Inheritance of property methods (set, get, didSet, willSet, etc.) in Swift

Question

My question is how to deal with inheritance of property methods such as set , get , didSet , willSet , ...?

Let's take an example: I want to override the setter method of a property in a Swift class. Here is what I want to achieve (which is obviously not working):

class A {
    var value: Int {get {...} set {...} }
}

class B: A {
    var value: Int {
        set(newValue) {
            // do some fancy stuff...
        }
    }
}

This is not working, too:

// in class B
override func setValue(newValue: Int) {
    // do some fancy stuff...
}

We can do in Swift something like this:

class A {
    var _value: Int = 0
    var value: Int {
        get {
            return _value
        }
        set {
            _value = newValue
        }
    }
}

class B: A {
    override var value: Int {
        get {
            return _value
        }
        set {
            _value = newValue + 1
        }
    }
}

let a = A()
a.value = 1
print(a.value) // => 1

let b = B()
b.value = 1 
print(b.value) // => 2

This approach is not very elegant because I have to implement also the getter methods, which is actually not necessary, because only the setter should be overridden.

Answer 1

You could do:

class A {
    var value: Int = 0
}

class B: A {
    override var value: Int {
        get { return super.value }
        set {
            // Something fancy...
            super.value = newValue
        }
    }
}

While you still have to implement a getter in B , you at least don't need _value , which helps keeps class A clean.

Answer 2

Unfortunately your only way to do this would be to create a separate get and set method if you really wanted to do this.

class A {
  private var value: Int = 0

 func getValue() -> Int { return value }
 func setValue(newValue : Int) {
   value = newValue
 }
}

By the nature of using computed properties you are basically doing a "shortcut" in some ways over making two separate methods - and as thus you would have to override both the set and get if you wanted to change things.

Answer 3

Depending on your usage an override of didSet could do the trick (based on your last example):

class B: A {
    override var value: Int {
        didSet {
            _value++
        }
    }
}