Discovering if a component is being displayed (of if it's in background), is it possible?

I need to run a method everytime the user switch the screen or return to the application (after a call, or pressing the "Home Button" and returning to the app, for example).

Is there any way to recognize this, like an event or method that is always executed when this happens?

开发者_如何学JAVAThank you guys in advance.

Implement those 2 methods in the UIApplicationDelegate :

- (void)applicationDidEnterBackground:(UIApplication *)application

- (void)applicationWillEnterForeground:(UIApplication *)application