Block code in a method does not get run - objective-c

Question

I have the code below to load a group of images and notify me via completionHandler when they are all done loading. However, I find that certain dispatch_group_leave won't be called at times and my guess is imageLoader is deallocated before the block gets to run. If I put a reference of imageLoader within the loadImageWithURL:completionHandler block, then everything works as intended.
Is my guess of the cause correct? What's the correct fix for this issue? I know ARC does block copy in most cases automatically, should I do a block copy in this case?
- (void)loadGroupImagesAsyncWithCompletion:(void(^)(NSError *))completionHandler {
dispatch_group_t group = dispatch_group_create();
int index = 0;
for (Item *item in items) {
char queueLabel[30] = {0};
sprintf(queueLabel, "loader%d", index);
dispatch_queue_t queue = dispatch_queue_create(queueLabel, NULL);
dispatch_group_enter(group);
dispatch_async(queue, ^{
ImageLoader *imageLoader = [[ImageLoader alloc] init];
[imageLoader loadImageWithURL:url completionHandler:^(UIImage *image, NSError *error) {
if (image) {
item.image = image;
}
//NOTE: if item object is referenced in this block,
//then there is no missed dispatch_group_leave call.
dispatch_group_leave(group);
}];
});
}
// Non-blocking wait
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// shouldn't take more than 5 secs to load all images
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)));
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler(nil);
});
});
}

Answer 1

Here's my guess. It's only a guess because you haven't posted the code of anything to do with ImageLoader.
If -loadImageWithURL:completionHandler: operates asynchronously i.e. it uses an async dispatch queue itself then you could be right that it is being deallocated before the load finishes. This is because its lifetime is just the scope of the for block it is declared in.
In fact, there is no reason why that method needs to do stuff asynchronously, because you have already got it in an asynchronous block. Just have a synchronous method and call dispatch_group_leave() after the method finishes.
EDIT
Given that you have no control over ImageLoader and -loadImageWithURL:completionHandler: operates asynchronously without any help from you, you should remove the dispatch_async wrapper around the call. You'll still have the problem of the ImageLoader being deallocated, but that can be avoided by putting each ImageLoader in a array when you create it.
The code would look something like this:
- (void)loadGroupImagesAsyncWithCompletion:(void(^)(NSError *))completionHandler
{
NSMutableArray* loaders = [[NSMutableArray alloc] init];
dispatch_group_t group = dispatch_group_create();
int index = 0;
for (Item *item in items) {
char queueLabel[30] = {0};
sprintf(queueLabel, "loader%d", index);
dispatch_queue_t queue = dispatch_queue_create(queueLabel, NULL);
dispatch_group_enter(group);
ImageLoader *imageLoader = [[ImageLoader alloc] init];
[imageLoader loadImageWithURL:url completionHandler:^(UIImage *image, NSError *error) {
if (image) {
item.image = image;
}
//NOTE: if item object is referenced in this block,
//then there is no missed dispatch_group_leave call.
dispatch_group_leave(group);
}];
[loaders addObject: imageLoader];
}
// Non-blocking wait
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// shouldn't take more than 5 secs to load all images
dispatch_group_wait(group, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC)));
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler(nil);
});
[loaders removeAllObjects];
});
}