Propiedades calculadas con get/set

struct Rectangle {
    var width: Double
    var height: Double
    var area: Double {             // read-only computed
        width * height
    }
}
let r = Rectangle(width: 3, height: 4)
print("area =", r.area)  // 12

struct Circle {
    var radius: Double            // stored
    var diameter: Double {        // computed read–write
        get { radius * 2 }
        set { radius = newValue / 2 }
    }
}
var c = Circle(radius: 5)
print(c.diameter)   // 10
c.diameter = 14
print(c.radius)     // 7

struct Percentage {
    private var _raw: Int = 0          // backing store
    var value: Int {
        get { _raw }
        set {                          // uses implicit newValue
            _raw = min(100, max(0, newValue))
        }
    }
}
var p = Percentage()
p.value = 120
print(p.value)  // 100
p.value = -5
print(p.value)  // 0

struct ScoreBoard {
    private(set) var highScore = 0   // public read, internal write
    mutating func update(_ v: Int) {
        if v > highScore { highScore = v }
    }
}
var s = ScoreBoard()
s.update(30)
print(s.highScore)
// s.highScore = 50  // error outside the type