两个 UIViewController 之间页面切换的生命周期
如下所示：
A push to B:
[A.navigationController pushViewController:B animated:animated];
- A viewWillDisappear:
- B viewWillAppear:
- A viewDidDisappear:
- B viewDidAppear:
B pop back to A:
[B.navigationController popViewControllerAnimated:animated];
- B viewWillDisappear:
- A viewWillAppear:
- B viewDidDisappear:
- A viewDidAppear:
