Why would you want a common abstraction for asynchrony? Well, compare these two. Might've gone a bit overboard, but the code was so easy with tasks I didn't realize what I was heading into when I was writing the callback hell version... Note: This would've been just as succinct using MAFuture or ReactiveCocoa or whatever. The important part is h…

async.m

// callback hell
- (void)fetch:(NSURL*)url saveWithCompletion:(void(^)())myCompletion errorHandler:(void(^)(NSError*))errorHandler
{
    [downloader fetchURL:url completion:^(NSData *data) {
        [parser parse:data completion:^(NSDictionary *parsed, NSError *err) {
            if(parsed) {
                [database save:completion^{
                    if(myCompletion)
                        myCompletion();
                }];
            } else {
                if(errorHandler) {
                    errorHandler(err);
                }
            }
        }];
    } errorHandler:^(NSError *err) {
        if(errorHandler)
            errorHandler(err);
    }];
}

- (void)fetchAllWithCompletion:(void(^)())completion error:(void(^)(NSArray*))errHandler
{
    int total = _allUrls.count;
    __block int soFar = 0;
    __block NSMutableArray *errors = [NSMutableArray new];
    for(id url in _allUrls) {
        [self fetch:url saveWithCompletion:^{
            soFar++;
            if(soFar == total) {
                if(errors.count > 0) {
                    if(errHandler) {
                        errHandler(errors);
                    }
                } else {
                    if(completion) {
                        completion();
                    }
                }
            }
        } errorHandler:^(NSError *err) {
            [errors addObject:err];
            soFar++;
            if(soFar == total) {
                if(errHandler)
                    errHandler(errors);
            }
        }];
    }
}

// SPTask
- (SPTask*)fetchAndSave:(NSURL*)url
{
    // error automatically propagates to caller.
    return [[[downloader fetchURL:url] chain:^(NSData *data) {
        return [parser parse:data];
    }] chain:^(NSDictionary *parsed) {
        return [database save];
    }];
}
- (SPTask*)fetchAll
{
    return [SPTask awaitAll:[_allUrls map:^(id url) {
        return [self fetchAndSave:url];
    }]];
}

yeah.. don't do that.
https://gist.github.com/seivan/b371906fdf405121d7a7 <- Though I don't clean up unexecuted blocks here. One way to do that is to still fire the notification but with a flag to return early. Could easily be abstracted to handle the return early another way.