为什么在viewDidLoad中调用presentingViewController返回nil?

ios7应用开发入门经典（第7版）p256页提到，对于modal类型的segue，可以通过presentingViewController和presentedViewController获取segue前后的controller。

但在ios8中实践发现presentingViewController返回了nil. 这是什么情况？查阅stackoverflow，得到如下答案：

When viewDidLoad is called, there is no guarantee that the view controller hierarchy is loaded in the navigation tree. Moving the logic to a later stage (for example: viewWillAppear) should resolve that issue as presentingController should be loaded by then.

http://stackoverflow.com/questions/26081661/presentingviewcontroller-is-nil-when-using-presentviewcontrolleranimatedcomple