[swift-users] didSet and time when propagation of mutation happens

[Diego Sánchez]

Hi all,

The following snippet summarises an issue I was investigating:

struct Observable<T> {
    var value: T {
        didSet {
            print("Observable.didSet")
            callback?()
        }
    }
    var callback: (() -> Void)?
}

class MyClass {
    var myString: Observable<String> {
        get {
            return _myString
        }
        set {
            print("MyClass.Setter")
            self._myString = newValue
        }
    }

    private var _myString: Observable<String>
    init (string: Observable<String>) {
        self._myString = string
        print("MyClass.init.end")
    }
}

let myClass = MyClass(string: Observable<String>(value: "1", callback: nil))
myClass.myString.callback = {
    print(myClass.myString.value)
}
myClass.myString.value = "2"

*Output:*
MyClass.init.end
MyClass.Setter
Observable.didSet
*1*
MyClass.Setter

Obviously I wasn't expecting to get "1", but "2" in the callback's print,
and this happens because MyClass.setter is called after Observable.didSet
completes.

Is it feasible that all the *set*s are called first, and then all the
*didSet*s?

Cheers,
Diego
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