swift : willSet + didSet 属性观察器

class StepCounter {
var totalSteps: Int = 0 {
willSet(newTotalSteps) {
println("About to set totalSteps to (newTotalSteps)")
}
didSet {
if totalSteps > oldValue {
println("Added (totalSteps - oldValue) steps")
}
}
}
}
let stepCounter = StepCounter()
stepCounter.totalSteps = 200
// About to set totalSteps to 200
// Added 200 steps
stepCounter.totalSteps = 360
// About to set totalSteps to 360
// Added 160 steps
stepCounter.totalSteps = 896
// About to set totalSteps to 896
// Added 536 steps