_KeyValueCodingAndObserving

This convenience method puts only a tiny bit of polish on Swift’s latest closure-based KVO implementation. Although there aren’t many obvious differences between this method and the one in Foundation, there are a few helpers, which are describe below.

First, it passes a remove condition, until, which is simpy a closure that gets called to determine whether to remove the observation closure. Returning true will remove the observer, otherwise, the observing will continue. Foundation‘s method automatically removes observation when the NSKeyValueObservation is released, but this requires you to save it somewhere in your view controller as a property, thereby cluttering up your view controller with plumbing-only members.

Second, this method attempts to slightly improve the clutter. You do not have to save the observer. @discardableResult allows you to disregard the returned object entirely.

Finally, a typical pattern is to remove observers when deinit is called on the object that owns the observed object. For instance, if you are observing a model object property on your view controller, you will probably want to stop observing when the view controller gets released from memory. Because this is a common pattern, there’s a convenient var available on all subclasses of NSObject named selfDeinits. Simply pass this as a parameter into the until paramaeter, and the observation will be removed when self is deallocated.

An example of calling this method:

<#someObject#>.observe(\.<#some.key.path#>, until: selfDeinits) { obj,change in
    <#do something#>
}