get widget observer for WidgetsBindingObserver

Question

I'm trying to add WidgetsBindingObserver accoidring to (https://medium.com/@phongyewtong/best-method-to-restart-reload-your-flutter-app-on-startup-2f787cecc6d6)

class MyApp extends StatefulWidget with WidgetsBindingObserver {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);   // The Error is right here
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialHomeApp();
  }

The Erros says The argument type '_MyAppState' can't be assigned to the parameter type 'WidgetsBindingObserver'.dartargument_type_not_assignable

Answer 1

您可以with WidgetsBindingObserver从MyApp转到_MyAppState
您可以在这里参考官方文档https://api.flutter.dev/flutter/widgets/WidgetsBindingObserver-class.html
代码片段

class MyApp extends StatefulWidget  {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

完整示例代码

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      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>[
            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),
      ),
    );
  }
}

Answer 2

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> with WidgetsBindingObserver {
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);   
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);

    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialHomeApp();
  }

在您的代码中更改上述内容。

WidgetsBindingObserver 是向 Widgets 层绑定注册的类的接口。它必须如上所示进行声明。请对您的代码进行必要的更改，如上所示。