A ReactiveCocoa 2.x `-shareWhileActive` backport

RACSignal+ShareWhileActiveBackport.m

- (RACSignal *)shareWhileActive {
    NSRecursiveLock *lock = [[NSRecursiveLock alloc] init];
    lock.name = @"com.github.ReactiveCocoa.shareWhileActive";

    // These should only be used while `lock` is held.
    __block NSUInteger subscriberCount = 0;
    __block RACDisposable *underlyingDisposable = nil;
    __block RACReplaySubject *inflightSubscription = nil;

    return [[RACSignal
        createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
            __block RACSignal*     signal     = nil;
            __block RACDisposable* disposable = nil;

            [lock lock];
            @onExit {
                [lock unlock];
                disposable = [signal subscribe:subscriber];
            };

            if (subscriberCount++ == 0) {
                // We're the first subscriber, so create the underlying
                // subscription.
                inflightSubscription = [RACReplaySubject subject];
                underlyingDisposable = [self subscribe:inflightSubscription];
            }

            signal = inflightSubscription;

            return [RACDisposable disposableWithBlock:^{
                [disposable dispose];

                [lock lock];
                @onExit {
                    [lock unlock];
                };

                NSCAssert(subscriberCount > 0, @"Mismatched decrement of subscriberCount (%lu)", (unsigned long)subscriberCount);
                if (--subscriberCount == 0) {
                    // We're the last subscriber, so dispose of the
                    // underlying subscription.
                    [underlyingDisposable dispose];
                    underlyingDisposable = nil;

                    // Also, release the inflightSubscription, since
                    // we won't need its stored values any longer.
                    inflightSubscription = nil;
                }
            }];
        }]
        setNameWithFormat:@"[%@] -shareWhileActive", self.name];
}