How to immediately stop decoding in GPUImage?-Collection of common programming errors

You should listen for UIApplicationWillResignActiveNotification and UIApplicationWillEnterForegroundNotification in the view controller where you are processing the movie:

// handles the user pressing the home or power button while the camera is onscreen
[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(didResignActive:)
                                             name: UIApplicationWillResignActiveNotification
                                           object:nil];

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(willEnterForeground:)
                                             name:UIApplicationWillEnterForegroundNotification
                                           object:nil];

Then you should add the appropriate methods to respond to the notifications:

-(void) didResignActive: (NSString *) notification {
    //stop processing the movie
}

-(void) willEnterForeground: (NSString *) notification {
    // start processing the movie
}

I’m pretty sure you can start processing by calling [movie startProcessing]; and stop processing with [movie endProcessing]; but I’m not positive.