RxJava2——Observable基本用例

最基本的调用

Observable.create(new ObservableOnSubscribe() {
            @Override
            public void subscribe(ObservableEmitter emitter) throws Exception {
                emitter.onNext("1");
                emitter.onNext("2");
                emitter.onNext("3");
                emitter.onNext("4");
                emitter.onNext("5");
                emitter.onComplete();

                XLog.d("subscribe over");

            }
        }).subscribe(new Observer() {
            @Override
            public void onSubscribe(Disposable d) {
                XLog.d("onSubscribe ");

            }

            @Override
            public void onNext(String s) {

                XLog.d("onNext " + s);
            }

            @Override
            public void onError(Throwable e) {
                XLog.d("onError ");

            }

            @Override
            public void onComplete() {
                XLog.d("onComplete ");

            }
        });

输出结果：基本按照顺序输出

onSubscribe
onNext 1
onNext 2
onNext 3
onNext 4
onNext 5
onComplete
subscribe over

onComplete/onError放中间

emitter.onNext("1");
emitter.onNext("2");
emitter.onNext("3");
//emitter.onComplete();
emitter.onError(new Exception());
emitter.onNext("4");
emitter.onNext("5");

输出结果：当中间有 onCompelete/onError 时，Observable会继续发送，但Obsever不会再接收事件了

onSubscribe
onNext 1
onNext 2
onNext 3
//onComplete
onError
subscribe over

连续两次调用onComplete

                emitter.onNext("1");
                emitter.onNext("2");
                emitter.onNext("3");
                emitter.onComplete();
                emitter.onComplete();
                emitter.onNext("4");
                emitter.onNext("5");

输出结果：和单次调用 onComplete一样，如果放中间，Observer将不会再接收事件，且无异常

onSubscribe
onNext 1
onNext 2
onNext 3
onComplete
subscribe over

连续两次调用onError

                emitter.onNext("1");
                emitter.onNext("2");
                emitter.onNext("3");
                emitter.onError(new Exception());
                emitter.onError(new Exception());
                emitter.onComplete();
                emitter.onNext("4");
                emitter.onNext("5");
                XLog.d("subscribe over");

输出结果：收到onError事件便截止

onSubscribe
onNext 1
onNext 2
onNext 3
onError

除此之外，连续 2 次调用 onError(),还报出了一个 UndeliverableException

09-22 12:45:18.181 8830-8830/www.ly.com.rxjavademo W/System.err: io.reactivex.exceptions.UndeliverableException: java.lang.Exception
        at io.reactivex.plugins.RxJavaPlugins.onError(RxJavaPlugins.java:367)
        at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onError(ObservableCreate.java:74)
        at www.ly.com.rxjavademo.MainActivity$2.subscribe(MainActivity.java:73)
        at io.reactivex.internal.operators.observable.ObservableCreate.subscribeActual(ObservableCreate.java:40)
        at io.reactivex.Observable.subscribe(Observable.java:12005)
        at www.ly.com.rxjavademo.MainActivity.onCreate(MainActivity.java:81)

onError 和 onComplete 同时出现

                emitter.onNext("1");
                emitter.onNext("2");
                emitter.onNext("3");
                emitter.onComplete();
                emitter.onError(new Exception());
                emitter.onNext("4");
                emitter.onNext("5");

输出结果：输出到onComplete截止

onSubscribe
onNext 1
onNext 2
onNext 3
onComplete

除此之外，同时也收到了 UndeliverableException 而终止进程

09-22 13:10:32.738 14353-14353/www.ly.com.rxjavademo W/System.err: io.reactivex.exceptions.UndeliverableException: java.lang.Exception
        at io.reactivex.plugins.RxJavaPlugins.onError(RxJavaPlugins.java:367)
09-22 13:10:32.739 14353-14353/www.ly.com.rxjavademo W/System.err:     at io.reactivex.internal.operators.observable.ObservableCreate$CreateEmitter.onError(ObservableCreate.java:74)
        at www.ly.com.rxjavademo.MainActivity$2.subscribe(MainActivity.java:73)
        at io.reactivex.internal.operators.observable.ObservableCreate.subscribeActual(ObservableCreate.java:40)
        at io.reactivex.Observable.subscribe(Observable.java:12005)
        at www.ly.com.rxjavademo.MainActivity.onCreate(MainActivity.java:81)

但如果把 onError 放在 onComplete 前面呢？则输出结果为：

onSubscribe
onNext 1
onNext 2
onNext 3
onError
subscribe over

说明onError 之前不能有 onComplete / onError 事件发生。

指定调度线程

Observable.create(new ObservableOnSubscribe() {
            @Override
            public void subscribe(ObservableEmitter emitter) throws Exception {
                emitter.onNext("1");
                emitter.onNext("2");
                emitter.onNext("3");
                emitter.onNext("4");
                emitter.onNext("5");
                XLog.d("subscribe over");

            }
        })
        .subscribeOn(Schedulers.newThread())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(new Observer() {
            @Override
            public void onSubscribe(Disposable d) {
                XLog.d("onSubscribe ");

            }

            @Override
            public void onNext(String s) {

                XLog.d("onNext " + s);
            }

            @Override
            public void onError(Throwable e) {
                XLog.d("onError ");

            }

            @Override
            public void onComplete() {
                XLog.d("onComplete ");

            }
        });

输出结果：被观察者在新的线程中执行发送事件，观察者在Android主线程中接收事件

Thread: main
onSubscribe
Thread: RxNewThreadScheduler-1
subscribe over
Thread: main
onNext 1
Thread: main
onNext 2
Thread: main
onNext 3
Thread: main
onNext 4
Thread: main
onNext 5

压力测试

被观察者限制快速发送数据，观察察者每休息 5 秒钟处理一个 onNext 事件

Observable.create(new ObservableOnSubscribe() {
            @Override
            public void subscribe(ObservableEmitter emitter) throws Exception {
                int i = 0;
                for (;;) {
                    emitter.onNext("send " + (i++));
                }

            }
        })
        .subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(new Observer() {
            @Override
            public void onSubscribe(Disposable d) {
                XLog.d("onSubscribe ");

            }

            @Override
            public void onNext(String s) {

                XLog.d("onNext " + s);
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onError(Throwable e) {
                XLog.d("onError ");

            }

            @Override
            public void onComplete() {
                XLog.d("onComplete ");

            }
        });

输出结果：内存溢出！！！在RxJava2中Observable是不支持背压的

09-22 13:36:52.613 20160-20169/www.ly.com.rxjavademo W/art: Throwing OutOfMemoryError "Failed to allocate a 28 byte allocation with 1864 free bytes and 1864B until OOM; failed due to fragmentation (required continguous free 4096 bytes for a new buffer where largest contiguous free 0 bytes)" (recursive case)

总结

正常情况下事件会按照预期逐个由被观察者发送给观察者
理论上来说，onComplete 和 onError 是互斥的，我们应该控制其只发送一个
onComplete / onError 一旦发送，被观察者还会继续发送事件，但观察者将不会再接收到任何事件
onComplete 可以发送多次，以收到第一次为结束，且不会有异常
onError 只能被发送一次，并且不能在有 onComplete 之后再发送 onError ，否则会报 UndeliverableException
被观察者发送事件的速度远远超过观察者处理的速度，最终会造成 OOM。原因是 Observable 不支持背压。