Question

嘿，我一直在Flutter中遇到这个问题，我不知道该如何处理。我试图浏览一些教程，然后我想这样做。

我想做的是在onPressed事件._increment上从FloatingActionButton调用，以便我可以递增_counter。但是，然后我得到那个错误。我想念什么？

void main() {
  runApp(MaterialApp(
    title: 'Flutter Tutorial',
    home: TutorialHome(),
  ));
}

class TutorialHome extends StatelessWidget {


  @override
  Widget build(BuildContext context) {
    // Scaffold is a layout for the major Material Components.
    SystemChrome.setEnabledSystemUIOverlays([]); //Hide Status Bar
    return Scaffold(
      appBar: AppBar(
        leading: IconButton(
          icon: Icon(Icons.menu),
          tooltip: 'Navigation menu',
          onPressed: null,
        ),
        title: Text('Example title'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.search),
            tooltip: 'Search',
            onPressed: null,
          ),
        ],
      ),
      // body is the majority of the screen.
      body: Center(
        child: Container(
          height: 100,
          width: 100,
          child: Column(
            children: <Widget>[
              Counter(),
            ]
          )
        )
      ),
      floatingActionButton: FloatingActionButton(
        tooltip: 'Add', // used by assistive technologies
        child: Icon(Icons.add),
        onPressed: _CounterState()._increment,
      ),
    );
  }
}


//class CounterIncrementor extends StatelessWidget {
//  CounterIncrementor({this.onPressed});
//
//  final VoidCallback onPressed;
//
//  @override
//  Widget build(BuildContext context) {
//    return RaisedButton(
//      onPressed: onPressed,
//      child: Text('Increment'),
//    );
//  }
//}

class Counter extends StatefulWidget {
  // This class is the configuration for the state. It holds the
  // values (in this case nothing) provided by the parent and used by the build
  // method of the State. Fields in a Widget subclass are always marked "final".

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

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    setState(() {
      // This call to setState tells the Flutter framework that
      // something has changed in this State, which causes it to rerun
      // the build method below so that the display can reflect the
      // updated values. If we changed _counter without calling
      // setState(), then the build method would not be called again,
      // and so nothing would appear to happen.
      print("Increasing counter value\n");
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance
    // as done by the _increment method above.
    // The Flutter framework has been optimized to make rerunning
    // build methods fast, so that you can just rebuild anything that
    // needs updating rather than having to individually change
    // instances of widgets.
    return Column(
      children: <Widget>[
        RaisedButton(
          onPressed: _increment,
          child: Text('Increment'),
        ),
        Text('Count: $_counter'),
      ],
    );
  }
}

这是错误：

 ************************ ERROR *************************
══╡ EXCEPTION CAUGHT BY GESTURE ╞═══════════════════════════════════════════════════════════════════
I/flutter (20840): The following assertion was thrown while handling a gesture:
I/flutter (20840): setState() called in constructor: _CounterState#452ba(lifecycle state: created, no widget, not
I/flutter (20840): mounted)
I/flutter (20840): This happens when you call setState() on a State object for a widget that hasn't been inserted into
I/flutter (20840): the widget tree yet. It is not necessary to call setState() in the constructor, since the state is
I/flutter (20840): already assumed to be dirty when it is initially created.

Answer 1

因此，检查代码后，我看到您有2个按钮：

FloatingActionButton

和

RaisedButton

两者之间的区别是

RaisedButton 会从 _CounterState 的实例对象调用方法 _increment ，并且该对象已被挂载（此_CounterState对象创建RaisedButton ）
FloatingActionButton 会调用该方法来创建一个新的 _CounterState（）对象，然后在该对象增量计数器上创建该对象。新状态与显示的内容无关。这只是一个新对象（未插入小部件树中）。

您在TutorialHome中实例化一个计数器

//...
child: Column(
                  children: <Widget>[
                    Counter(), 
                  ]
              )

//...

此Counter负责为其自身创建状态（_CounterState对象）。

根据要求更新：

这不是干净的代码，仅用于演示。

下面是一个示例，该示例如何使用流控制器（您的计数器和从其他位置按下的按钮）来设置两个对象之间的信息交换。

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Flutter Tutorial',
    home: TutorialHome(),
  ));
}

class TutorialHome extends StatelessWidget {

  StreamController<void> buttonPressStream = StreamController<bool>.broadcast();

  @override
  Widget build(BuildContext context) {
    // Scaffold is a layout for the major Material Components.

    return Scaffold(
      appBar: AppBar(
        leading: IconButton(
          icon: Icon(Icons.menu),
          tooltip: 'Navigation menu',
          onPressed: null,
        ),
        title: Text('Example title'),
        actions: <Widget>[
          IconButton(
            icon: Icon(Icons.search),
            tooltip: 'Search',
            onPressed: null,
          ),
        ],
      ),
      // body is the majority of the screen.
      body: Center(
          child: Container(
              height: 100,
              width: 100,
              child: Column(
                  children: <Widget>[
                    Counter(buttonPressStream),
                  ]
              )
          )
      ),
      floatingActionButton: FloatingActionButton(
        tooltip: 'Add', // used by assistive technologies
        child: Icon(Icons.add),
        onPressed: () => buttonPressStream.add(null),
      ),
    );
  }
}


//class CounterIncrementor extends StatelessWidget {
//  CounterIncrementor({this.onPressed});
//
//  final VoidCallback onPressed;
//
//  @override
//  Widget build(BuildContext context) {
//    return RaisedButton(
//      onPressed: onPressed,
//      child: Text('Increment'),
//    );
//  }
//}

class Counter extends StatefulWidget {
  // This class is the configuration for the state. It holds the
  // values (in this case nothing) provided by the parent and used by the build
  // method of the State. Fields in a Widget subclass are always marked "final".

  final StreamController<void> buttonPressStream;

  const Counter(this.buttonPressStream);

  @override
  _CounterState createState() => _CounterState(buttonPressStream);
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  StreamController<void> buttonPressStream;
  _CounterState(this.buttonPressStream);

  void _increment() {
    setState(() {
      // This call to setState tells the Flutter framework that
      // something has changed in this State, which causes it to rerun
      // the build method below so that the display can reflect the
      // updated values. If we changed _counter without calling
      // setState(), then the build method would not be called again,
      // and so nothing would appear to happen.
      print("Increasing counter value\n");
      _counter++;
    });
  }

  @override
  void initState() {
    super.initState();
    buttonPressStream.stream.listen( (_) {
      setState(() {});
    });
  }



  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance
    // as done by the _increment method above.
    // The Flutter framework has been optimized to make rerunning
    // build methods fast, so that you can just rebuild anything that
    // needs updating rather than having to individually change
    // instances of widgets.
    return Column(
      children: <Widget>[
        RaisedButton(
          onPressed: _increment,
          child: Text('Increment'),
        ),
        Text('Count: $_counter'),
      ],
    );
  }
}

在另一个类Flutter上的事件上调用函数

1 个答案: