冷热rx-java可观察

我自己对“热的和冷的可观察的”的理解是不稳定的，但这是我到目前为止所了解的！

冷观测

考虑一个返回rx-java Observable的API：

import obs.Util;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import rx.Observable;
import rx.schedulers.Schedulers;public class Service1 {private static final Logger logger = LoggerFactory.getLogger(Service1.class);public Observable<String> operation() {return Observable.<String>create(s -> {logger.info("Start: Executing slow task in Service 1");Util.delay(1000);s.onNext("data 1");logger.info("End: Executing slow task in Service 1");s.onCompleted();}).subscribeOn(Schedulers.computation());}
}

现在，首先要注意的是，典型的Observable在订阅之前不会做任何事情：

所以基本上，如果我要这样做：

Observable<String> op1 = service1.operation();

除非通过以下方式在Observable上进行订阅，否则不会打印或返回任何内容：

Observable<String> op1 = service1.operation();CountDownLatch latch = new CountDownLatch(1);op1.subscribe(s -> logger.info("From Subscriber 1: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());latch.await();

因此，现在，如果此Observable上有多个订阅，将会发生什么：

Observable<String> op1 = service1.operation();CountDownLatch latch = new CountDownLatch(3);op1.subscribe(s -> logger.info("From Subscriber 1: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());op1.subscribe(s -> logger.info("From Subscriber 2: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());op1.subscribe(s -> logger.info("From Subscriber 3: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());latch.await();

有了冷的可观察到的代码，代码将再次被调用并再次发出项目，我在机器上得到了这个代码：

06:04:07.206 [RxComputationThreadPool-2] INFO  o.b.Service1 - Start: Executing slow task in Service 1
06:04:07.208 [RxComputationThreadPool-3] INFO  o.b.Service1 - Start: Executing slow task in Service 1
06:04:08.211 [RxComputationThreadPool-2] INFO  o.b.BasicObservablesTest - From Subscriber 2: data 1
06:04:08.211 [RxComputationThreadPool-1] INFO  o.b.BasicObservablesTest - From Subscriber 1: data 1
06:04:08.211 [RxComputationThreadPool-3] INFO  o.b.BasicObservablesTest - From Subscriber 3: data 1
06:04:08.213 [RxComputationThreadPool-2] INFO  o.b.Service1 - End: Executing slow task in Service 1
06:04:08.214 [RxComputationThreadPool-1] INFO  o.b.Service1 - End: Executing slow task in Service 1
06:04:08.214 [RxComputationThreadPool-3] INFO  o.b.Service1 - End: Executing slow task in Service 1

热可观察–使用ConnectableObservable

另一方面，Hot Observable确实不需要订阅即可开始发射项目。一种实现Hot Observable的方法是使用ConnectableObservable ，它是一个Observable，它在调用connect方法之前不会发出项目，但是一旦开始发出项目，它的任何订阅者只能从订阅点获取项目。因此，再次回顾前面的示例，但使用ConnectableObservable代替：

Observable<String> op1 = service1.operation();ConnectableObservable<String> connectableObservable =  op1.publish();CountDownLatch latch = new CountDownLatch(3);connectableObservable.subscribe(s -> logger.info("From Subscriber 1: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());connectableObservable.subscribe(s -> logger.info("From Subscriber 2: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());connectableObservable.subscribe(s -> logger.info("From Subscriber 3: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());connectableObservable.connect();latch.await();

并打印以下内容：

06:07:23.852 [RxComputationThreadPool-3] INFO  o.b.Service1 - Start: Executing slow task in Service 1
06:07:24.860 [RxComputationThreadPool-3] INFO  o.b.ConnectableObservablesTest - From Subscriber 1: data 1
06:07:24.862 [RxComputationThreadPool-3] INFO  o.b.ConnectableObservablesTest - From Subscriber 2: data 1
06:07:24.862 [RxComputationThreadPool-3] INFO  o.b.ConnectableObservablesTest - From Subscriber 3: data 1
06:07:24.862 [RxComputationThreadPool-3] INFO  o.b.Service1 - End: Executing slow task in Service 1

热点可观察–使用主题

将冷的Observable转换为热的另一种方法是使用Subject 。主题既表现为可观察者，又表现为观察者，有不同类型的主题具有不同的行为。在这里，我使用一个名为PublishSubject的Subject，它具有Pub / Sub行为–这些项目被发送给所有在其上监听的订阅者。因此，随着PublishSubject的引入，代码如下所示：

Observable<String> op1 = service1.operation();PublishSubject<String> publishSubject = PublishSubject.create();op1.subscribe(publishSubject);CountDownLatch latch = new CountDownLatch(3);publishSubject.subscribe(s -> logger.info("From Subscriber 1: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());publishSubject.subscribe(s -> logger.info("From Subscriber 2: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());publishSubject.subscribe(s -> logger.info("From Subscriber 3: {}", s),e -> logger.error(e.getMessage(), e),() -> latch.countDown());latch.await();

了解如何将PublishSubject作为Observable的订阅者引入，而其他订阅者如何订阅PublishSubject。输出将类似于ConnectableObservable的输出。

从本质上来说，这就是我对“热可观察”的理解程度。因此，总而言之，Cold和Hot Observable之间的区别在于订户何时获得发射的项目以及何时发射项目–使用Cold Observable，它们在订阅并通常获得所有发射的项目时发射，一个Hot Observable，项目将在没有订阅服务器的情况下发出，而订阅者通常会在订阅点之后获得项目。