When is viewwillappear and viewdidappear of the IOS View Controller called?

From: http://stackoverflow.com/questions/5277940/why-does-viewwillappear-not-get-called-when-an-app-comes-back-from-the-backgroun

When Will viewwillappear and viewdidappear be called?

Q:

When I press a View Controller Into A uinavigationcontroller,

1. When will viewwillappear and viewdidappear be triggered?

2. What causes failure to trigger viewwillappear and viewdidappear?

A:

When you call pushviewcontroller: animated to push a View Controller to uinavigationcontroller, uinavigationcontroller automatically calls these methods. Similarly, when you use tabs, uitabbarcontroller directly calls these methods. When you use presentmodalviewcontroller, the method is also called. These methods are also called when a View Controller is added to a window. In these cases, I have never encountered any failure in calling these methods.

Remember, these methods are called only when the Controller pushed or presented is in these specific circumstances. In other cases, it will not be called. For example, you add the view of your view controller as a subview of a view rather than a subview of a window. According to Apple's official documentation, the view controller is only used for full-screen views, typically using the method mentioned above. You can ignore Apple's suggestion to associate one View Controller with another view as a subview, but you need to manually call viewwillappear and viewdidappear of the nested controller in the View Controller as the container.