In Swift 4, how do I remove a block-based KVO observer?

swift key-value-observing swift4
Guilherme picture Guilherme · Oct 5, 2017 · Viewed 11.3k times · Source

If I store an observer like this:

let observer: NSKeyValueObservation = foo.observe(\.value, options: [.new]) { (foo, change) in
    print(change.newValue)
}

How do I remove/disable/cleanup observer once I no longer need it?

My foo instance does not have any remove-like method that receives an NSKeyValueObservation instance, the observer itself doesn't have any remove-like either.

Answer

matt picture matt · Oct 5, 2017

In iOS 11, you don't have to. Just let the observer go out of scope. There is no penalty any longer for letting an observer die before the observed or for letting the observed die before the observer, so you have no actual work to do.

On the other hand, if you really want to unregister the observer, remove it from whatever is retaining it, or tell it to invalidate. (Something must be retaining it, because if you don't persist the observer, it will die and your observer function will never be called.)

(You say "if I store an observer like this", but the way you are storing it, with let, is a somewhat silly way to store the observer. It would be better to put it in a Set from which you can remove it later, or at least store it in a Optional var that you can later set to nil.)

Related questions

Swift 4 approach for observeValue(forKeyPath:...)

I've been trying to find an example, but what I've seen doesn't work in my case. What would be the equivalent of the following code: object.addObserver(self, forKeyPath: "keyPath", options: [.new], context: nil) override public func observeValue( forKeyPath keyPath: …

Read More
Is key-value observation (KVO) available in Swift?

If so, are there any key differences that weren't otherwise present when using key-value observation in Objective-C?

Read More
Adding observer for KVO without pointers using Swift

In Objective-C, I would normally use something like this: static NSString *kViewTransformChanged = @"view transform changed"; // or static const void *kViewTransformChanged = &kViewTransformChanged; [clearContentView addObserver:self forKeyPath:@"transform" options:NSKeyValueObservingOptionNew context:&kViewTransformChanged]; I have two overloaded methods to choose from …

Read More