RxJava的消息发送和线程切换实现原理

RxJava是一个在Java虚拟机上的响应式扩展，通过使用可观察的序列将异步和基于事件的程序组合起来的一个库。

它扩展了观察者模式来支持数据/事件序列，并且添加了操作符，这些操作符允许你声明性地组合序列，同时抽象出要关注的问题：比如低级线程、同步、线程安全和并发数据结构等。

RxJava相信大家都非常了解吧，今天分享一下RxJava的消息发送和线程源码的分析。最后并分享一个相关demo,让大家更加熟悉我们天天都在用的框架。

消息订阅发送

首先让我们看看消息订阅发送最基本的代码组成：

 Observable observable = Observable.create(new ObservableOnSubscribe<String>() {
      @Override
      public void subscribe(ObservableEmitter<String> emitter) throws Exception {
        emitter.onNext("Jack1");
        emitter.onNext("Jack2");
        emitter.onNext("Jack3");
        emitter.onComplete();
      }
    });

    Observer<String> observer = new Observer<String>() {
      @Override
      public void onSubscribe(Disposable d) {
        Log.d(TAG, "onSubscribe");
      }

      @Override
      public void onNext(String s) {
        Log.d(TAG, "onNext : " + s);
      }

      @Override
      public void onError(Throwable e) {
        Log.d(TAG, "onError : " + e.toString());
      }

      @Override
      public void onComplete() {
        Log.d(TAG, "onComplete");
      }
    };

    observable.subscribe(observer);

代码很简单,observable为被观察者,observer为观察者,然后通过observable.subscribe(observer),把观察者和被观察者关联起来。被观察者发送消息(emitter.onNext("内容")),观察者就可以在onNext()方法里回调出来。

我们先来看Observable，创建是用Observable.create()方法进行创建，源码如下：

public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
  ObjectHelper.requireNonNull(source, "source is null");
  return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
}

public static <T> T requireNonNull(T object, String message) {
  if (object == null) {
     throw new NullPointerException(message);
  }
  return object;
 }

public static <T> Observable<T> onAssembly(@NonNull Observable<T> source) {
  Function<? super Observable, ? extends Observable> f = onObservableAssembly;
  if (f != null) {
     return apply(f, source);
  }
  return source;
}

可以看出，create()方法里最主要的还是创建用ObservableOnSubscribe传入创建了一个ObservableCreate对象并且保存而已。

public final class ObservableCreate<T> extends Observable<T> {
  final ObservableOnSubscribe<T> source;

  public ObservableCreate(ObservableOnSubscribe<T> source) {
    this.source = source;
  }

}

接着是创建Observer，这比较简单只是单纯创建一个接口对象而已

public interface Observer<T> {
  void onSubscribe(@NonNull Disposable d);

  void onNext(@NonNull T t);

  void onError(@NonNull Throwable e);

  void onComplete();
}

订阅发送消息

observable.subscribe(observer)的subscribe方法如下：

public final void subscribe(Observer<? super T> observer) {
  ObjectHelper.requireNonNull(observer, "observer is null");
  try {
    observer = RxJavaPlugins.onSubscribe(this, observer);
    ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");
    subscribeActual(observer);
  } catch (NullPointerException e) { // NOPMD
    throw e;
  } catch (Throwable e) {
    Exceptions.throwIfFatal(e);
    RxJavaPlugins.onError(e);
    NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
    npe.initCause(e);
    throw npe;
  }
}

//ObjectHelper.requireNonNull()方法
public static <T> T requireNonNull(T object, String message) {
  if (object == null) {
     throw new NullPointerException(message);
  }
  return object;
}

//RxJavaPlugins.onSubscribe()方法
public static <T> Observer<? super T> onSubscribe(@NonNull Observable<T> source, @NonNull Observer<? super T> observer) {
  BiFunction<? super Observable, ? super Observer, ? extends Observer> f = onObservableSubscribe;
  if (f != null) {
    return apply(f, source, observer);
  }
  return observer;
}

从上面源码可以看出requireNonNull()只是做非空判断而已，而RxJavaPlugins.onSubscribe()也只是返回最终的观察者而已。所以关键代码是抽象方法subscribeActual(observer);那么subscribeActual对应哪个代码段呢？

还记得Observable.create()创建的ObservableCreate类吗，这就是subscribeActual()具体实现类，源码如下：

protected void subscribeActual(Observer<? super T> observer) {
  CreateEmitter<T> parent = new CreateEmitter<T>(observer);
  observer.onSubscribe(parent);
  try {
    source.subscribe(parent);
  } catch (Throwable ex) {
    Exceptions.throwIfFatal(ex);
    parent.onError(ex);
  }
}

