Swift: Declaring a constant in a subclass but still have a reference to it in the superclass?

Question

Let's say you had the class Apple with several subclasses.

class Apple {
  let color = ""
}

class redDelicious :Apple {
  let color = "Red"
}

class grannySmith :Apple {
  let color = "Green"
}

func eatApple (appleToEat: Apple)
{
  if appleToEat.color == "Red" {
    //do something
  } else if appleToEat.color == "Green"
  {
    //do something else
  }
}

The problem is that swift won't let me call the "color" property but I want to make sure that's defined in the subclasses. I also want to make sure every apple has a color so that I can call the color property on each sublcass of Apple. What would be the best solution in this case?

Answer 1

You could do:

class Apple {
    let color: String
    init(color: String) {
        self.color = color
    }
}

class RedDelicious: Apple {
    init() {
        super.init(color: "Red")
    }
}

Or instead, you could use read-only computed properties:

class Apple {
    var color: String { return "" }
}

class RedDelicious: Apple {
    override var color: String { return "Red" }
}

If color can only be certain values it may be worth using an enum, for example:

class Apple {
    enum Color {
        case Red, Green
    }

    let color: Color
    init(color: Color) {
        self.color = color
    }
}

class RedDelicious: Apple {
    init() {
        super.init(color: .Red)
    }
}

In your eatApple function, you would then do:

func eatApple (appleToEat apple: Apple) {
    switch apple.color {
        case .Red  : // ...
        case .Green: // ... 
    }
}