Fetch JSON from URL with RAC

JSON_RAC.m

- (RACSignal *)fetchJSONFromURL:(NSURL *)url {
    
    NSParameterAssert(url);
    
    return [[RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
        
        NSURLSession * session = [NSURLSession sharedSession];
        NSURLSessionDataTask * dataTask = [session dataTaskWithURL:url completionHandler:^(NSData * data, NSURLResponse * response, NSError * error) {
            
            if (data) {
                NSError * jsonError = nil;
                id json = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&jsonError];
                
                if (json) {
                    [subscriber sendNext:json];
                }
                else {
                    [subscriber sendError:jsonError];
                }
            }
            else {
                [subscriber sendError:error];
            }
            
            [subscriber sendCompleted];
        }];
        
        [dataTask resume];
        
        return [RACDisposable disposableWithBlock:^{
            [dataTask cancel];
        }];
        
    }] doError:^(NSError * error) {
        NSLog(@"%@", error);
    }];
}