Flutter-notification和notificatioLisener

Notification源码：

abstract class Notification {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const Notification();

  /// Applied to each ancestor of the [dispatch] target.
  ///
  /// The [Notification] class implementation of this method dispatches the
  /// given [Notification] to each ancestor [NotificationListener] widget.
  ///
  /// Subclasses can override this to apply additional filtering or to update
  /// the notification as it is bubbled (for example, increasing a `depth` field
  /// for each ancestor of a particular type).
  @protected
  @mustCallSuper
  bool visitAncestor(Element element) {
    if (element is StatelessElement) {
      final StatelessWidget widget = element.widget;
      if (widget is NotificationListener<Notification>) {
        if (widget._dispatch(this, element)) // that function checks the type dynamically
          return false;
      }
    }
    return true;
  }

  /// Start bubbling this notification at the given build context.
  ///
  /// The notification will be delivered to any [NotificationListener] widgets
  /// with the appropriate type parameters that are ancestors of the given
  /// [BuildContext]. If the [BuildContext] is null, the notification is not
  /// dispatched.
  void dispatch(BuildContext target) {
    // The `target` may be null if the subtree the notification is supposed to be
    // dispatched in is in the process of being disposed.
    target?.visitAncestorElements(visitAncestor);
  }

  @override
  String toString() {
    final List<String> description = <String>[];
    debugFillDescription(description);
    return '$runtimeType(${description.join(", ")})';
  }

  /// Add additional information to the given description for use by [toString].
  ///
  /// This method makes it easier for subclasses to coordinate to provide a
  /// high-quality [toString] implementation. The [toString] implementation on
  /// the [Notification] base class calls [debugFillDescription] to collect
  /// useful information from subclasses to incorporate into its return value.
  ///
  /// If you override this, make sure to start your method with a call to
  /// `super.debugFillDescription(description)`.
  @protected
  @mustCallSuper
  void debugFillDescription(List<String> description) { }
}

其中dispatch(context)是分发通知的，它会从参数context位置处开始沿着widget树向上冒泡通知。

也可以自定义Notification，比如class MyNotification extends Notification( final String msg; MyNotification(this.msg);)

NotificationListener是用来监听Notification的，源码如下：

/// A widget that listens for [Notification]s bubbling up the tree.
///
/// Notifications will trigger the [onNotification] callback only if their
/// [runtimeType] is a subtype of `T`.
///
/// To dispatch notifications, use the [Notification.dispatch] method.
class NotificationListener<T extends Notification> extends StatelessWidget {
  /// Creates a widget that listens for notifications.
  const NotificationListener({
    Key key,
    @required this.child,
    this.onNotification,
  }) : super(key: key);

  /// The widget directly below this widget in the tree.
  ///
  /// This is not necessarily the widget that dispatched the notification.
  ///
  /// {@macro flutter.widgets.child}
  final Widget child;

  /// Called when a notification of the appropriate type arrives at this
  /// location in the tree.
  ///
  /// Return true to cancel the notification bubbling. Return false (or null) to
  /// allow the notification to continue to be dispatched to further ancestors.
  ///
  /// The notification's [Notification.visitAncestor] method is called for each
  /// ancestor, and invokes this callback as appropriate.
  ///
  /// Notifications vary in terms of when they are dispatched. There are two
  /// main possibilities: dispatch between frames, and dispatch during layout.
  ///
  /// For notifications that dispatch during layout, such as those that inherit
  /// from [LayoutChangedNotification], it is too late to call [State.setState]
  /// in response to the notification (as layout is currently happening in a
  /// descendant, by definition, since notifications bubble up the tree). For
  /// widgets that depend on layout, consider a [LayoutBuilder] instead.
  final NotificationListenerCallback<T> onNotification;

  bool _dispatch(Notification notification, Element element) {
    if (onNotification != null && notification is T) {
      final bool result = onNotification(notification);
      return result == true; // so that null and false have the same effect
    }
    return false;
  }

  @override
  Widget build(BuildContext context) => child;
}

其中属性child是子widget，onNotification是监听到了通知产生的回调。

一个例子：

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
void main() => runApp(BuilderApp());

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

class NotificationRoute extends StatefulWidget {
  @override
  NotificationRouteState createState() {
    return new NotificationRouteState();
  }
}

class NotificationRouteState extends State<NotificationRoute> {
  String _msg="";
  @override
  Widget build(BuildContext context) {
    //监听通知
    return NotificationListener<MyNotification>(
      onNotification: (notification) {
        setState(() {
          _msg+=notification.msg+"  ";
        });
        return true;
      },
      child: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
//          RaisedButton(
//           onPressed: () => MyNotification("Hi").dispatch(context),
//           child: Text("Send Notification"),
//          ),
            Builder(
              builder: (context) {
                return RaisedButton(
                  //按钮点击时分发通知
                  onPressed: () => MyNotification("Hi").dispatch(context),
                  child: Text("Send Notification"),
                );
              },
            ),
            Text(_msg)
          ],
        ),
      ),
    );
  }
}

class MyNotification extends Notification {
  MyNotification(this.msg);
  final String msg;
}

onNotification的方法需要返回bool值，返回true，表示当前事件不在向上传递，false表示继续向上传递