Good practices for tracking multiple REST call completions within one function - iphone

Question

I am using the MKNetworkKit library to make REST calls to a server. This particular snippet sets is called when a user wants to make a photo favorite or not favorite. It is pretty straight forward objective-c block code if we are only setting favorite flag for a single photo. However I have impelmented a multiselect mechanism in the GUI so that a user can select several photos and favorite them all at once.
The function I wrote to do this works just fine, but it just doesn't feel very clean to me.
What I dislike:
Tracking all of the callbacks with a block counter. I am wondering if there is a more elegant way to handle this.
The same code exists in both completion blocks. However that is just how MKNetworkKit is used (one block for success, one for error). I suppose if I made this an instance method, I could handle it by calling another i-method, but that seems just as messy to do all the setup. I'd like to keep this as a handy class method utility.
Suggestions?
+(BOOL)updateAssets:(NSArray*)assets
isFavorite:(BOOL)isFavorite
completion:(MyAssetCompletion)completion // (BOOL success)
{
assert(assets);
if (assets == nil || assets.count == 0) return NO;
__block BOOL bError = NO;
__block NSInteger counter = 0; // Use a counter to track number of completed REST calls
for(MyAsset *asset in assets){
MyUpdateAssetForm *form = [[MyUpdateAssetForm alloc] init];
form.isPrivate = isFavorite ? #(1) : #(0);
[[MyRESTEngine sharedInstance] updateAssetWithUUID:asset.UUID
withForm:form
completionBlock:^{
counter++;
if(counter == assets.count){
completion(bError == NO);
}
} errorBlock:^(NSError *error, NSString *additionalInfo) {
bError = bError || YES;
counter++;
if(counter == assets.count){
completion(bError == NO);
}
}];
}
return YES;
}

Answer 1

You can try this (untested):
(By the way, your current implementation is not thread safe as the counter might be incremented from more then one thread in a non-atomic manner)
#see NSConditionLock
- (BOOL) updateAssets:(NSArray*)assets
isFavorite:(BOOL)isFavorite
completion:(bool_block_t)completion
{
assert(assets && [assets count]);
__block NSMutableArray* failed = [NSMutableArray new];
__block NSConditionLock* lock = [[NSConditionLock alloc] initWithCondition:[assets count]];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
^{
[lock lockWhenCondition:0];
completion([failed count] == 0);
});
NSNumber* val = (isFavorite ? #1 : #0);
for (MyAsset* asset in assets) {
MyUpdateAssetForm* form = [[MyUpdateAssetForm alloc] init];
form.isPrivate = val;
[self updateAssetWithUUID:asset.UUID
withForm:form
completionBlock:^{[lock unlockWithCondition:[lock condition] - 1];}
errorBlock:^(NSError *error, NSString *additionalInfo)
{
[lock lock];
[failed addObject:asset];
[lock unlockWithCondition:[lock condition] - 1];
}];
}
return YES;
}