Flutter:如何从单独的 Widget 方法调用 onPressed() 中的 setState()

Question

我是这个Flutter的新手

我正在尝试在 onPressed 时更改 FlatButton 文本。我的页面设计在单独的Widget方法中，所以无法在onPressed中添加setState()。

我搜索了很多。但是，找不到。请帮我解决这个问题。

import 'package:flutter/material.dart';

int number = 10;

class SecondRoute extends StatelessWidget {
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Lists'),
        backgroundColor: new Color(0xFF000000),
      ),
      body: DynamicChange(),
    );
  }
}

class DynamicChange extends StatefulWidget {
  @override
  StateDynamic createState() => StateDynamic();
} // Class SecondRoute

class StateDynamic extends State<DynamicChange> {
  Widget build(BuildContext context) {
    return new Scaffold(
      body: thisScreenNeeds(),
    );
  }
}

Widget thisScreenNeeds() {
  return Container(
    margin: EdgeInsets.only(top: 100, left: 50),
      child: FlatButton(
    child: Text(
      number.toString(),
      textAlign: TextAlign.center,
      style: (TextStyle(
          fontWeight: FontWeight.bold, fontSize: 60, color: Colors.green)),
    ),
    onPressed: () {
       // Unable to add, setState(). Throwing error.
    },
  ));
}

尝试 1

onPressed: () {
      // Error: The member 'setState' can only be used within instance members of subclasses
      StateDynamic().setState(() {
        number = number + 1;
   });
},

尝试 2

class StateDynamic extends State<DynamicChange> {

.....
.........

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

.....
.......

}

onPressed: StateDynamic()._incrementCounter,

// Run Time Error: 
   This happens when you call setState() on a State object for a 
   widget that hasn't been inserted into the widget tree yet. 
   It is not necessary to call setState() in the constructor, 
   since the state is already assumed to be dirty when it is 
   initially created.

Answer 1

将方法保留在 State 类中。最好也将 number 变量也保留在该类中。您只能在 State 类中直接setState，这就是原因。

在你的情况下，解决方案

class SecondRoute extends StatelessWidget {
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Lists'),
        backgroundColor: new Color(0xFF000000),
      ),
      body: DynamicChange(),
    );
  }
}

class DynamicChange extends StatefulWidget {
  @override
  StateDynamic createState() => StateDynamic();
} // Class SecondRoute

class StateDynamic extends State<DynamicChange> {
  int number = 10;

  Widget build(BuildContext context) {
    return new Scaffold(
      body: thisScreenNeeds(),
    );
  }

  Widget thisScreenNeeds() {
    return Container(
        margin: EdgeInsets.only(top: 100, left: 50),
        child: FlatButton(
          child: Text(
            number.toString(),
            textAlign: TextAlign.center,
            style: (TextStyle(
                fontWeight: FontWeight.bold,
                fontSize: 60,
                color: Colors.green)),
          ),
          onPressed: () {
            setState(() {
              number++;
            });
          },
        ));
  }
}

如果你需要完全分离小部件(正如我首先理解的那样)，你可以通过构造函数发送 VoidCallback 参数。喜欢:

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  String text = 'Some text';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        children: <Widget>[
          Text(text),
          ButtonWidget(stateSetter),
        ],
      ),
    );
  }

  void stateSetter() {
    setState(() {
      text = 'Changed text';
    });
  }
}

class ButtonWidget extends StatelessWidget {
  final VoidCallback stateSetter;

  ButtonWidget(this.stateSetter);

  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      onPressed: this.stateSetter,
    );
  }
}