Poor man's generator using ReactiveCocoa 3.0

RAC-generator.m

static RACSignal *BodyImageURLs(NSString *postBody)
{
    NSRange (^lastRangeFromMatch)(NSTextCheckingResult *) = ^(NSTextCheckingResult *match) {
        if (match == nil) {
            return NSMakeRange(NSNotFound, 0);
        }
        else {
            return [match rangeAtIndex:match.numberOfRanges - 1];
        }
    };

    return [RACSignal create:^(id <RACSubscriber> subscriber) {
        [ImageTagRegularExpression() enumerateMatchesInString:postBody
                options:0
                  range:NSMakeRange(0, postBody.length)
             usingBlock:^(NSTextCheckingResult *match, NSMatchingFlags flags, BOOL *stop) {
                 if (subscriber.disposable.disposed) {
                     *stop = YES;
                     return;
                 }

                 NSRange imageURLRange = lastRangeFromMatch(match);

                 if (imageURLRange.location != NSNotFound) {
                     NSString *urlString = [postBody substringWithRange:imageURLRange];

                     NSURL *url = [NSURL URLWithString:urlString];
                     [subscriber sendNext:url];
                 }
           }];

        [subscriber sendCompleted];
    }];
}

RACSignal *generator = BodyImageURLs(whateverStringWithHTML);

// Usage:
NSURL *firstURL = [generator first];
// only first regexp match will be processed

NSArray *firstThreeItems = [[generator take:3] array];