RACSubject 與 RACReplaySubject

 RACReplaySubject *replaySubject = [RACReplaySubject subject];
    [replaySubject sendNext:@"RACReplaySubject可以先發(fā)送消息再訂閱"];
    [replaySubject subscribeNext:^(id x) {
        NSLog(@"%@", x);
    }];
    
    
    RACSubject *subject = [RACSubject subject];
    [subject subscribeNext:^(id x) {
        NSLog(@"%@", x);
    }];
    [subject sendNext:@"subject只能先訂閱再發(fā)送消息"];