UIViewController生命周期

琢磨清楚UIViewController的生命周期是蛮有用的，这意味着你可以使其在正确的阶段做正确的事情。

参看UIViewController的生命周期及iOS程序执行顺序 蛮有价值的一篇文章。

此外，补上几点来自于官方的说明：

- (void)loadView

If you use Interface Builder to create your views and initialize the view controller, you must not override this method.

You can override this method in order to create your views manually. If you choose to do so, assign the root view of your view hierarchy to the view property. 

Your custom implementation of this method should not call super

- (void)viewDidLoad

This method is called regardless of whether the view hierarchy was loaded from a nib file or created programmatically in the loadView method. 

其他：在dealloc中注销订阅的NSNotification以及KVO。