从上面的代码可以看出，首先创建了一个CreateEmitter对象并传入observer，然后回到observer的onSubscribe()方法，而source就是我们之前创建ObservableCreate传入的ObservableOnSubscribe对象。

class CreateEmitter<T> extends AtomicReference<Disposable>
  implements ObservableEmitter<T>, Disposable {

 }

而CreateEmitter又继承ObservableEmitter接口，又回调ObservableOnSubscribe的subscribe方法，对应着我们的：

Observable observable = Observable.create(new ObservableOnSubscribe<String>() {
   @Override
   public void subscribe(ObservableEmitter<String> emitter) throws Exception {
      emitter.onNext("Jack1");
      emitter.onNext("Jack2");
      emitter.onNext("Jack3");
      emitter.onComplete();
   }
});

当它发送消息既调用emitter.onNext()方法时，既调用了CreateEmitter的onNext()方法：

public void onNext(T t) {
  if (t == null) {
    onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
    return;
  }
  if (!isDisposed()) {
    observer.onNext(t);
  }
}

可以看到最终又回调了观察者的onNext()方法，把被观察者的数据传输给了观察者。有人会问

isDisposed()是什么意思，是判断要不要终止传递的，我们看emitter.onComplete()源码：

public void onComplete() {
  if (!isDisposed()) {
    try {
      observer.onComplete();
    } finally {
      dispose();
    }
  }
}

public static boolean dispose(AtomicReference<Disposable> field) {
    Disposable current = field.get();
    Disposable d = DISPOSED;
    if (current != d) {
      current = field.getAndSet(d);
      if (current != d) {
        if (current != null) {
          current.dispose();
        }
        return true;
      }
    }
    return false;
 }

public static boolean isDisposed(Disposable d) {
    return d == DISPOSED;
}

dispose()方法是终止消息传递，也就付了个DISPOSED常量，而isDisposed()方法就是判断这个常量而已。这就是整个消息订阅发送的过程，用的是观察者模式。

线程切换

在上面模板代码的基础上，线程切换只是改变了如下代码：

observable.subscribeOn(Schedulers.io())
     .observeOn(AndroidSchedulers.mainThread())
     .subscribe(observer);

下面我们对线程切换的源码进行一下分析，分为两部分：subscribeOn()和observeOn()

subscribeOn()

首先是subscribeOn()源码如下：

public final Observable<T> subscribeOn(Scheduler scheduler) {
  ObjectHelper.requireNonNull(scheduler, "scheduler is null");
  return RxJavaPlugins.onAssembly(new ObservableSubscribeOn<T>(this, scheduler));
}

我们传进去了一个Scheduler类，Scheduler是一个调度类，能够延时或周期性地去执行一个任务。

Scheduler有如下类型：

接着就没什么了，只是返回一个ObservableSubscribeOn对象而已。

observeOn()

首先看源码如下：

public final Observable<T> observeOn(Scheduler scheduler) {
  return observeOn(scheduler, false, bufferSize());
}

public final Observable<T> observeOn(Scheduler scheduler, boolean delayError, int bufferSize) {
  ObjectHelper.requireNonNull(scheduler, "scheduler is null");
  ObjectHelper.verifyPositive(bufferSize, "bufferSize");
  return RxJavaPlugins.onAssembly(new ObservableObserveOn<T>(this, scheduler, delayError, bufferSize));
}

这里也是没什么，只是最终返回一个ObservableObserveOn对象而已。

接着还是像原来那样调用subscribe()方法进行订阅，看起来好像整体变化不大，就是封装了一些对象而已，不过着恰恰是RxJava源码的精华，当他再次调用subscribeActual()方法时，已经不是之前的ObservableCreate()里subscribeActual方法了，而是最先调用ObservableObserveOn的subscribeActual()方法，对应源码如下：

protected void subscribeActual(Observer<? super T> observer) {
  if (scheduler instanceof TrampolineScheduler) {
    source.subscribe(observer);
  } else {
    Scheduler.Worker w = scheduler.createWorker();
    source.subscribe(new ObserveOnObserver<T>(observer, w, delayError, bufferSize));
  }
}

在这里有两点要讲，一点是ObserveOnObserver是执行观察者的线程，后面还会详解，然后就是source.subscribe，这个source.subscribe调的是ObservableSubscribeOn的subscribe方法，而subscribe方法因为继承的也是Observable，是Observable里的方法，所以和上面的ObservableCreate一样的方法，所以会调用ObservableSubscribeOn里的subscribeActual()方法，对应的代码如下：

public void subscribeActual(final Observer<? super T> s) {
  final SubscribeOnObserver<T> parent = new SubscribeOnObserver<T>(s);
  s.onSubscribe(parent);
  parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
}

