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。
