将变量从有状态小部件传递到状态

Question

我尝试传递一个变量“int counter = 0;”在状态小部件中，但我不知道为什么我无法检索它，应用程序将我返回为“Null”，但我将所有变量都传递给了构造函数，（MyHomePage 类扩展了 StatefulWidget）。

import 'package:flutter/material.dart';

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, this.counter}) : super(key: key);
  final String title;
  int counter= 0;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    setState(() {
      widget.counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${widget.counter}',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

感谢您的帮助

Answer 1

Widgets（包括StatefulWidgets）应该始终是不可变的，不应该在其生命周期内改变。事实上，如果您尝试将非final 变量添加到StatefulWidget，您应该会看到警告，因为它是Widget 的子类型，具有@immutable 注释。

很少有任何逻辑实际上发生在StatefulWidget 子类中。相反，所有状态和相关逻辑都应该在 State 类中。

您应该像这样将counter 移动到状态类中：

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

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

class _MyHomePageState extends State<MyHomePage> {
  int counter = 0;

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

  // the rest is the same, except using counter instead of widget.counter
}

Answer 2

让我们先分析一下代码

 home: MyHomePage(title: 'Flutter Demo Home Page'),
 // You did not pass integer to the counter field, dart pass null to counter field


class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title, this.counter
/* here null will be initialized to the counter field */
}) : super(key: key);
  final String title;
  int counter= 0; // now it is null
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

因此，您会得到 null

如何正确传入你的场景：

import 'package:flutter/material.dart';

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, this.counter=0}) : super(key: key);
  final String title;
  int counter;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    setState(() {
      widget.counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${widget.counter}',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}