observeOn()与subscribeOn()的详解
Rxjava 提供了subscribeOn()方法来用于每个observable对象的操作符在哪个线程上运行
Rxjava 提供了ObserveOn()方法来用于每个Subscriber(Observer)对象的操作符在哪个线程上运行
线程切换的时候subscribeOn()只被执行一次 。如果出现多次,那么以第一次出现是用的那个线程为准。 ObserverOnobserveOn()改变调用它之后代码的线程
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为什么多次调用subscribeOn()却只有第一个起作用?
为什么多次调用observeOn()却可以切换到不同线程
observeOn()后能不能再次调用subscribeOn()?
如果你有这些疑问,那接下来的内容必定能解决你心头的疑惑
subscribeOn()和observeOn()的区别
subscribeOn()和observeOn()都是用来切换线程用的
- subscribeOn()改变调用它之前代码的线程
- observeOn()改变调用它之后代码的线程
这里给出下面示例中用到的两个函数
//用指定的名称新建一个线程
public static Scheduler getNamedScheduler(final String name) {return Schedulers.from(Executors.newCachedThreadPool(new ThreadFactory() {@Overridepublic Thread newThread(@android.support.annotation.NonNull Runnable runnable) {return new Thread(runnable, name);}}));}//打印当前线程的名称
public static void threadInfo(String caller) {System.out.println(caller + " => " + Thread.currentThread().getName());}
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一、subscribeOn()
在讲解他的原理之前,先来一个简单的例子,有个感性认识,学起来更轻松
先说结论:subscribeOn 作用于该操作符之前的 Observable 的创建操符作以及 doOnSubscribe 操作符 ,换句话说就是 doOnSubscribe 以及 Observable 的创建操作符总是被其之后最近的 subscribeOn 控制 。没看懂不要紧,看下面代码和图你就懂了。
Observable.create(new Observable.OnSubscribe<String>() {@Overridepublic void call(Subscriber<? super String> subscriber) {threadInfo("OnSubscribe.call()");subscriber.onNext("RxJava");}}).subscribeOn(getNamedScheduler("create之后的subscribeOn")).doOnSubscribe(() -> threadInfo(".doOnSubscribe()-1")).subscribeOn(getNamedScheduler("doOnSubscribe1之后的subscribeOn")).doOnSubscribe(() -> threadInfo(".doOnSubscribe()-2")).subscribe(s -> {threadInfo(".onNext()");System.out.println(s + "-onNext");});
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结果如下:
.doOnSubscribe()-2 => main
.doOnSubscribe()-1 => doOnSubscribe1之后的subscribeOn
OnSubscribe.call() => create之后的subscribeOn
.onNext() => create之后的subscribeOn
RxJava-onNext
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3号框中的.doOnSubscribe(() -> threadInfo(“.doOnSubscribe()-2”)) 的之后由于没有subscribeOn操作符所以回调到该段代码被调用的线程(即主线程)
由于 subscribe 之前 没有 使用observeOn 指定Scheduler,所以.onNext()的线程是和OnSubscribe.call()使用相同的Scheduler 。
下面通过源码来分析一下:
1、示例代码:
Observable.create(new Observable.OnSubscribe<String>() {@Overridepublic void call(Subscriber<? super String> subscriber) {subscriber.onNext("a");subscriber.onNext("b");subscriber.onCompleted();}}).subscribeOn(Schedulers.io()).subscribe(new Observer<String>() {@Overridepublic void onCompleted() {}@Overridepublic void onError(Throwable e) {}@Overridepublic void onNext(String integer) {System.out.println(integer);}});
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运行如下:
a
b
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2、subscribeOn()源代码
public final Observable<T> subscribeOn(Scheduler scheduler) {if (this instanceof ScalarSynchronousObservable) {return ((ScalarSynchronousObservable<T>)this).scalarScheduleOn(scheduler);}return create(new OperatorSubscribeOn<T>(this, scheduler));}
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很明显,会走if之外的方法。
在这里我们可以看到,又创建了一个OperatorSubscribeOn对象,但创建时传入的参数为OperatorSubscribeOn(this,scheduler),我们看一下此对象以及其对应的构造方法
3、create()的源代码:
public static <T> Observable<T> create(OnSubscribe<T> f) {return new Observable<T>(hook.onCreate(f));}
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我们看到这个方法,使用OperatorSubscribeOn这个类,来创建一个新的Observable,那就把它叫做Observable_2,把原来的Observable叫做Observable_1
4、OperatorSubscribeOn类的源代码:
