How to stop preloading process running in concurrent thread? - ios

Question

In my game, after play button is pressed, game view controller is presented and couple of assets start to preload in background for later use in game.
My question is, how to stop preloading process when home button is pressed but my preloading process is still running?
Now I've to wait till all preloading is done for proper deallocating...
ViewController.m
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
// preload intro sequences
dispatch_sync(concurrentQueue, ^{
[weak_self.spaceSun prepareAnimationArrays];
});
dispatch_async(concurrentQueue, ^{
[weak_self.sailor prepareAnimationArrays];
});
SpaceSun, Sailor.m
- (void)prepareAnimationArrays
{
_introArray = [NSMutableArray new];
_introArray = [self.animator generateCachedImageArrayWithFilename:#"circleSun" extension:#".png" andImageCount:10];
self.isAnimationArrayCached = YES;
}
- (NSMutableArray *)generateCachedLoopedImageArrayWithFilename:(NSString *)filename
extension:
(NSString *)extension
andImageCount:
(int)count
{
_imagesArray = [[NSMutableArray alloc] init];
_fileExtension = extension;
_animationImageName = filename;
_imageCount = count;
for (int i = 0; i < _imageCount; i++)
{
NSString *tempImageName = [NSString stringWithFormat:#"%#%i", filename, i];
NSString *imagePath = [[NSBundle mainBundle] pathForResource:tempImageName ofType:extension];
UIImage *frameImage = [self loadRetinaImageIfAvailable:imagePath];
UIGraphicsBeginImageContextWithOptions(frameImage.size, NO, [UIScreen mainScreen].scale);
CGRect rect = CGRectMake(0, 0, frameImage.size.width, frameImage.size.height);
[frameImage drawInRect:rect];
UIImage *renderedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[_imagesArray addObject:renderedImage];
_precachedImagesCount++;
if (_didPreCachePicture)
_didPreCachePicture();
if (_isDoublingFrames)
{
[_imagesArray addObject:renderedImage];
}
else if (_isTriplingFrames)
{
[_imagesArray addObject:renderedImage];
[_imagesArray addObject:renderedImage];
}
if (i == _imageCount - 1)
{
// we have 5 images
// 12345 already in array
// let's add 432
for (int j = _imageCount - 2; j > 0; j--)
{
tempImageName = [NSString stringWithFormat:#"%#%i", filename, j];
imagePath = [[NSBundle mainBundle] pathForResource:tempImageName ofType:extension];
frameImage = [self loadRetinaImageIfAvailable:imagePath];
UIGraphicsBeginImageContextWithOptions(frameImage.size, NO, [UIScreen mainScreen].scale);
rect = CGRectMake(0, 0, frameImage.size.width, frameImage.size.height);
[frameImage drawInRect:rect];
renderedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[_imagesArray addObject:renderedImage];
_precachedImagesCount++;
if (_didPreCachePicture)
_didPreCachePicture();
if (_isDoublingFrames)
{
[_imagesArray addObject:renderedImage];
}
else if (_isTriplingFrames)
{
[_imagesArray addObject:renderedImage];
[_imagesArray addObject:renderedImage];
}
}
}
}
return _imagesArray;
}

Answer 1

You cannot cancel a block that has been scheduled using dispatch_async. Instead of using GCD you should prefer the higher-level NSOperation API. Create multiple NSOperation subclasses for your tasks and schedule them on a NSOperationQueue.
If you have tasks that need to finish earlier than others specify dependencies between the operations with addDependency:.
To cancel the preloading process simply call:
[self.operationQueue cancelAllOperations];
In order to benefit from the cancelation feature exposed by operation queues, you should regularly check the isCancelled property for longer-running operations:
- (void)main
{
for (int i = 0; i < _imageCount; i++) {
if (self.isCancelled) break;
// do your processing
}
}