func swap(var a: Int, var b: Int){
    let c = a
    a = b
    b = c
}
var a = 3
var b = 2
swap(a, b) // a = 3, b = 2

func swap(inout a: Int, inout b: Int){
    let c = a
    a = b
    b = c
}
var a = 3
var b = 2
swap(a, b) // a = 2, b = 3

class StepCounter {
    var totalSteps: Int = 0
    willSet(newTotalSteps){
        println("About to set totalSteps to \(newTotalSteps)") // you can use newValue if you does not set a name with newTotalSteps
    }

    didSet{
        if totalSteps > oldValue {
            println("Added \(totalSteps - oldValue) steps")
        }
    }
}

let stepCounter = StepCounter() // no output since it is the initialiser, not change the value.
stepCounter.totalSteps = 200 // print both the willSet‘s and didSet‘s output.

    3.可选类型

class Optional{
    var a: Int?
    var b: Int
}

a = 10
// optional binding.
if let c = a {
    println("\(c)")
} else {
    println("a is nil")
}

// forced unwrapping, might cause an runtime error if a is nil.
let d = a!

    4.计算属性的setter

struct Circle {
    var radius: Float
    var diameter: Float {
        get{
            return 2.0 * radius
        }
        set{
            radius = newValue / 2.0
        }
    }
}

struct Circle {
    var radius: Float
    var diameter: Float {
        get{
            return 2.0 * radius
        }
        set(newDiameter){
            radius = newDiameter / 2.0
        }
    }
}