget) {
    super.didUpdateWidget(oldWidget);
    _start = widget.count;
  }

  @override
  Widget build(BuildContext context) {
    return Text(this._start.toString());
  }
}

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

class MyApp extends StatelessWidget {
  @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>[
            MyWidget(),
            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),
      ),
    );
  }
}