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