上面代码中，首先把ObserveOnObserver返回给来的用SubscribeOnObserver“包装”起来，然后在回调Observer的onSubscribe()，就是对应模板代码的onSubscribe()方法。

接着看SubscribeTask类的源码：

final class SubscribeTask implements Runnable {
  private final SubscribeOnObserver<T> parent;
  SubscribeTask(SubscribeOnObserver<T> parent) {
    this.parent = parent;
  }
  @Override
  public void run() {
    source.subscribe(parent);
  }
}

其中的source.subscribe(parent)，就是我们执行子线程的回调方法，对应我们模板代码里的被观察者的subscribe()方法。它放在run()方法里，并且继承Runnable,说明这个类主要是线程运行。接着看scheduler.scheduleDirect()方法对应的源码如下：

public Disposable scheduleDirect(@NonNull Runnable run) {
  return scheduleDirect(run, 0L, TimeUnit.NANOSECONDS);
}

public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
  final Worker w = createWorker();
  final Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
  DisposeTask task = new DisposeTask(decoratedRun, w);
  w.schedule(task, delay, unit);
  return task;
}

在这里，createWorker()也是一个抽象方法，调用的是我们的调度类对应的Schedulers类里面的方法，这里是IoScheduler类，

public final class IoScheduler extends Scheduler{

  final AtomicReference<CachedWorkerPool> pool;

  //省略....

  public Worker createWorker() {
    return new EventLoopWorker(pool.get());
  }

  static final class EventLoopWorker extends Scheduler.Worker {
    private final CompositeDisposable tasks;
    private final CachedWorkerPool pool;
    private final ThreadWorker threadWorker;

    final AtomicBoolean once = new AtomicBoolean();

    EventLoopWorker(CachedWorkerPool pool) {
      this.pool = pool;
      this.tasks = new CompositeDisposable();
      this.threadWorker = pool.get();
    }

    //省略....

    @NonNull
    @Override
    public Disposable schedule(@NonNull Runnable action, long delayTime, @NonNull TimeUnit unit) {
      if (tasks.isDisposed()) {
        // don't schedule, we are unsubscribed
        return EmptyDisposable.INSTANCE;
      }
      return threadWorker.scheduleActual(action, delayTime, unit, tasks);
    }
  }

}

 static final class CachedWorkerPool implements Runnable {

  //省略....

  ThreadWorker get() {
    if (allWorkers.isDisposed()) {
      return SHUTDOWN_THREAD_WORKER;
    }
    while (!expiringWorkerQueue.isEmpty()) {
      ThreadWorker threadWorker = expiringWorkerQueue.poll();
      if (threadWorker != null) {
        return threadWorker;
      }
    }

    ThreadWorker w = new ThreadWorker(threadFactory);
    allWorkers.add(w);
    return w;
   }
   //省略....
}

这就是IoScheduler的createWorker()的方法，其实最主要的意思就是获取线程池，以便于生成子线程，让SubscribeTask()可以运行。然后直接调用 w.schedule(task, delay, unit)方法让它在线程池里执行。上面中那ThreadWorker的源码如下：

static final class ThreadWorker extends NewThreadWorker {
  private long expirationTime;
  ThreadWorker(ThreadFactory threadFactory) {
    super(threadFactory);
    this.expirationTime = 0L;
  }

  //省略代码....
 }

public class NewThreadWorker extends Scheduler.Worker implements Disposable {
  private final ScheduledExecutorService executor;

  public NewThreadWorker(ThreadFactory threadFactory) {
    executor = SchedulerPoolFactory.create(threadFactory);
  }

  public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, @NonNull TimeUnit unit, @Nullable DisposableContainer parent) {
    Runnable decoratedRun = RxJavaPlugins.onSchedule(run);

    ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, parent);

    if (parent != null) {
      if (!parent.add(sr)) {
        return sr;
      }
    }

    Future<?> f;
    try {
      if (delayTime <= 0) {
        f = executor.submit((Callable<Object>)sr);
      } else {
        f = executor.schedule((Callable<Object>)sr, delayTime, unit);
      }
      sr.setFuture(f);
    } catch (RejectedExecutionException ex) {
      if (parent != null) {
        parent.remove(sr);
      }
      RxJavaPlugins.onError(ex);
    }

    return sr;
  }
}

可以看到，这就调了原始的javaAPI来进行线程池操作。

然后最后一环在子线程调用source.subscribe(parent)方法，然后回调刚开始创建的ObservableCreate的subscribeActual(),既：

protected void subscribeActual(Observer<? super T> observer) {
    CreateEmitter<T> parent = new CreateEmitter<T>(observer);
    observer.onSubscribe(parent);
    try {
      source.subscribe(parent);
    } catch (Throwable ex) {
      Exceptions.throwIfFatal(ex);
      parent.onError(ex);
    }
}

