What method is called when iPhone is locked?

I want my app to stop locating the user when the app is open, but the phone is locked, to cut down on battery usage. What delegate method is called when the user locks the phone while an app is still running and acti开发者_C百科ve?

I don't know if there's a special one just for locking, but applicationWillResignActive: is definitely called when the phone is locked (or receives a call, etc.)