属性观察器， willSet和didSet的使用
var maxValue = 999
var mineValue = -999
var number = 0 {
willSet{
print(“将从\(number)变成\(newValue)”)
}
didSet{
if number > maxValue{
number = maxValue
}else if number < mineValue {
number = mineValue
}
print(“已经从\(oldValue)变成\(number)”)
}
}
number = 1000
number
