在iOS5之前，将view controller的view加入到另一个vc的view中时，如下[parent_vc.view addSubview:sub_vc.view];
sub_vc的view appear,disappear等方法是不会被调用的。

在iOS5之后view controller内部，增加了一个属性：childViewControllers。
调用addChildViewController,就是把sub vc 加入到childViewControllers，这样，parent vc发生view appear，dispappear事件时，就轮循childViewControllers数组，依次调用其对应
方法就可以了。

removeFromParentViewController则是把一个vc从sub vc数组中去掉。
这样，这个vc就不会收到appear等事件了。

下面是UIViewController头文件相关的代码

/*
  The methods in the UIContainerViewControllerProtectedMethods and the UIContainerViewControllerCallbacks
  categories typically should only be called by subclasses which are implementing new container view
  controllers. They may be overridden but must call super.
*/
@interface UIViewController (UIContainerViewControllerProtectedMethods)

// An array of children view controllers. This array does not include any presented view controllers.
@property(nonatomic,readonly) NSArray *childViewControllers NS_AVAILABLE_IOS(5_0);

/*
  If the child controller has a different parent controller, it will first be removed from its current parent
  by calling removeFromParentViewController. If this method is overridden then the super implementation must
  be called.
*/
- (void)addChildViewController:(UIViewController *)childController NS_AVAILABLE_IOS(5_0);

/*
  Removes the the receiver from its parent's children controllers array. If this method is overridden then
  the super implementation must be called.
*/
- (void) removeFromParentViewController NS_AVAILABLE_IOS(5_0);

/*
  This method can be used to transition between sibling child view controllers. The receiver of this method is
  their common parent view controller. (Use [UIViewController addChildViewController:] to create the
  parent/child relationship.) This method will add the toViewController's view to the superview of the
  fromViewController's view and the fromViewController's view will be removed from its superview after the
  transition completes. It is important to allow this method to add and remove the views. The arguments to
  this method are the same as those defined by UIView's block animation API. This method will fail with an
  NSInvalidArgumentException if the parent view controllers are not the same as the receiver, or if the
  receiver explicitly forwards its appearance and rotation callbacks to its children. Finally, the receiver
  should not be a subclass of an iOS container view controller. Note also that it is possible to use the
  UIView APIs directly. If they are used it is important to ensure that the toViewController's view is added
  to the visible view hierarchy while the fromViewController's view is removed.
*/
- (void)transitionFromViewController:(UIViewController *)fromViewController toViewController:(UIViewController *)toViewController duration:(NSTimeInterval)duration options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion NS_AVAILABLE_IOS(5_0);

// If a custom container controller manually forwards its appearance callbacks, then rather than calling
// viewWillAppear:, viewDidAppear: viewWillDisappear:, or viewDidDisappear: on the children these methods
// should be used instead. This will ensure that descendent child controllers appearance methods will be
// invoked. It also enables more complex custom transitions to be implemented since the appearance callbacks are
// now tied to the final matching invocation of endAppearanceTransition.
- (void)beginAppearanceTransition:(BOOL)isAppearing animated:(BOOL)animated __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_5_0);
- (void)endAppearanceTransition __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_5_0);

@end