RACSubject的子類，說這個主要為了下篇文章，重寫了RACSubject的四個個重要方法：subscribe，sendNext，sendCompleted， sendError

- (void)sendNext:(id)value {
    @synchronized (self) {
//用數(shù)組存了下sendNext的值
        [self.valuesReceived addObject:value ?: RACTupleNil.tupleNil];
        [super sendNext:value];
//這個數(shù)組有個額度，當超過這個額度，就把前面的清掉。邏輯簡單 
        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的狀態(tài)
        self.hasCompleted = YES;
        [super sendCompleted];
    }
}

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

好的，這時候如果有人訂閱了這個信號，看看subscribe的操作

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

    RACDisposable *schedulingDisposable = [RACScheduler.subscriptionScheduler schedule:^{
        @synchronized (self) {
//好的。如果這時候以前已經(jīng)被訂閱過了，那valuesReceived里面就有值了，就不需要要再走一次didsubribe了，直接sendNext了。這個還是很方便的，如果didsubribe里面有網(wǎng)絡(luò)請求，那多次訂閱不使用這個就會發(fā)很多次請求。
            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 {
//如果沒被訂閱過，那就老老實實的走一次didsubscriber
                RACDisposable *subscriptionDisposable = [super subscribe:subscriber];
                [compoundDisposable addDisposable:subscriptionDisposable];
            }
        }
    }];

    [compoundDisposable addDisposable:schedulingDisposable];

    return compoundDisposable;
}