Proprietà memorizzate e calcolate, observer

struct Rectangle {
    // stored properties (hold data)
    var width: Double
    var height: Double

    // computed property (derived on access)
    var area: Double {
        return width * height   // read-only computed
    }
}
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 Counter {
    var value: Int = 0 {
        willSet { print("will set to", newValue) }
        didSet  { print("did set from", oldValue) }
    }
}
var cnt = Counter()
cnt.value = 5
cnt.value = 9

struct Score {
    var raw: Int = 0 {
        didSet {
            if raw < 0 { raw = 0 }
            if raw > 100 { raw = 100 }
        }
    }
    var label: String {
        // computed: derive a label from current raw
        if raw >= 90 { return "A" }
        if raw >= 80 { return "B" }
        if raw >= 70 { return "C" }
        return "D"
    }
}
var s = Score()
s.raw = 120
print(s.raw, s.label)   // 100 A
s.raw = -5
print(s.raw, s.label)   // 0 D