Controller Container

几个API有点混沌，简单整理一下：

• addChildViewController: Adds the specified view controller as a child of the current view controller.
  这个API跟view没有关系，是一个controller之间的关系。这个API也同时调用 willMoveToParentViewController:self。
• willMoveToParentViewController: Called just before the view controller is added or removed from a container view controller. 这个API必须在removeFromParentViewController前调用，用作willMoveToParentViewController:nil。
• removeFromParentViewController: Removes the view controller from its parent. 这个API同时调用了didMoveToParentViewController。
• didMoveToParentViewController: Called after the view controller is added or removed from a container view controller。
• transitionFromViewController:toViewController:options:completionHandler: 这个API只完成了view的设置，对controller的设置还需要调用前面几种API。

Reference

• Apple Developer Document: UIViewController