Swift property observer in protocol extension?

Question

Consider the following:

protocol ViewControllable: class {
  typealias VM: ViewModellable
  var vm: VM! { get }
  func bind()
}

extension ViewControllable {
  var vm: VM! {
    didSet {
      bind()
    }
  }
}

I'm trying to observe vm property and call bind whenever it is injected. But this doesn't compile with error saying:

Extensions may not contain stored properties

which makes sense since protocol cannot enforce properties to be stored or computed.

Is this possible to accomplish without introducing class inheritance?

In other words, Can I observe the change of a property inside protocol extension?

note that you can indeed do this very easily, if you use an associated property. Here's a recent post from me on exactly how to make an associated property: example That is quite fresh as of writing this, so hopefully it's pretty stable. — Fattie
Regarding this question, a perfect example of where you could use this is the example used in this QA: stackoverflow.com/questions/41910120/… — Fattie
@Fattie can you please fix that link. I currently can't view your post or example. — TMin
Ah you mean the first link ! Note that SO closed down the "documentation" feature. Unfortunately, the example is gone forever. Sorry ! — Fattie
Here it is, I think: web.archive.org/web/20170221052848/http://stackoverflow.com/… — koen

Rob Napier Rob Napier · Accepted Answer · 2015-11-23T04:27:33

No, this is explicitly disallowed. See Extension: Computed Properties:

Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties.

Keep in mind that if this were legal, it would add some non-trivial confusion about order of execution. Imagine there were several extensions that added didSet, and the actual implementation also had a didSet. What order should they run in? This doesn't mean it's impossible to implement, but it could be somewhat surprising if we had it.

Swift property observer in protocol extension?

1 Answers