RACSubject

1.RACSubject继承RACSignal，与RACSignal不同的是，RACSubject自身就可以信号的创建，订阅与发送

    //创建信号
    RACSubject *subject = [RACSubject subject];

   //订阅信号
    [subject subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@", x);
    }];
    
  //发送信号
    [subject sendNext:@"信号"];

2.RACReplaySubject继承RACSubject，与RACSubject不同，RACReplaySubject关心历史信号，会进行输出

    RACReplaySubject *subject =[RACReplaySubject subject];
    [subject sendNext:@"RACReplaySubject"];
    [subject subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@", x);
    }];
    
    RACSubject *subject2 = [RACSubject subject];
    [subject2 sendNext:@"RACSubject"];
    [subject2 subscribeNext:^(id  _Nullable x) {
        NSLog(@"%@", x);
    }];

运行结果：

可见RACSubject对订阅前的信号进行了忽略。