gistfile1.m

- (void) start
{
    if(self.isCancelled)
    {
        self.finished = YES;
        return;
    }
    self.executing = YES;
    
    __block UIBackgroundTaskIdentifier delayedTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^
    {
        [[UIApplication sharedApplication] endBackgroundTask:delayedTask] ;
        delayedTask = UIBackgroundTaskInvalid ;
        self.executing = NO;
        self.finished = YES;
        self.failed = YES;
    }];
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

    NSLog(@"Sleeping long running task") ;
    [NSThread sleepForTimeInterval:5] ;
    NSLog(@"Waking long running task") ;
        
    @autoreleasepool
    {
        GPUImageTransformFilter *transformerFilter = [[GPUImageTransformFilter alloc] init];
        [transformerFilter forceProcessingAtSize:_size];
        _resizedImage = [transformerFilter imageByFilteringImage:_originalImage];
        NSLog(@"Got resized image: %@" , _resizedImage) ;
        if(! _resizedImage)
        {
            self.failed = YES;
        }
        else
        {
            _originalImage = nil;
        }
    }
        
    self.executing = NO;
    self.finished = YES;

    [[UIApplication sharedApplication] endBackgroundTask:delayedTask] ;
    delayedTask = UIBackgroundTaskInvalid ;

    }); // dispatch_async
}