Swift语言精要 - 属性

class Rectangle {
    var width: Double = 0.0
    var height: Double = 0.0
    var area : Double {
        // computed getter
        get {
            return width * height
        }
        // computed setter
        set {
            // Assume equal dimensions (i.e., a square)
            width = sqrt(newValue)
            height = sqrt(newValue)
        }
    }
}

var rect = Rectangle()
rect.width = 3.0
rect.height = 4.5
rect.area // = 13.5
rect.area = 9 // width & height now both 3.0

class PropertyObserverExample {
    var number : Int = 0 {
        willSet(newNumber) {
            println("About to change to \(newNumber)")
        }
        didSet(oldNumber) {
            println("Just changed from \(oldNumber) to \(self.number)!")
        }
    }
}

var observer = PropertyObserverExample()
observer.number = 4
// prints "About to change to 4", then "Just changed from 0 to 4!"

class SomeExpensiveClass {
    init(id : Int) {
        println("Expensive class \(id) created!")
    }
}

class LazyPropertyExample {
    var expensiveClass1 = SomeExpensiveClass(id: 1)
    lazy var expensiveClass2 = SomeExpensiveClass(id: 2)    
    init() {
        println("First class created!")
    }
}

var lazyExample = LazyPropertyExample()
// prints "Expensive class 1 created", then "First class created!"
lazyExample.expensiveClass1 // prints nothing, it's already created
lazyExample.expensiveClass2 // prints "Expensive class 2 created!"