How to tell if blocks in loop all have completed executing?

Question

I have a loop set up that downloads a series a images which I will later use for to animate using the animationImages property of UIImageView. I wo

Answer 1

Create a dispatch group, in the for loop enter the group, in the completion block leave the group. Then you can use dispatch_group_notify to find out when all blocks have completed:

dispatch_group_t    group = dispatch_group_create();

for (PFObject *pictureObject in objects){

    PFFile *imageFile = [pictureObject objectForKey:@"image"];
    NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
    NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];

    dispatch_group_enter(group);
    [tokenImageView setImageWithURLRequest:imageRequest placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
        [self.downloadedUIImages addObject:image]; //This is a mutableArray that will later be set to an UIImageView's animnationImages
        dispatch_group_leave(group);
    } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
        NSLog(@"Error %@", error);
        dispatch_group_leave(group);
    }];

}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // do your completion stuff here
});

Answer 2

for (PFObject *pictureObject in objects){

    PFFile *imageFile = [pictureObject objectForKey:@"image"];
    NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
    NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];

    [tokenImageView setImageWithURLRequest:imageRequest placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
       [self.downloadedUIImages addObject:image]; //This is a mutableArray that will later be set to an UIImageView's animnationImages
       if([[objects lastObject] isEqual:pictureObject]) {
           [self animateImages];
       }
    } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
        NSLog(@"Error %@", error);
        if([[objects lastObject] isEqual:pictureObject]) {
           [self animateImages];
       }
    }];

}

- (void)animateImages {
    //do animation here.
}

Answer 3

Count how many you've completed. The challenging part is making it thread safe. I recommend creating an atomic counter class for that.

Generic solution!

+ (void)runBlocksInParallel:(NSArray *)blocks completion:(CompletionBlock)completion {
    AtomicCounter *completionCounter = [[AtomicCounter alloc] initWithValue:blocks.count];
    for (AsyncBlock block in blocks) {
        block(^{
            if ([completionCounter decrementAndGet] == 0) {
                if (completion) completion();
            }
        });
    }
    if (blocks.count == 0) {
        if (completion) completion();
    }
}

---

NSMutableArray *asyncBlocks = [NSMutableArray array];
for (PFObject *pictureObject in objects){
    [asyncBlocks addObject:^(CompletionBlock completion) {
        PFFile *imageFile = [pictureObject objectForKey:@"image"];
        NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
        NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];

        [tokenImageView setImageWithURLRequest:imageRequest placeholderImage:nil success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
           [self.downloadedUIImages addObject:image]; //This is a mutableArray that will later be set to an UIImageView's animnationImages
        } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
            NSLog(@"Error %@", error);
        } completion:completion];
    }];
}
[BlockRunner runBlocksInParallel:[asyncBlocks copy] completion:^{
    //Do your final completion here!
}];

Answer 4

Set up a property and initialize it to the number of cycles - objects.count. In the completion of the block, lower the number down. When you reach zero, you are done.