RACSubject的子类，说这个主要为了下篇文章，重写了RACSubject的四个个重要方法：subscribe，sendNext，sendCompleted， sendError

- (void)sendNext:(id)value {
    @synchronized (self) {
//用数组存了下sendNext的值
        [self.valuesReceived addObject:value ?: RACTupleNil.tupleNil];
        [super sendNext:value];
//这个数组有个额度，当超过这个额度，就把前面的清掉。逻辑简单 
        if (self.capacity != RACReplaySubjectUnlimitedCapacity && self.valuesReceived.count > self.capacity) {
            [self.valuesReceived removeObjectsInRange:NSMakeRange(0, self.valuesReceived.count - self.capacity)];
        }
    }
}

- (void)sendCompleted {
    @synchronized (self) {
//存下commplete的状态
        self.hasCompleted = YES;
        [super sendCompleted];
    }
}

- (void)sendError:(NSError *)e {
    @synchronized (self) {
//存下error的状态
        self.hasError = YES;
        self.error = e;
        [super sendError:e];
    }
}

好的，这时候如果有人订阅了这个信号，看看subscribe的操作

- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    RACCompoundDisposable *compoundDisposable = [RACCompoundDisposable compoundDisposable];

    RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{
        @synchronized (self) {
//好的。如果这时候以前已经被订阅过了，那valuesReceived里面就有值了，就不需要要再走一次didsubribe了，直接sendNext了。这个还是很方便的，如果didsubribe里面有网络请求，那多次订阅不使用这个就会发很多次请求。
            for (id value in self.valuesReceived) {
                if (compoundDisposable.disposed) return;

                [subscriber sendNext:(value == RACTupleNil.tupleNil ? nil : value)];
            }

            if (compoundDisposable.disposed) return;
            if (self.hasCompleted) {
                [subscriber sendCompleted];
            } else if (self.hasError) {
                [subscriber sendError:self.error];
            } else {
//如果没被订阅过，那就老老实实的走一次didsubscriber
                RACDisposable *subscriptionDisposable = [super subscribe:subscriber];
                [compoundDisposable addDisposable:subscriptionDisposable];
            }
        }
    }];

    [compoundDisposable addDisposable:schedulingDisposable];

    return compoundDisposable;
}