How to send notifications asynchronously using NotificationQueue

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

Any notifications posted using NotificationCenter are delivered synchronously, which means all observers get notified simultaneously and execute all their code before control gets passed back to the the poster of the notification.

While that’s often what you want, sometimes it can be problematic because processing repeated notifications during busy periods can slow your app down. Fortunately, Apple gives us an alternative in the shape of NotificationQueue: an asynchronous system that queues up notifications at different urgencies, and can even coalesce similar messages to avoid repetition.

You can try it out using this code:

let notification = Notification(name: Notification.Name("MyValueChanged"))
NotificationQueue.default.enqueue(notification, postingStyle: .whenIdle, coalesceMask: .none, forModes: nil)

That will enqueue the “MyValueChanged” notification to be delivered when your app is idle, and without coalescing. You can also use .asap for your posting style to deliver in the next run loop, and .now, which will cause the notification to be delivered synchronously.

The reason the .now posting style is important is because of the ability of NotificationQueue to coalesce notifications – i.e., join them together. If you specify .onName for the coalesceMask property it will automatically merge any notifications of the same name, which stops observers being overloaded by repeated notifications.