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How to post messages using NotificationCenter

Swift version: 5.0

Paul Hudson    @twostraws   

iOS notifications are a simple and powerful way to send data in a loosely coupled way. That is, the sender of a notification doesn't have to care about who (if anyone) receives the notification, it just posts it out there to the rest of the app and it could be picked up by lots of things or nothing depending on your app's state.

As a basic example, you might want various parts of your app to do some work when the user logs in – you might want some views to refresh, you might want a database to update itself, and so on. To do this, just post a notification name like this:

let nc = NotificationCenter.default
nc.post(name: Notification.Name("UserLoggedIn"), object: nil)

Note: it is preferable, for type safety, to define your notification names as static strings that belong to a class or struct or other global form so that you don't make a typo and introduce bugs.

To register to catch a notification being posted, use this:

nc.addObserver(self, selector: #selector(userLoggedIn), name: Notification.Name("UserLoggedIn"), object: nil)

That will call a userLoggedIn() method when your notification is posted.

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Available from iOS 2.0

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