Swift Programming: getter/setter in stored property

Question

How do I overwrite the setter of stored property in Swift?

In Obj-C, I can overwrite its setter, but Swift doesn't seem to be happy about getter/setters being used for stored property.

Say I have a Card class with a property called rank. I don't want the client to give it any invalid value, therefore, in objective-C, I can overwrite setRank so that it performs additional check. But willSet in Swift doesn't seem to help because newValue is constant and it makes no sense to assign rank because setter will be called in a loop.

Answer 1

Ok. Reading through Apples documentation on Swift I found this:

If you assign a value to a property within its own didSet observer, the new value that you assign will replace the one that was just set.

So all you have to do is this:

var rank: Int = 0 {
    didSet {
        // Say 1000 is not good for you and 999 is the maximum you want to be stored there
        if rank >= 1000  {
            rank = 999
        }
    }
}

Answer 2

You can't override get/set for a stored property but you can use property observers willSet/didSet:

var totalSteps: Int = 0 {
    willSet(newTotalSteps) {
        println("About to set totalSteps to \(newTotalSteps)")
    }
    didSet {
        if totalSteps > oldValue  {
            println("Added \(totalSteps - oldValue) steps")
        }
    }
}

The default parameter names are newValue for willSet and oldValue for didSet, or you can name them yourself as in willSet(newTotalSteps).

Answer 3

get and set are for computed properties (they don't have any backing store). (In my opinion, the keyword 'var' is confusing here)

• willSet and didSet are called for an instance variable (Use didSet to override any changes)
• set and get are purely for computed properties

Answer 4

If you don't want to use didSet, which has the problem that the property's value is temporarily wrong, you should wrap a computed property around it.

private var _foo:Int = 0
var foo:Int {
    get {
        return _foo
    }
    set {
        if(newValue > 999) {
            _foo = 999
        } else {
            _foo = newValue
        }
    }
}

Or:

private var _foo:Int = 0
var foo:Int {
    get {
        return _foo
    }
    set {
        guard newValue <= 999 else {
            _foo = 999
            return
        }
        _foo = newValue
    }
}