Flutter - 使用通知向其他小部件发送消息

Question

I would like to use a flutter Notification in a StatefulWidget to communicate with another StatefulWidget widget. I have posted an example below that I feel should be working but it does not. When you click the "+" icon button, it should send out the notification to the subPage widget. Currently, when you click the button, nothing seems to happen. I would expect the onTitlePush() function to be executed. This is my first time trying Notifications and I must have something set up incorrectly. I am using this in a larger app but the code below is just a sample of the implementation. Could you let me know where I went wrong?

import 'package:flutter/material.dart';

void main() => runApp(TestApp());

class TestApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Notificaton Test',
      home: MainPage(),
    );
  }
}

class MyNotification extends Notification {
  final String title;

  const MyNotification({this.title});
}

class MainPage extends StatefulWidget {
  @override
  MainPageState createState() {
    return new MainPageState();
  }
}

class MainPageState extends State<MainPage> {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Basic AppBar'),
          actions: <Widget>[
            // action button
            IconButton(
              icon: new Icon(Icons.add),
              onPressed: () {
                MyNotification(title: "Updated Text!")..dispatch(context);
              },
            ),
            // action button
          ],
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: new SubPage(),
        ),
      ),
    );
  }
}

class SubPage extends StatefulWidget {
  @override
  SubPageState createState() {
    return new SubPageState();
  }
}

class SubPageState extends State<SubPage> {
  String _text = "Click the Plus Icon";
  @override
  Widget build(BuildContext context) {
    return NotificationListener<MyNotification>(
      onNotification: onTitlePush,
      child: new Center(
          child: new Text(_text, style: new TextStyle(fontSize: 40.0))
      ),
    );
  }
  bool onTitlePush(MyNotification notification) {
    print("New item ${notification.title}");

    setState(() { _text = notification.title; });
    return true;
  }
}

Answer 1

Notifications are the opposite of BuildContext. With notifications, it is children that sends value to their parents.

Notifications are not global . Widgets that are not parent of the dispatcher cannot listen to the said notification.

Therefore, what you are trying to achieve in fact cannot be done with notifications.

There are a few alternatives:

• Use Listenable such as ValueListenable How to set state from another widget?
• Use Streams Trigger a function from a widget to a State object
• Refactor your Widget tree so that listeners are parents of the dispatcher