Removing Notification.visitAncestor

Summary

Notifications are more efficient by traversing only ancestors that are notification listeners.

Context

The notification API traversed the element tree in order to locate a notification receiver. This led to some unfortunate performance characteristics:

  • If there was no receiver for a given notification type, the entire element tree above the notification dispatch point would be traversed and type checked.
  • For multiple notifications in a given frame (which is common for scroll views) we ended up traversing the element tree multiple times.

If there were multiple or nested scroll views on a given page, the situation was worsened significantly - each scroll view would dispatch multiple notifications per frame. For example, in the Dart/Flutter Devtools flamegraph page, we found that about 30% of CPU time was spent dispatching notifications.

In order to reduce the cost of dispatching notifications, we have changed notification dispatch so that it only visits ancestors that are notification listeners, reducing the number of elements visited per frame.

However, the old notification system exposed the fact that it traversed each element as part of its API via Notification.visitAncestor. This method is no longer supported as we no longer visit all ancestor elements.

Description of change

Notification.visitAncestor has been removed. Any classes that extend Notification should no longer override this method.

** If you don’t implement a custom Notification that overrides Notification.visitAncestor, then no changes are required. **

Migration guide

If you have a subclass of Notification that overrides Notification.visitAncestor, then you must either delete the override or opt-into old style notification dispatch with the following code.

Code before migration:

import 'package:flutter/widgets.dart';

class MyNotification extends Notification {

  @override
  bool visitAncestor(Element element) {
    print('Visiting $element');
    return super.visitAncestor(element);
  }
}

void methodThatSendsNotification(BuildContext? context) {
  MyNotification().dispatch(context);
}

Code after migration:

import 'package:flutter/widgets.dart';

class MyNotification extends Notification {

  bool visitAncestor(Element element) {
    print('Visiting $element');
    if (element is ProxyElement) {
      final Widget widget = element.widget;
      if (widget is NotificationListener<MyNotification>) {
        return widget.onNotification?.call(notification) ?? true;
      }
    }
    return true;
  }
}

void methodThatSendsNotification(BuildContext? context) {
  context?.visitAncestor(MyNotification().visitAncestor);
}

Note that this performs poorly compared to the new default behavior of Notification.dispatch.

Timeline

Landed in version: 2.12.0-4.1 In stable release: not yet

References

API documentation:

Relevant issues:

Relevant PRs: