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Implement NSNotificationCenter using Swift!

If NSNotificationCenter didn’t exist, how might you implement it?

You have to think quite deeply about NSNotificationCenter, what it does and how it might work.


NSNotificationCenter: A notification dispatch mechanism that enables the broadcast of information to registered observers.

Observers: An object that subscribes to an observable and responds the the message(s) that are send.

Observable: In this case NSNotification, and object than can be subscribed to.

Subscribe: An observer subscribes to an observable in order to register to be sent messages.

The problem

NSNotification broadcasts information to the observers when they are subscribed to it. In all probability, NSNotification is probably a Singleton. When you add an observer, it gets moved into what is probably an array of observers (delegate).

When you remove an observer, it is removed from the array of observers.

When you post a notification, NSNotification loops through the array of observers and initiates the callback method assigned with each delegate in the array.

The Approach

Since we are using Swift, a protocol-based approach will be helpful to us.

The subscriber class will use a simple callback mechanism for receiving the messages from NSNotificationCenter where we declare an Observer:

which is then involved in the following subscriber class:

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