public final class OperatorSubscribeOn<T> implements OnSubscribe<T> {final Scheduler scheduler;final Observable<T> source;public OperatorSubscribeOn(Observable<T> source, Scheduler scheduler) {this.scheduler = scheduler;this.source = source;}@Overridepublic void call(final Subscriber<? super T> subscriber) {final Worker inner = scheduler.createWorker();subscriber.add(inner);inner.schedule(new Action0() {@Overridepublic void call() {final Thread t = Thread.currentThread();Subscriber<T> s = new Subscriber<T>(subscriber) {@Overridepublic void onNext(T t) {subscriber.onNext(t);}@Overridepublic void onError(Throwable e) {try {subscriber.onError(e);} finally {inner.unsubscribe();}}@Overridepublic void onCompleted() {try {subscriber.onCompleted();} finally {inner.unsubscribe();}}@Overridepublic void setProducer(final Producer p) {subscriber.setProducer(new Producer() {@Overridepublic void request(final long n) {if (t == Thread.currentThread()) {p.request(n);} else {inner.schedule(new Action0() {@Overridepublic void call() {p.request(n);}});}}});}};source.unsafeSubscribe(s);}});}
}
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- OperatorSubscribeOn类implements 了Onsubscribe接口,并实现call()方法
- OperatorSubscribeOn的构造方法,
- 保存了Observable对象,就是调用了subscribeOn()方法的Observable对象
- 并保存了Scheduler对象。
这里做个总结。
把Observable.create()创建的称之为Observable_1,OnSubscribe_1。
把subscribeOn()创建的称之为Observable_2,OnSubscribe_2
Observable_1是由示例代码的第1、2行创建的
OperatorSubscribeOn类是implements Onsubscribe接口的,所以可以当做Onsubscribe类使用。(OnSubscribe_2)
并且OnSubscribe_2中保存了Observable_1的应用,即source。(在OperatorSubscribeOn源代码的第8行)
在subscribeOn()源代码的倒数第二行,
create(new OperatorSubscribeOn<T>(this, scheduler))
返回新创建的Observable_2对象。
4.1、分析call()方法。
- inner.schedule()改变了线程,此时Action的call()运行在指定的线程中。
- 把示例代码中的Subscriber包装了一层,赋给对象S(Subscriber_2)。见上面代码21行。
- source.unsafeSubscribe(s);,
- 注意:source是Observable_1对象,这里的s就是Subscriber_2
- 因为调用过subscribeOn(Schedulers.io())后,返回Observable_2对象,所以示例代码第13行代码的subscribe()就是Observable_2.subscribe(),也就是执行OnSubscribe_2的call()方法(即OperatorSubscribeOn类的源代码的第12行)。
4.2 看一下source.unsafeSubscribe(s);(第65行)代码都做了什么
这里的source就是Observable_1,s是Subscriber_2
unsafeSubscribe()源代码:
public final Subscription unsafeSubscribe(Subscriber<? super T> subscriber) {try {// new Subscriber so onStart itsubscriber.onStart();// allow the hook to intercept and/or decoratehook.onSubscribeStart(this, onSubscribe).call(subscriber);return hook.onSubscribeReturn(subscriber);} catch (Throwable e) {// special handling for certain Throwable/Error/Exception typesExceptions.throwIfFatal(e);// if an unhandled error occurs executing the onSubscribe we will propagate ittry {subscriber.onError(hook.onSubscribeError(e));} catch (Throwable e2) {Exceptions.throwIfFatal(e2);// if this happens it means the onError itself failed (perhaps an invalid function implementation)// so we are unable to propagate the error correctly and will just throwRuntimeException r = new RuntimeException("Error occurred attempting to subscribe [" + e.getMessage() + "] and then again while trying to pass to onError.", e2);// TODO could the hook be the cause of the error in the on error handling.hook.onSubscribeError(r);// TODO why aren't we throwing the hook's return value.throw r;}return Subscriptions.unsubscribed();}}
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关键代码:
hook.onSubscribeStart(this, onSubscribe).call(subscriber);
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该方法即调用了OnSubscribe_1.call()方法。
注意,此时的call()方法在我们指定的线程中运行。起到了改变线程的作用。
对于以上线程,我们可以总结,其有如下流程:
Observable.create() : 创建了Observable_1和OnSubscribe_1;
subscribeOn(): 创建Observable_2和OperatorSubscribeOn(OnSubscribe_2),同时OperatorSubscribeOn保存了Observable_1的引用。
示例代码中的subscribe(Observer) 实际上就是调用Observable_2.subscribe(Observer):
- 调用OperatorSubscribeOn的call()。call()改变了线程的运行,并且调用了Observable_1.unsafeSubscribe(s);
- Observable_1.unsafeSubscribe(s);,该方法的实现中调用了OnSubscribe_1的call()。
这样就实现了在指定线程运行OnSubscribe的call()函数,无论我们的subscribeOn()放在哪里,他改变的是subscribe()的过程,而不是onNext()的过程。
那么如果有多个subscribeOn(),那么线程会怎样执行呢。如果按照我们的逻辑,有以下程序
