细说ReactiveCocoa的冷信号与热信号（三）：怎么处理冷信号与热信号

第一篇文章中我们介绍了冷信号与热信号的概念，前一篇文章我们也讨论了为什么要区分冷信号与热信号，下面我会先为大家揭晓热信号的本质，再给出冷信号转换成热信号的方法。

揭示热信号的本质

在ReactiveCocoa中，究竟什么才是热信号呢？冷信号是比较常见的，map一下就会得到一个冷信号。但在RAC中，好像并没有“hot signal”这个单独的说法。原来在RAC的世界中，所有的热信号都属于一个类——RACSubject。接下来我们来看看究竟它为什么这么“神奇”。

在RAC2.5文档的框架概述中，有着这样一段描述：

A subject, represented by the RACSubject class, is a signal that can be manually controlled.

Subjects can be thought of as the “mutable” variant of a signal, much like NSMutableArray is for NSArray. They are extremely useful for bridging non-RAC code into the world of signals.

For example, instead of handling application logic in block callbacks, the blocks can simply send events to a shared subject instead. The subject can then be returned as a RACSignal, hiding the implementation detail of the callbacks.

Some subjects offer additional behaviors as well. In particular, RACReplaySubject can be used to buffer events for future subscribers, like when a network request finishes before anything is ready to handle the result.

从这段描述中，我们可以发现Subject具备如下三个特点：

1. Subject是“可变”的。
2. Subject是非RAC到RAC的一个桥梁。
3. Subject可以附加行为，例如RACReplaySubject具备为未来订阅者缓冲事件的能力。

从第三个特点来看，Subject具备为未来订阅者缓冲事件的能力，那也就说明它是自身是有状态的。根据上文的介绍，Subject是符合热信号的特点的。为了验证它，我们再来做个简单实验：

    RACSubject *subject = [RACSubject subject];
    RACSubject *replaySubject = [RACReplaySubject subject];
    
    [[RACScheduler mainThreadScheduler] afterDelay:0.1 schedule:^{
        // Subscriber 1
        [subject subscribeNext:^(id x) {
            NSLog(@"Subscriber 1 get a next value: %@ from subject", x);
        }];
        [replaySubject subscribeNext:^(id x) {
            NSLog(@"Subscriber 1 get a next value: %@ from replay subject", x);
        }];
        
        // Subscriber 2
        [subject subscribeNext:^(id x) {
            NSLog(@"Subscriber 2 get a next value: %@ from subject", x);
        }];
        [replaySubject subscribeNext:^(id x) {
            NSLog(@"Subscriber 2 get a next value: %@ from replay subject", x);
        }];
    }];
    
    [[RACScheduler mainThreadScheduler] afterDelay:1 schedule:^{
        [subject sendNext:@"send package 1"];
        [replaySubject sendNext:@"send package 1"];
    }];
    
    [[RACScheduler mainThreadScheduler] afterDelay:1.1 schedule:^{
        // Subscriber 3
        [subject subscribeNext:^(id x) {
            NSLog(@"Subscriber 3 get a next value: %@ from subject", x);
        }];
        [replaySubject subscribeNext:^(id x) {
            NSLog(@"Subscriber 3 get a next value: %@ from replay subject", x);
        }];
        
        // Subscriber 4
        [subject subscribeNext:^(id x) {
            NSLog(@"Subscriber 4 get a next value: %@ from subject", x);
        }];
        [replaySubject subscribeNext:^(id x) {
            NSLog(@"Subscriber 4 get a next value: %@ from replay subject", x);
        }];
    }];
    
    [[RACScheduler mainThreadScheduler] afterDelay:2 schedule:^{
        [subject sendNext:@"send package 2"];
        [replaySubject sendNext:@"send package 2"];
    }];

按照时间线来解读一下上述代码：

1. 0s时创建subject与replaySubject这两个subject。
2. 0.1s时Subscriber 1分别订阅了subject与replaySubject。
3. 0.1s时Subscriber 2也分别订阅了subject与replaySubject。
4. 1s时分别向subject与replaySubject发送了"send package 1"这个字符串作为值。
5. 1.1s时Subscriber 3分别订阅了subject与replaySubject。
6. 1.1s时Subscriber 4也分别订阅了subject与replaySubject。
7. 2s时再分别向subject与replaySubject发送了"send package 2"这个字符串作为值。

接下来看一下输出的结果：

2015-09-28 13:35:22.855 RACDemos[13646:1269269] Start
2015-09-28 13:3