Flutter 入門 - 狀態管理

import 'package:flutter/material.dart';
import 'package:flutter_redux_example/screens/MyHomePage.dart';
void main() => runApp(new MyApp());
class MyApp extends StatefulWidget {
  
  @override
  _MyAppState createState() => new _MyAppState();
}
class _MyAppState extends State<MyApp> {
  int counter;
  @override
  void initState() {
    super.initState();
    counter = counter ?? 0;
  }
  void _decrementCounter(_) {
    setState(() {
      counter--;
      print('decrement: $counter');
    });
  }
  
  void _incrementCounter(_) {
    setState(() {
      counter++;
      print('increment: $counter');
    });
  }
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(
        title: 'My Home Page',
        counter: counter,
        decrementCounter: _decrementCounter,
        incrementCounter: _incrementCounter,
      ),
    );
  }
}
複製代碼

如上所示， _decrementCounter() 和 _incrementCounter()分別是遞增和遞減counter的方法，咱們將他們當作MyHomePage的構造函數。

MyHomePage.dart 以下：

import 'package:flutter/material.dart';
import 'package:flutter_redux_example/screens/MySecondPage.dart';
class MyHomePage extends StatefulWidget {
  MyHomePage({
    Key key,
    this.title,
    this.counter,
    this.decrementCounter,
    this.incrementCounter
  }) : super(key: key);
  final String title;
  final int counter;
  final ValueChanged<void> decrementCounter;
  final ValueChanged<void> incrementCounter;
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
  void _onPressed() {
    widget.incrementCounter(null);
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text('You have pushed the button this many times:'),
            new Text(
              widget.counter.toString(),
              style: Theme.of(context).textTheme.display1,
            ),
            new RaisedButton(
              child: new Text('Next Screen'),
              onPressed: () {
                Navigator.push(
                  context,
                  new MaterialPageRoute(
                    builder: (context) => new MySecondPage(
                      widget.decrementCounter,
                      title: 'My Second Page',
                      counter: widget.counter,
                    ),
                  ),
                );
              },
            )
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _onPressed,
        tooltip: 'Increment',
        child: new Icon(Icons.add),
      ),
    );
  }
}
複製代碼

如上所示，咱們將decrementCounter()做爲MySecondPage構造函數的參數，向下傳遞。

MySecondPage.dart 文件以下所示：

import 'package:flutter/material.dart';

class MySecondPage extends StatefulWidget {
  MySecondPage(
    this.decrementCounter, 
    {Key key, this.title, this.counter}
  ): super(key: key);
  final String title;
  final int counter;
  final ValueChanged<void> decrementCounter;
  @override
  _MySecondPageState createState() => new _MySecondPageState();
}
class _MySecondPageState extends State<MySecondPage> {
  void onPressed() {
    widget.decrementCounter(null);
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text('You have pushed the button this many times :'),
            new Text(
              super.widget.counter.toString(),
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
    floatingActionButton: new FloatingActionButton(
      onPressed: onPressed,
      tooltip: 'Decrement',
      child: new Icon(Icons.indeterminate_check_box),
      backgroundColor: Colors.red),
    );
  }
}
複製代碼