Observable.just("ss") .subscribeOn(Schedulers.io()) // ----1---.subscribeOn(Schedulers.newThread()) //----2----.subscribe(new Action1<String>() {@Overridepublic void call(String s) {}});
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那么,我们根据之前的源码分析其执行逻辑。
Observable.just(“ss”),创建Observable,OnSubscribe
Observable_1.subscribeOn(Schedulers.io()):创建Observable_1,OperatorSubscribeOn_1并保存Observable的引用。
Observable_2.subscribeOn(Schedulers.newThread()):创建Observable_2,OperatorSubscribeOn_2并保存Observable_1的引用。
Observable_3.subscribe():
- 调用OperatorSubscribeOn_2.call(),改变线程为Schedulers.newThread()。
- 调用OperatorSubscribeOn_1.call(),改变线程为Schedulers.io()。
- 调用OnSubscribe.call(),此时call()运行在Schedulers.io()。
根据以上逻辑分析,会按照1的线程进行执行。
二、observeOn()
先说结论:observeOn作用于该操作符之后操作符直到出现新的observeOn操作符
举个例子:
Observable.just("RxJava").observeOn(getNamedScheduler("map之前的observeOn")).map(s -> {threadInfo(".map()-1");return s + "-map1";}).map( s -> {threadInfo(".map()-2");return s + "-map2";}).observeOn(getNamedScheduler("subscribe之前的observeOn")).subscribe(s -> {threadInfo(".onNext()");System.out.println(s + "-onNext");});
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结果如下:
.map()-1 => map之前的observeOn
.map()-2 => map之前的observeOn
.onNext() => subscribe之前的observeOn
RxJava-map1-map2-onNext
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下面通过源码来进行分析:
1、observeOn()源码
public final Observable<T> observeOn(Scheduler scheduler) {return observeOn(scheduler, RxRingBuffer.SIZE);}public final Observable<T> observeOn(Scheduler scheduler, int bufferSize) {return observeOn(scheduler, false, bufferSize);}public final Observable<T> observeOn(Scheduler scheduler, boolean delayError, int bufferSize) {if (this instanceof ScalarSynchronousObservable) {return ((ScalarSynchronousObservable<T>)this).scalarScheduleOn(scheduler);}return lift(new OperatorObserveOn<T>(scheduler, delayError, bufferSize));}
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这里引出了lift()函数
public final <R> Observable<R> lift(final Operator<? extends R, ? super T> operator) {return new Observable<R>(new OnSubscribeLift<T, R>(onSubscribe, operator));}
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关于lift的详细介绍,如果不明白lift的原理,参考这里:RxJava 之二—— Lift()详解
用OperatorObserveOn对象,创建OnSubscribeLift对象(实现了OnSubscribe接口),接着创建Observable对象。为了加以区分,这里我们把OnSubscribeLift叫做OnSubscribe_2,Observable叫做Observable_2。
2、OperatorObserveOn代码:
public final class OperatorObserveOn<T> implements Operator<T, T> {private final Scheduler scheduler;private final boolean delayError;private final int bufferSize;/*** @param scheduler the scheduler to use* @param delayError delay errors until all normal events are emitted in the other thread?*/public OperatorObserveOn(Scheduler scheduler, boolean delayError) {this(scheduler, delayError, RxRingBuffer.SIZE);}/*** @param scheduler the scheduler to use* @param delayError delay errors until all normal events are emitted in the other thread?* @param bufferSize for the buffer feeding the Scheduler workers, defaults to {@code RxRingBuffer.MAX} if <= 0*/public OperatorObserveOn(Scheduler scheduler, boolean delayError, int bufferSize) {this.scheduler = scheduler;this.delayError = delayError;this.bufferSize = (bufferSize > 0) ? bufferSize : RxRingBuffer.SIZE;}@Overridepublic Subscriber<? super T> call(Subscriber<? super T> child) {if (scheduler instanceof ImmediateScheduler) {// avoid overhead, execute directlyreturn child;} else if (scheduler instanceof TrampolineScheduler) {// avoid overhead, execute directlyreturn child;} else {ObserveOnSubscriber<T> parent = new ObserveOnSubscriber<T>(scheduler, child, delayError, bufferSize);parent.init();return parent;}}public static <T> Operator<T, T> rebatch(final int n) {return new Operator<T, T>() {@Overridepublic Subscriber<? super T> call(Subscriber<? super T> child) {ObserveOnSubscriber<T> parent = new ObserveOnSubscriber<T>(Schedulers.immediate(), child, false, n);parent.init();return parent;}};}/** Observe through individual queue per observer. */static final class ObserveOnSubscriber<T> extends Subscriber<T> implements Action0 {final Subscriber<? super T> child;final Scheduler.Worker recursiveScheduler;final NotificationLite<T> on;final boolean delayError;final Queue<Object> queue;/** The emission threshold that should trigger a replenishing request. */final int limit;// the status of the current streamvolatile boolean finished;final AtomicLong requested = new AtomicLong();final AtomicLong counter = new AtomicLong();/** * The single exception if not null, should be written before setting finished (release) and read after* reading finished (acquire).*/Throwable error;/** Remembers how many elements have been emitted before the requests run out. */long emitted;// do NOT pass the Subscriber through to couple the subscription chain ... unsubscribing on the parent should// not prevent anything downstream from consuming, which will happen if the Subscription is chainedpublic ObserveOnSubscriber(Scheduler scheduler, Subscriber<? super T> child, boolean delayError, int bufferSize) {this.child = child;this.recursiveScheduler = scheduler.createWorker();this.delayError = delayError;this.on = NotificationLite.instance();int calculatedSize = (bufferSize > 0) ? bufferSize : RxRingBuffer.SIZE;// this formula calculates the 75% of the bufferSize, rounded up to the next integerthis.limit = calculatedSize - (calculatedSize >> 2);if (UnsafeAccess.isUnsafeAvailable()) {queue = new SpscArrayQueue<Object>(calculatedSize);} else {queue = new SpscAtomicArrayQueue<Object>(calculatedSize);}// signal that this is an async operator capable of receiving this manyrequest(calculatedSize);}void init() {// don't want this code in the constructor because `this` can escape through the // setProducer callSubscriber<? super T> localChild = child;localChild.setProducer(new Producer() {@Overridepublic void request(long n) {if (n > 0L) {BackpressureUtils.getAndAddRequest(requested, n);schedule();}}});localChild.add(recursiveScheduler);localChild.add(this);}@Overridepublic void onNext(final T t) {if (isUnsubscribed() || finished) {return;}if (!queue.offer(on.next(t))) {onError(new MissingBackpressureException());return;}schedule();}@Overridepublic void onCompleted() {if (isUnsubscribed() || finished) {return;}finished = true;schedule();}@Overridepublic void onError(final Throwable e) {if (isUnsubscribed() || finished) {RxJavaHooks.onError(e);return;}error = e;finished = true;schedule();}protected void schedule() {if (counter.getAndIncrement() == 0) {recursiveScheduler.schedule(this);}}// only execute this from schedule()@Overridepublic void call() {long missed = 1L;long currentEmission = emitted;// these are accessed in a tight loop around atomics so// loading them into local variables avoids the mandatory re-reading// of the constant fieldsfinal Queue<Object> q = this.queue;final Subscriber<? super T> localChild = this.child;final NotificationLite<T> localOn = this.on;// requested and counter are not included to avoid JIT issues with register spilling// and their access is is amortized because they are part of the outer loop which runs// less frequently (usually after each bufferSize elements)for (;;) {long requestAmount = requested.get();while (requestAmount != currentEmission) {boolean done = finished;Object v = q.poll();boolean empty = v == null;if (checkTerminated(done, empty, localChild, q)) {return;}if (empty) {break;}localChild.onNext(localOn.getValue(v));currentEmission++;if (currentEmission == limit) {requestAmount = BackpressureUtils.produced(requested, currentEmission);request(currentEmission);currentEmission = 0L;}}if (requestAmount == currentEmission) {if (checkTerminated(finished, q.isEmpty(), localChild, q)) {return;}}emitted = currentEmission;missed = counter.addAndGet(-missed);if (missed == 0L) {break;}}}boolean checkTerminated(boolean done, boolean isEmpty, Subscriber<? super T> a, Queue<Object> q) {if (a.isUnsubscribed()) {q.clear();return true;}if (done) {if (delayError) {if (isEmpty) {Throwable e = error;try {if (e != null) {a.onError(e);} else {a.onCompleted();}} finally {recursiveScheduler.unsubscribe();}}} else {Throwable e = error;if (e != null) {q.clear();try {a.onError(e);} finally {recursiveScheduler.unsubscribe();}return true;} elseif (isEmpty) {try {a.onCompleted();} finally {recursiveScheduler.unsubscribe();}return true;}}}return false;}}
}
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虽然代码很长,但是也就是三部分
- 构造函数,
- 实现Operator所继承的Func1中的call()函数
- 静态内部类ObserveOnSubscriber< T>
下面来逐一分析:
因为调用Observable.等函数而需要创建的称之为Observable_1,Subscriber_1。
因为调用observeOn()而创建的称之为Observable_2,Subscriber_2
2.1、创建OperatorObserveOn对象
上面这段代码,主要功能就是创建OperatorObserveOn对象
既然是Operator,那么它的职责就是把一个Subscriber转换成另外一个Subscriber,
2.2、OperatorObserveOn对象中的call()函数返回ObserveOnSubscriber对象(Subscriber_2)
我们来看下call函数都做了什么:
ObserveOnSubscriber是一个静态类(第53行),创建一个ObserveOnSubscriber类(继承Subscriber< T>(Subscriber_2))(OperatorObserveOn代码第35行),在参数中传入Subscriber_1(即局部变量child)和scheduler(指定线程)等参数。
调用了observeOn(),在subscribe()中调用onSubscribe.call(subscriber);时,就会调用上面代码第27行的call(),结果被传入到ObserveOnSubscriber的onNext()(第118行)。(如果不明白,请看RxJava 之二—— Lift()详解)
public void onNext(final T t) {if (isUnsubscribed() || finished) {return;}if (!queue.offer(on.next(t))) {onError(new MissingBackpressureException());return;}schedule();
}
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这里做了两件事,
- 把执行的结果缓存到一个队列里,这里的on对象,不是Subscriber_1。
- 调用schedule()启动传入的线程所创建的worker
2.3、schedule()代码:
protected void schedule() {if (counter.getAndIncrement() == 0) {recursiveScheduler.schedule(this);}
}
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- recursiveScheduler 就是之前我们传入的Scheduler,就是在observeOn()传入的指定线程,例如:AndroidScheluders.mainThread()
2.4、我们看下在scheduler()中调用的call()方法代码,call()方法只能由scheduler()去调用执行
@Override
public void call() {...final Subscriber<? super T> localChild = this.child;for (;;) {...boolean done = finished;Object v = q.poll();boolean empty = v == null;if (checkTerminated(done, empty, localChild, q)) {return;}if (empty) {break;}localChild.onNext(localOn.getValue(v));...}if (emitted != 0L) {request(emitted);}
}
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OK,在Scheduler启动后, 我们在Observable.subscribe(a)传入的a就是这里的localChild(即Subscriber_1,是在第35行代码传递进来的) , 我们看到,在call中终于调用了它的onNext方法,把真正的结果传了出去,此时是工作在observeOn()指定的线程。
那么总结起来的结论就是:
- observeOn 对调用之前的序列默不关心,也不会要求之前的序列运行在指定的线程上
- observeOn 对之前的序列产生的结果先缓存起来,然后再在指定的线程上,推送给最终的subscriber
下面给出两次调用observeOn()的示意图
复杂情况
我们经常多次使用subscribeOn()切换线程,那么以后是否可以组合observeOn()和subscribeOn()达到自由切换的目的呢?
subscribeOn()改变的是subscribe()这句调用所在的线程,大多数情况,产生内容和消费内容是在同一线程的,所以改变了产生内容所在的线程,就改变了消费内容所在的线程。
对subscribeOn()的调用是自下向上,所以连续多次调用subscribeOn(),结果会被最上面的subscribeOn()覆盖。(生成和消费都会被覆盖)
observeOn()之上有subscribeOn()调用
observeOn()的工作原理是把消费结果先缓存,再切换到新线程上让原始消费者消费,它和生产者是没有一点关系的,就算subscribeOn()调用了,也只是改变observeOn()这个消费者所在的线程,和OperatorObserveOn中存储的原始消费者一点关系都没有,它还是由observeOn()控制。observeOn()之下有subscribeOn()调用
这也不会改变observeOn()所指定的消费线程,因为observeOn()是自上而下调用,对subscribeOn()的调用是自下向上,在observeOn()指定的线程会覆盖下面subscribeOn()指定线程来去消费
用一张图来解释当多个 subscribeOn() 和 observeOn() 混合使用时,线程调度是怎么发生的(由于图中对象较多,相对于上面的图对结构做了一些简化调整):
参考:http://blog.csdn.net/jdsjlzx/article/details/51685769
http://blog.csdn.net/jdsjlzx/article/details/51686152
https://segmentfault.com/a/1190000004856071
https://gank.io/post/560e15be2dca930e00da1083
你真的会用RxJava么?RxJava线程变换之observeOn与subscribeOn
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