[移动开发]RxJava 两种生产和消费模式，（冷）cold和（热）hot

RxJava目前有两种发布和订阅模式

第一种 cold模式，这种模式观察者订阅被观察模式时，被观察者的动作会重放，举例说明：

@NonNull
		Flowable<@NonNull Object> observeOn =Flowable.create(e -> {
			e.onNext(1); Thread.sleep(1000);
			e.onNext(2); Thread.sleep(1000);
			e.onNext(3); Thread.sleep(1000);
			e.onNext(4); Thread.sleep(1000);
			e.onNext(5); Thread.sleep(1000);
			e.onNext(6); Thread.sleep(1000);
		}, BackpressureStrategy.BUFFER).observeOn(Schedulers.computation());
		Thread.sleep(1000 * 1);
		observeOn.subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		Thread.sleep(1000 * 1);
		observeOn.subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));

这种会大音两次1，2，3，4，5，6，即使时不同的线程，第二次会等第一次完成过后开始（因为未设置subscribeOn，所以是单线程的，第二次会在第一次完成后再开始）。而且是当观察者订阅被观察者的时候触发被观察者的动作。

RxComputationThreadPool-1 + next + 1
RxComputationThreadPool-1 + next + 2
RxComputationThreadPool-1 + next + 3
RxComputationThreadPool-1 + next + 4
RxComputationThreadPool-1 + next + 5
RxComputationThreadPool-1 + next + 6
RxComputationThreadPool-2 + next + 1
RxComputationThreadPool-2 + next + 2
RxComputationThreadPool-2 + next + 3
RxComputationThreadPool-2 + next + 4
RxComputationThreadPool-2 + next + 5
RxComputationThreadPool-2 + next + 6

我们create中lambda的方法会再次运行

相似的ReplaySubject

还有一种当订阅后会重新处理已发送的数据ReplaySubject

ReplaySubject<Integer> replaySubject = ReplaySubject.create(1);
		replaySubject.onNext(1);
		replaySubject.onNext(2);
		replaySubject.onNext(3);
		System.out.println("subscribe 1");
		replaySubject.observeOn(Schedulers.computation()).subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		replaySubject.onNext(4);
		replaySubject.onNext(5);
		System.out.println("subscribe 2");
		replaySubject.observeOn(Schedulers.computation()).subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		replaySubject.onNext(6);

控制台输出为

subscribe 1
subscribe 2
RxComputationThreadPool-1 + next + 1
RxComputationThreadPool-1 + next + 2
RxComputationThreadPool-1 + next + 3
RxComputationThreadPool-1 + next + 4
RxComputationThreadPool-1 + next + 5
RxComputationThreadPool-1 + next + 6
RxComputationThreadPool-2 + next + 1
RxComputationThreadPool-2 + next + 2
RxComputationThreadPool-2 + next + 3
RxComputationThreadPool-2 + next + 4
RxComputationThreadPool-2 + next + 5
RxComputationThreadPool-2 + next + 6

但是Flowable.create和ReplaySubject的模式不太一样。
对于Flowable来说，新的subscribe来临时，会重新执行create方法里面的FlowableOnSubscribe的apply方法。
对于ReplaySubject是把前面onNext()的数据保存到list中，然后新的subscribe来临时重新遍历list消费。这里需要注意ReplaySubject有内存泄漏的风险。见io.reactivex.rxjava3.subjects.ReplaySubject.buffer。

第二种是HOT模式。

1、使用cold + publish()方法修改cold为hot。

@NonNull
		ConnectableFlowable<@NonNull Object> publish = Flowable.create(e -> {
			e.onNext(1); Thread.sleep(1000);System.out.println(1+ " " + Thread.currentThread().getName());
			e.onNext(2); Thread.sleep(1000);System.out.println(2+ " " + Thread.currentThread().getName());
			e.onNext(3); Thread.sleep(1000);System.out.println(3+ " " + Thread.currentThread().getName());
			e.onNext(4); Thread.sleep(1000);System.out.println(4+ " " + Thread.currentThread().getName());
			e.onNext(5); Thread.sleep(1000);System.out.println(5+ " " + Thread.currentThread().getName());
			e.onNext(6); Thread.sleep(1000);System.out.println(6+ " " + Thread.currentThread().getName());
		}, BackpressureStrategy.BUFFER).subscribeOn(Schedulers.computation()).publish();
		System.out.println("connect");
		publish.connect();
		System.out.println("subscribe 1");
		publish.subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		Thread.sleep(3000 * 1);
		System.out.println("subscribe 2");
		publish.observeOn(Schedulers.computation()).subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));

下面是输出,可以看到没有出现重放。

connect
subscribe 1
main + next + 1
1 RxComputationThreadPool-1
RxComputationThreadPool-1 + next + 2
2 RxComputationThreadPool-1
RxComputationThreadPool-1 + next + 3
3 RxComputationThreadPool-1
RxComputationThreadPool-1 + next + 4
subscribe 2
4 RxComputationThreadPool-1
RxComputationThreadPool-1 + next + 5
RxComputationThreadPool-2 + next + 5
5 RxComputationThreadPool-1
RxComputationThreadPool-1 + next + 6
RxComputationThreadPool-2 + next + 6
6 RxComputationThreadPool-1

2、或者使用Subject对象
这里使用了PublishSubject，他的观察者只处理订阅过后的数据。subject包含了其他类型的对象，可以参考RxJava 的 Subject

		PublishSubject<Integer> publishSubject = PublishSubject.create();
		publishSubject.onNext(1);
		publishSubject.onNext(2);
		publishSubject.onNext(3);
		System.out.println("subscribe 1");
		publishSubject.observeOn(Schedulers.computation()).subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		publishSubject.onNext(4);
		publishSubject.onNext(5);
		System.out.println("subscribe 2");
		publishSubject.observeOn(Schedulers.computation()).subscribe(e -> System.out.println(Thread.currentThread().getName() + " + next + " + e));
		publishSubject.onNext(6);

执行过后的打印

subscribe 1
subscribe 2
RxComputationThreadPool-1 + next + 4
RxComputationThreadPool-1 + next + 5
RxComputationThreadPool-1 + next + 6
RxComputationThreadPool-2 + next + 6