rac中热信号的源头，继承于RACSignal
用法比较简单，如下：

    RACSubject *subject = [RACSubject subject];
    [subject subscribeNext:^(id x) {
        NSLog(@"订阅者1%@", x);
    }];
    
    [subject sendNext:@"subject1"];
    
    [subject subscribeNext:^(id x) {
        NSLog(@"订阅者2%@", x);
    }];
    
    [subject sendNext:@"subject2"];
2016-01-28 11:32:40.028 demo[7131:42883] 订阅者1subject1
2016-01-28 11:32:40.029 demo[7131:42883] 订阅者1subject2
2016-01-28 11:32:40.029 demo[7131:42883] 订阅者2subject2

看log可以看出racsubject和racsignal有本质的区别，不关心历史值，错过就错过了，具体原因看源码：

//初始化没什么好说的
+ (instancetype)subject {
    return [[self alloc] init];
}

- (id)init {
    self = [super init];
    if (self == nil) return nil;

    _disposable = [RACCompoundDisposable compoundDisposable];
    _subscribers = [[NSMutableArray alloc] initWithCapacity:1];
    
    return self;
}

- (void)dealloc {
    [self.disposable dispose];
}

#pragma mark Subscription

- (RACDisposable *)subscribe:(id<RACSubscriber>)subscriber {
    NSCParameterAssert(subscriber != nil);

    RACCompoundDisposable *disposable = [RACCompoundDisposable compoundDisposable];
    subscriber = [[RACPassthroughSubscriber alloc] initWithSubscriber:subscriber signal:self disposable:disposable];
//核心代码就是把订阅者加入到subscribers这个数组中
    NSMutableArray *subscribers = self.subscribers;
    @synchronized (subscribers) {
        [subscribers addObject:subscriber];
    }
    
    return [RACDisposable disposableWithBlock:^{
        @synchronized (subscribers) {
            // Since newer subscribers are generally shorter-lived, search
            // starting from the end of the list.
            NSUInteger index = [subscribers indexOfObjectWithOptions:NSEnumerationReverse passingTest:^ BOOL (id<RACSubscriber> obj, NSUInteger index, BOOL *stop) {
                return obj == subscriber;
            }];

            if (index != NSNotFound) [subscribers removeObjectAtIndex:index];
        }
    }];
}

- (void)enumerateSubscribersUsingBlock:(void (^)(id<RACSubscriber> subscriber))block {
    NSArray *subscribers;
    @synchronized (self.subscribers) {
        subscribers = [self.subscribers copy];
    }

    for (id<RACSubscriber> subscriber in subscribers) {
        block(subscriber);
    }
}

#pragma mark RACSubscriber

- (void)sendNext:(id)value {
    [self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) {
//当sendnext的时候，就会遍历这个subscribers数组，然后把值发送出去，所以对历史值不关心。
        [subscriber sendNext:value];
    }];
}

- (void)sendError:(NSError *)error {
    [self.disposable dispose];
    
    [self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) {
        [subscriber sendError:error];
    }];
}

- (void)sendCompleted {
    [self.disposable dispose];
    
    [self enumerateSubscribersUsingBlock:^(id<RACSubscriber> subscriber) {
        [subscriber sendCompleted];
    }];
}

- (void)didSubscribeWithDisposable:(RACCompoundDisposable *)d {
    if (d.disposed) return;
    [self.disposable addDisposable:d];

    @weakify(self, d);
    [d addDisposable:[RACDisposable disposableWithBlock:^{
        @strongify(self, d);
        [self.disposable removeDisposable:d];
    }]];
}