我有一辆车和司机。它们相互引用。在汽车的init()中，我创建一个驱动程序并将其分配给驱动程序成员。驱动程序成员具有doSet方法，该方法应该设置驾驶员的车，从而将它们彼此相互连接起来。
class GmDriver {
var car: GmCar! = nil
}
class GmCar {
var driver: GmDriver {
didSet {
driver.car = self
}
}
init() {
driver = GmDriver()
}
}
let myCar = GmCar()
println(myCar.driver.car) // nil
但是，didSet从不触发。为什么？
Apple Documentation：
The willSet and didSet observers of superclass properties are called when a property is set in a subclass initializer,after the superclass initializer has been called. They are not called while a class is setting its own properties,before the superclass initializer has been called.
