How do I restrict a property to a range?

Question

There are a cases where I might want to model data where it makes sense for a value to be restricted to a given range.

For example, if I want to represent a "mammal", I might want to restrict a legs property to 0–4.

My first attempt is shown below:

class Mammal {
    var _numLegs:Int?

    var numLegs:Int {
    get {
        return _numLegs!
    }
    set {
        if 0...4 ~= newValue {
            self._numLegs = newValue
        }
        else {
            self._numLegs = nil
        }
    }
    }
}

However, this seems unsatisfactory since all properties are "public" there is nothing stopping the customer of the class from setting Mammal._numLegs to some arbitrary value.

Any better ways to do it?

Answer 1

Just for fun I decided to write a snippet with @jackWu's suggestion (+1) in order to try that didSet thing:

class Mammal {
    var numLegs:UInt? {
        didSet { if numLegs? > 4 { numLegs = nil } }
    }

    init() {
        numLegs = nil
    }
}

It works perfectly. As soon as you try to set numLegs to 5 or greater, boom, it gets nilled automatically. Please note that I used Uint to avoid negative leg quantities :)

I really like the elegance of didSet.

Answer 2

In this specific case, you want a property observer, you could implement it like this:

class Mammal {
    init () {
        numLegs = 0
        super.init()
    }
    var numLegs:Int {
        didSet: {
            if !(numLegs ~= 0...4) {
               numLegs = max(0,min(numLegs,4)) // Not sure if this is what you want though
            }   
        }
    }
}

Looking at this though, I'm not sure if this would recurse and call didSet again...I guess it wouldn't be too bad because it would pass the check the second time