进行消息的订阅绑定。

当我们在调用 emitter.onNext(内容)时，是在io线程里的，那回调的onNext()又是什么时候切换的？那就是前面为了整个流程流畅性没讲的在observeOn()里的ObserveOnObserver是执行观察者的线程的过程。

class ObserveOnObserver<T> extends BasicIntQueueDisposable<T>
  implements Observer<T>, Runnable {

    //省略代码....

    ObserveOnObserver(Observer<? super T> actual, Scheduler.Worker worker, boolean delayError, int bufferSize) {
      this.actual = actual;
      this.worker = worker;
      this.delayError = delayError;
      this.bufferSize = bufferSize;
    }

    @Override
    public void onSubscribe(Disposable s) {
      if (DisposableHelper.validate(this.s, s)) {
        this.s = s;
        if (s instanceof QueueDisposable) {
          @SuppressWarnings("unchecked")
          QueueDisposable<T> qd = (QueueDisposable<T>) s;
          int m = qd.requestFusion(QueueDisposable.ANY | QueueDisposable.BOUNDARY);
          if (m == QueueDisposable.SYNC) {
            sourceMode = m;
            queue = qd;
            done = true;
            actual.onSubscribe(this);
            schedule();
            return;
          }
          if (m == QueueDisposable.ASYNC) {
            sourceMode = m;
            queue = qd;
            actual.onSubscribe(this);
            return;
          }
        }
        queue = new SpscLinkedArrayQueue<T>(bufferSize);
        actual.onSubscribe(this);
      }
    }

    @Override
    public void onNext(T t) {
      if (done) {
        return;
      }
      if (sourceMode != QueueDisposable.ASYNC) {
        queue.offer(t);
      }
      schedule();
    }  

    void schedule() {
      if (getAndIncrement() == 0) {
        worker.schedule(this);
      }
    }
    //省略代码....
  }

当调用emitter.onNext(内容)方法，会调用上面的onNext()方法，然后在这个方法里会把数据压入一个队列，然后执行worker.schedule(this)方法，work是什么呢，还记得AndroidSchedulers.mainThread()吗，这个对应这个HandlerScheduler这个类，所以createWorker()对应着：

private static final class MainHolder {
    static final Scheduler DEFAULT = new HandlerScheduler(new Handler(Looper.getMainLooper()));
}

public Worker createWorker() {
  return new HandlerWorker(handler);
}

private static final class HandlerWorker extends Worker {
    private final Handler handler;
    private volatile boolean disposed;

    HandlerWorker(Handler handler) {
      this.handler = handler;
    }

    @Override
    public Disposable schedule(Runnable run, long delay, TimeUnit unit) {
      if (run == null) throw new NullPointerException("run == null");
      if (unit == null) throw new NullPointerException("unit == null");
      if (disposed) {
        return Disposables.disposed();
      }
      run = RxJavaPlugins.onSchedule(run);
      ScheduledRunnable scheduled = new ScheduledRunnable(handler, run);
      Message message = Message.obtain(handler, scheduled);
      message.obj = this; // Used as token for batch disposal of this worker's runnables.
      handler.sendMessageDelayed(message, unit.toMillis(delay));
      if (disposed) {
        handler.removeCallbacks(scheduled);
        return Disposables.disposed();
      }
      return scheduled;
    }
}

在next()方法里，运用android自带的Handler消息机制，通过把方法包裹在Message里，同通过handler.sendMessageDelayed()发送消息，就会在ui线程里回调Next()方法，从而实现从子线程切换到android主线程的操作。我们在主线程拿到数据就可以进行各种在主线程的操作了。

总结一下：

ObservableCreate 一> ObservableSubscribeOn 一> ObservableObserveOn为初始化顺序

当调用observable.subscribe(observer)时的执行顺序
ObservableObserveOn 一> ObservableSubscribeOn 一> ObservableCreate

当发送消息的执行顺序
ObservableCreate 一> ObservableSubscribeOn 一> ObservableObserveOn

以上就是消息订阅和线程切换的源码的所有讲解了。

为了让你们理解更清楚，我仿照RxJava写了大概的消息订阅和线程切换的最基本代码和基本功能，以帮助你们理解

https://github.com/jack921/RxJava2Demo

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持我们。