Rx系列：RxJava之4種Subject

4種Subject 分別爲：PublishSubject、ReplaySubject、BehaviorSubject、AsyncSubject

Subject將觀察者和被觀察者結合爲一體，是觀察者和被觀察者的代理。

（1）AsyncSubject

    AsyncSubject asyncSubject = AsyncSubject.create();
    asyncSubject.onNext("A");
    asyncSubject.onNext("B");
    asyncSubject.subscribe(new Consumer<String>() {
        @Override
        public void accept(String s) throws Exception {
            System.out.println("接收發射過來的數據："+s);
        }
    }, new Consumer<Throwable>() {
        @Override
        public void accept(Throwable throwable) throws Exception {

        }
    }, new Action() {
        @Override
        public void run() throws Exception {
            System.out.println("執行完成！");
        }
    });

    asyncSubject.onNext("C");
    asyncSubject.onComplete();

日誌如下：

也就是說，AsyncSubject 無論發射多少條數據，無論在訂閱前發射還是在訂閱後發射，都只會收到最後一條發射的數據。

（2）BehaviorSubject

    BehaviorSubject behaviorSubject = BehaviorSubject.create();
    behaviorSubject.onNext("A");
    behaviorSubject.onNext("B");
    behaviorSubject.subscribe(new Consumer<String>() {
        @Override
        public void accept(String s) throws Exception {
            System.out.println("接收發射過來的數據："+s);
        }
    }, new Consumer<Throwable>() {
        @Override
        public void accept(Throwable throwable) throws Exception {

        }
    }, new Action() {
        @Override
        public void run() throws Exception {
            System.out.println("執行完成！");
        }
    });

    behaviorSubject.onNext("C");
    behaviorSubject.onComplete();

日誌如下：

也就是說，BehaviorSubjec只會接收到訂閱前最後一條發射的數據以及訂閱之後所有的數據。

（3）ReplaySubject

    ReplaySubject replaySubject = ReplaySubject.create();
    replaySubject.onNext("A");
    replaySubject.onNext("B");
    replaySubject.subscribe(new Consumer<String>() {
        @Override
        public void accept(String s) throws Exception {
            System.out.println("接收發射過來的數據："+s);
        }
    }, new Consumer<Throwable>() {
        @Override
        public void accept(Throwable throwable) throws Exception {

        }
    }, new Action() {
        @Override
        public void run() throws Exception {
            System.out.println("執行完成！");
        }
    });

    replaySubject.onNext("C");
    replaySubject.onComplete();

日誌如下：

也就是說，ReplaySubject會接收到全部數據。

這裏需要補充一下：
ReplaySubject可以限制緩存的大小，也可以限制緩存的時間

    ReplaySubject replaySubject = ReplaySubject.createWithSize(1);
    ReplaySubject replaySubject = ReplaySubject.createWithTime(1000, TimeUnit.MILLISECONDS, Schedulers.newThread());
    ReplaySubject replaySubject = ReplaySubject.createWithTimeAndSize(1000, TimeUnit.MILLISECONDS, Schedulers.newThread(), 1);

• createWithSize 限制可以接收訂閱前發射數據的條數；
• createWithTime 限制時間內可以接收訂閱前發射的數據；
• createWithTimeAndSize 限制時間內可以接收訂閱前發射數據的條數；

（4）PublishSubject

    PublishSubject publishSubject = PublishSubject.create();

    publishSubject.onNext("A");
    publishSubject.onNext("B");
    publishSubject.subscribe(new Consumer<String>() {
        @Override
        public void accept(String s) throws Exception {
            System.out.println("接收發射過來的數據："+s);
        }
    }, new Consumer<Throwable>() {
        @Override
        public void accept(Throwable throwable) throws Exception {

        }
    }, new Action() {
        @Override
        public void run() throws Exception {
            System.out.println("執行完成！");
        }
    });

    publishSubject.onNext("C");
    publishSubject.onComplete();

日誌如下：

也就是說，PublishSubject只會接收到訂閱之後的所有數據。

參考：https://www.jianshu.com/p/117fcda8df97