RxJava 如何接受最多 5 个项目的任何订阅

Question

In RxJAVA we can replay old entries to new subscribers. I would like to know if there is a mechanism that I can replay parametric number of entries to subscribers.

        Subscriber<String> firstSubscriber = new Subscriber<String>() {
            @Override
            public void onCompleted() {
                // TODO Auto-generated method stub
                
            }

            @Override
            public void onError(Throwable e) {
                // TODO Auto-generated method stub
                
            }

            @Override
            public void onNext(String t) {
                System.out.println("First Subscriber gets -> " + t);
            }
        };
        
        Subscriber<String> secondSubscriber = new Subscriber<String>() {
            @Override
            public void onCompleted() {
                // TODO Auto-generated method stub
                
            }

            @Override
            public void onError(Throwable e) {
                // TODO Auto-generated method stub
                
            }

            @Override
            public void onNext(String t) {
                System.out.println("Second Subscriber gets -> " + t);
            }
        };
        
        ReplaySubject<String> replaySubject = ReplaySubject.create();
        replaySubject.onNext("one");
        replaySubject.onNext("two");
        replaySubject.onNext("three");
        replaySubject.onNext("four");
        replaySubject.subscribe(firstSubscriber);
        replaySubject.onNext("five");
        replaySubject.onNext("six");
        replaySubject.subscribe(secondSubscriber);

Result:

• First Subscriber gets -> one
• First Subscriber gets -> two
• First Subscriber gets -> three
• First Subscriber gets -> four
• First Subscriber gets -> five
• First Subscriber gets -> six
• Second Subscriber gets -> one
• Second Subscriber gets -> two
• Second Subscriber gets -> three
• Second Subscriber gets -> four
• Second Subscriber gets -> five
• Second Subscriber gets -> six

What I would like to do is getting parametric number of entries. For example firstSubscriber will get the latest 2 entries when it subscribe

Answer 1

If you want fully dynamic way of limited replay, you could defer the subscription and check the ReplaySubject how many items it currently has, then skip those you don't want:

public static <T> Observable<T> replayDynamicSize(ReplaySubject<T> subject, int n) {
    return Observable.defer(() -> {
         int s = Math.max(0, subject.size() - n);
         return skip > 0 ? subject.skip(s) : subject;
    });
}

ReplaySubject<String> replaySubject = ReplaySubject.create();

Observable<String> four = replayDynamicSize(replaySubject, 4);
Observable<String> two = replayDynamicSize(replaySubject, 2);

        replaySubject.onNext("one");
        replaySubject.onNext("two");
        replaySubject.onNext("three");
        replaySubject.onNext("four");

four.subscribe(firstSubscriber);

        replaySubject.onNext("five");
        replaySubject.onNext("six");

two.subscribe(secondSubscriber);

One consideration here is that ReplaySubject should be bounded by itself to not retain every possible value if you don't plan to replay more than some amount of values.