Completion blocks in for loop - ios

Question

Currently I am trying to do some async and concurrent tasks, and I am using Azures blob to upload all the images, however the concern is that, for every blob I need to get a SASURL and then upload the images. Also the another side towards it is that I want to have all the operations of the images completed to be uploaded, and hence send a final upload to the database. Although I can send the operation to the database earlier, without having the confirmation of the images completed, but I just wanted to make sure, that the operation does gets completed.
Below is the code for the SASURL block.
- (void)storageServiceBlob:(NSArray*)images
{
StorageService *storageService = [StorageService getInstance];
NSLog(#"%#",[storageService containers]);
NSLog(#"%#",[storageService blobs]);
for (int i = 0; i < [images count]; i++) {
NSString *file_name = [images objectAtIndex:i];
NSString *result = [self imageName:file_name];
NSLog(#"Final: %#", result);
[storageService getSasUrlForNewBlob:result forContainer:#"misccontainer" withCompletion:^(NSString *sasUrl) {
NSLog(#"%#",sasUrl);
[self postBlobWithUrl:sasUrl Image:[images objectAtIndex:i]];
}];
}
}
I want to use gcd in group somehow to determine that after all the completion blocks is called in a group, it executes Post method. Is there anyway to do this in gcd?

Answer 1

There are many ways you could do this. Here's one:
- (void)storageServiceBlob:(NSArray *)imageFilenames
{
StorageService *storageService = [StorageService getInstance];
__block NSMutableSet *remainingImageFilenames = [NSMutableSet setWithArray:imageFilenames];
for (NSString *imageFilename in imageFilenames) {
NSString *imageName = [self imageNameForImageFilename:imageFilename];
[storageService getSasUrlForNewBlob:imageName forContainer:#"misccontainer" withCompletion:^(NSString *sasUrl) {
[self postBlobWithUrl:sasUrl imageFilename:imageFileName];
[remainingImageFilenames removeObject:imageFilename];
if ([remainingImageFilenames count] == 0) {
// you're done, do your thing
}
}];
}
}
A few tips:
Be careful with your naming. There seems to be some ambiguity there.
Generally, idiomatic method name parameters start with a lower-case letter, e.g. myMethodWithThis:andThat:, not MyMethodWithThis:AndThat:.
Fast enumeration, e.g. for (id obj in array) is your friend. Learn and use it.
You can shortcut [array objectAtIndex:1] as array[1].

Answer 2

If you have access to the queue that the requests are going in then you can issue a barrier block.
When you have an async queue a barrier block will sit and wait to be executed until all of the blocks issued before it have run.
If you don't have access to the queue then your best bet is to keep a count.