KVO within Swift Extension

Question

How would I go about getting my extension to observe a property on the class it's extending while also being able to remove it at a later time?

For example:

I have a UIView, and I'd like to listen for frame changes within my UIView extension. When these changes occur I need to run some code that alters the views appearance based on the new frame.

The problem I'm having is, that while I can set up an observer, I can't figure out a way to remove it. There's no deinit() and I'd like the observer to hang around for the lifecycle of the UIView.

I'd rather not have to put this removal responsibility on the developer, and I don't want to subclass.

It doesn't even have to be KVO, if there's a better way I'd love to know about it.

Answer 1

The usual solution to this is to use an associated object as a deallocation spy. Associated objects are released when their attached object is deallocated, so you can run code in their deinit that you want to fire when the attached object goes away. That said, doing this really well in a multi-threaded world is a little tricky to say the least. I don't recommend building your own. Instead, I suggest PMKVObserver which will handle this for you. Even if you do choose to build your own, you should study how PMKVObserver does its magic so you're not overly naïve in your implementation.

Doing this sloppily (not worrying about multi-threaded race conditions) is not too hard in Swift, but to do it really well is better done in ObjC and bridged to Swift. Again, consult the code for the tricky corner cases.

Answer 2

While

Extensions can add new convenience initializers to a class, but they cannot add new designated initializers or deinitializers to a class. Designated initializers and deinitializers must always be provided by the original class implementation.

So I think you cannot handle it in any nice way without subclassing.