How to make iPhone app continue working when I lock screen?

-For example, I wanna keep recording video with iPhone camera, however, I should save my battery change while filming!It doesn't work that way. Even though apps are multi-tasking in the background, once you lock your screen, every halts.You cant do that sadly. :-/ Cause when you lock the screen, it goes into a sleep mode. So there is no way at all.