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

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 would like to know when all the blocks inside my loops have finished executing so I could begin the animation, and was wondering how I may be able to tell when they are finished completing? Thanks!
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
} failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
NSLog(#"Error %#", error);
}];
}
//When I know all the blocks have finished downloading, I will then to animate the downloaded images.
Edit: having issue with Error -999
I am encountering the following issue when executing the code in the provided answer: Domain=NSURLErrorDomain Code=-999 "The operation couldn’t be completed. (NSURLErrorDomain error -999.)"
A quick search reveals that Error -999 means "another request is made before the previous request is completed" ... which is certainly the case here since I am making several requests in quick succession. The recommended fix suggested here didn't work for me as it will only successfully download one UIImage (the last one requested) , with the previous ones failing. I was wondering if there is workaround here or in AFNetworking that I ought to consider? Thanks!
Edit 2: working code based on #David's solution
for (PFObject *pictureObject in objects){
PFFile *imageFile = [pictureObject objectForKey:#"image"];
NSURL *imageFileURL = [[NSURL alloc] initWithString:imageFile.url];
NSURLRequest *imageRequest = [NSURLRequest requestWithURL:imageFileURL];
AFHTTPRequestOperation *requestOperation = [[AFHTTPRequestOperation alloc] initWithRequest:imageRequest];
requestOperation.responseSerializer = [AFImageResponseSerializer serializer];
dispatch_group_enter(group);
[requestOperation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
NSLog(#"Response: %#", responseObject);
UIImage *retrivedImage = (UIImage *)responseObject;
[self.downloadedUIImages addObject:retrivedImage];
dispatch_group_leave(group);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
NSLog(#"Image error: %#", error);
dispatch_group_leave(group);
}];
[requestOperation start];
counter ++;
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(#"Horray everything has completed");
NSLog(#"What is here %#", self.downloadedUIImages);
NSLog(#"Done");
});

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

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 3

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.

Answer 4

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.
}