Simple Reactive: KVO

This is the first example in a series of Reactive design patterns for Swift that have no dependencies. See here for details.

My initial thought when considering how to build a reactive model without any dependencies was to use iOS Foundation’s KVO, or Key Value Observing. Baked into anything inheriting from NSObject is a mechanism to observe changes at key paths of a specific object.

For example, let’s suppose we have a Person model that has a first and last name. If we want to observe changes to either attribute, we need only mark them as dynamic, make them available in objective-c, and tell the instance that we want to be told when the value changes.

class Person: NSObject {
    @objc dynamic var firstName: String
    @objc dynamic var lastName: String


And then in the ViewModel, where we want to build a string for our View to observe:

class PersonViewModel: NSObject {
    private var firstNameObserver: NSKeyValueObservation?
    private var lastNameObserver: NSKeyValueObservation?


    private func observePersonAttributes() {
        firstNameObserver = person.observe(\Person.firstName, options: [.initial, .new], changeHandler: {[unowned self] object, change in
            self.firstNameChanged(change.newValue as? String)
        lastNameObserver = person.observe(\Person.lastName, options: [.initial, .new], changeHandler: {[unowned self] object, change in
            self.lastNameChanged(change.newValue as? String)


KVO is (probably) already there! It will manage your observers, it will send events on change.

You can also specify if you want your callback executed with the initial value or only new ones. This may seem like not a big deal, but as we explore other patterns for reactive programming, it’s a pretty big plus.


Doing this has quite a few requirements, so let’s walk through them.

Objective C

KVO depends entirely on meta-programming, because it observes changes to objects at key paths. Swift doesn’t like meta-programming, so this all needs to happen in the Objective-C under the hood. This requires a simple dynamic declaration for each observable property, as well as an @objc export.

In practice, this doesn’t matter too much. You can still write your app in Swift, and leverage the compiler to make sure you’re observing real keypaths. But it does mean some extra casting.

Note: Swift makes this a bit safer than it used to, telling the compiler to verify that the key path used actually exists (previously it was a string, which of course guarantees a mistake at least once).

Type Constraints

…but the real bummer about requiring objective-c is that your observable types need to be representable in objc. So, if, for example, you want to observe changes to a Swift enum, you’ll need to instead use monitor its raw value.


KVO’s observe method lives in NSObject. So your models need to extend NSObject. That’s a drag.

Other Notes

  • Observing will continue until your NSKeyValueObservation instance is deallocated. This is important! If you do not retain a reference to it, it will stop observing; if you retain a reference to it, your callback better be able to run
  • If you don’t know the difference between unowned and weak (or - gasp! - either), then take the time now to read up on the difference. Because if you refer to self in your callback, you probably want it to be weak or unowned, else you’ll have circular references
  • Have I mentioned the importance of retaining or destroying references?
  • This one was too obvious to list as a cost, but this does require Foundation. That doesn’t matter for iOS/macOS projects, but if you’re running a swift app elsewhere, that may not be desirable.

Rating: B-

The fact that all of this comes right out of the box is pretty hard to beat. But I really don’t want to introduce dependencies if I don’t need them. And the fact that NSObject#observe, the method used above, was introduced within the last year, makes me very wary that there may be other interface changes. Furthermore, for many of my projects, the inability to use Swift types not available in Objective-C is a deal breaker.

#swift #ios #mobile #programming #simple-reactive

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