【viewDidAppear在何时调用】
If the view belonging to a view controller is added to a view hierarchy directly, the view controller will not receive this message. If you insert or add a view to the view hierarchy, and it has a view controller, you should send the associated view controller this message directly. Failing to send the view controller this message will prevent any associated animation from being displayed.
如果包含在一个viewController中的view被直接添加到一个view上，就像这样：
[view1 addSubView: viewController.view];
这样我们的viewController就不会收到viewDidAppear:的消息。按这种方式添加视图的话，我们一般需要手动发送这个消息，也就是调用viewController的viewDidAppear方法。否则的话相关的动画都无法正常显示。
像navigationController的push,或者是presentModelViewController的方法不属于上述方式。而且有明显的动画显示（弹出效果），个人认为是自动调用viewDidAppear方法的。
