Shorthand setter declaration for a subscript of an array in Swift

Question

In my last question, I asked how to write setter for a subscript of a computed property in Swift. I think my question was not substantial enough to be understood. The answer giv

Answer 1

I did think of another way, but I don't like it much. Use an extension to add another subscript to setter to Array - a setter that allows you to supply a callback. The problem with this approach is that you have remember to supply the callback! This puts the onus on whoever sets into the index, which is probably not what you had in mind. Still, it has the advantage, once configured, of brevity - and it takes O(0) time:

extension Array {
    subscript(ix:Int, f:(T,T,Int)->Void) -> T {
        set {
            let old = self[ix]
            self[ix] = newValue
            f(old,newValue,ix)
        }
        get {
            return self[ix]
        }
    }
}

class Thing {
    var arr : [Int] = [1, 2, 3]
    func f (oldv:Int, newv:Int, ix:Int) {
        println("changed \(oldv) to \(newv) at \(ix)")
    }
}

var t = Thing()
t.arr[1,t.f] = 100 // "changed 2 to 100 at 1"