[移动开发]Rxjava 线程切换原理

前言

rxjava 可以很方便的进行线程切换， 那么rxjava是如何进行线程切换的呢？阅读本文可以了解下rxjava 是如何进行线程切换的及线程切换的影响点。

一个简单的代码：

Observable.create(new ObservableOnSubscribe<String>() {
    @Override
    public void subscribe(ObservableEmitter<String> e) throws Exception {
        Log.d("WanRxjava ", "subscrib  td ==" + Thread.currentThread().getName());
        e.onNext("我在发送next");
        e.onComplete();
    }
}).subscribeOn(Schedulers.io())
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(new Observer<String>() {
            @Override
            public void onSubscribe(Disposable d) {
                Log.d("WanRxjava ", "onSubscribe td ==" + Thread.currentThread().getName());
            }

            @Override
            public void onNext(String value) {
                Log.d("WanRxjava ", "onNext td ==" + Thread.currentThread().getName());
            }

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onComplete() {
                Log.d("WanRxjava ", "onComplete td ==" + Thread.currentThread().getName());
            }
        });

如上代码，实现了线程切换和观察者被观察者绑定的逻辑。我们分四部分看上述代码逻辑create、subscribeOn、observeOn、subscribe

1.create

create 顾名思议是 创建被观察者，这里有一个参数是 ObservableOnSubscribe，这是个接口类，我们看下create 的源码：

@SchedulerSupport(SchedulerSupport.NONE)
public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
    ObjectHelper.requireNonNull(source, "source is null");
    return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
}

将ObservableOnSubscribe 传入后 又调用了 new ObservableCreate(source)

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

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

ObservableCreate 有一个变量是 source,这里只是将传入的ObservableOnSubscribe 赋值给source,也就是做了一层包装，然后返回。

2.subscribeOn

调用完create后返回了 ObservableCreate（Observable），然后继续调用subscribeOn,传入了一个变量 Schedulers.io()

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

我们看到调用了new ObservableSubscribeOn(this, scheduler) 将自身和 scheduler 传入

public final class ObservableSubscribeOn<T> extends AbstractObservableWithUpstream<T, T> {
    final Scheduler scheduler;

    public ObservableSubscribeOn(ObservableSource<T> source, Scheduler scheduler) {
        super(source);
        this.scheduler = scheduler;
    }
}

ObservableSubscribeOn 将scheduler 和 create 返回的对象又包装了一层 返回ObservableSubscribeOn

3.observeOn

有一个参数是 Scheduler

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

ObservableSubscribeOn（observable）又调用了observeOn，然后调用了new ObservableObserveOn(this, scheduler, delayError, bufferSize)

public final class ObservableObserveOn<T> extends AbstractObservableWithUpstream<T, T> {
    final Scheduler scheduler;
    final boolean delayError;
    final int bufferSize;
    public ObservableObserveOn(ObservableSource<T> source, Scheduler scheduler, boolean delayError, int bufferSize) {
        super(source);
        this.scheduler = scheduler;
        this.delayError = delayError;
        this.bufferSize = bufferSize;
    }
}

又是一个包装,将ObservableSubscribeOn 和 scheduler 包装成 ObservableObserveOn

4.subscribe

上述最后一步即调用ObservableObserveOn.subscribe,传入参数是一个 observer

//ObservableObserveOn.java
@SchedulerSupport(SchedulerSupport.NONE)
@Override
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);
        // can't call onError because no way to know if a Disposable has been set or not
        // can't call onSubscribe because the call might have set a Subscription already
        RxJavaPlugins.onError(e);

        NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
        npe.initCause(e);
        throw npe;
    }
}

可以看到调用subscribe 后调用了subscribeActual(observer);将observer 传入

我们看下 subscribeActual(observer)

//ObservableObserveOn.java
@Override
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));
    }
}

上面的if 先不管，主要看下下面的逻辑，调用了 scheduler.createWorker()，这个scheduler 是 observeOn 传入的，然后调用

new ObserveOnObserver(observer, w, delayError, bufferSize)；将worker /observer 又做了一次包装。

//ObservableObserveOn 内部类
static final class ObserveOnObserver<T> extends BasicIntQueueDisposable<T>
implements Observer<T>, Runnable {

    private static final long serialVersionUID = 6576896619930983584L;
    final Observer<? super T> actual;
    final Scheduler.Worker worker;
    final boolean delayError;
    final int bufferSize;

    SimpleQueue<T> queue;

    Disposable s;

    Throwable error;
    volatile boolean done;

    volatile boolean cancelled;

    int sourceMode;

    boolean outputFused;

    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();
    }

    @Override
    public void onError(Throwable t) {
        if (done) {
            RxJavaPlugins.onError(t);
            return;
        }
        error = t;
        done = true;
        schedule();
    }

    @Override
    public void onComplete() {
        if (done) {
            return;
        }
        done = true;
        schedule();
    }

    @Override
    public void dispose() {
        if (!cancelled) {
            cancelled = true;
            s.dispose();
            worker.dispose();
            if (getAndIncrement() == 0) {
                queue.clear();
            }
        }
    }

    @Override
    public boolean isDisposed() {
        return cancelled;
    }

    void schedule() {
        if (getAndIncrement() == 0) {
            worker.schedule(this);
        }
    }

    void drainNormal() {
        int missed = 1;

        final SimpleQueue<T> q = queue;
        final Observer<? super T> a = actual;

        for (;;) {
            if (checkTerminated(done, q.isEmpty(), a)) {
                return;
            }

            for (;;) {
                boolean d = done;
                T v;

                try {
                    v = q.poll();
                } catch (Throwable ex) {
                    Exceptions.throwIfFatal(ex);
                    s.dispose();
                    q.clear();
                    a.onError(ex);
                    return;
                }
                boolean empty = v == null;

                if (checkTerminated(d, empty, a)) {
                    return;
                }

                if (empty) {
                    break;
                }

                a.onNext(v);
            }

            missed = addAndGet(-missed);
            if (missed == 0) {
                break;
            }
        }
    }

    void drainFused() {
        int missed = 1;

        for (;;) {
            if (cancelled) {
                return;
            }

            boolean d = done;
            Throwable ex = error;

            if (!delayError && d && ex != null) {
                actual.onError(error);
                worker.dispose();
                return;
            }

            actual.onNext(null);

            if (d) {
                ex = error;
                if (ex != null) {
                    actual.onError(ex);
                } else {
                    actual.onComplete();
                }
                worker.dispose();
                return;
            }

            missed = addAndGet(-missed);
            if (missed == 0) {
                break;
            }
        }
    }

    @Override
    public void run() {
        if (outputFused) {
            drainFused();
        } else {
            drainNormal();
        }
    }

    boolean checkTerminated(boolean d, boolean empty, Observer<? super T> a) {
        if (cancelled) {
            queue.clear();
            return true;
        }
        if (d) {
            Throwable e = error;
            if (delayError) {
                if (empty) {
                    if (e != null) {
                        a.onError(e);
                    } else {
                        a.onComplete();
                    }
                    worker.dispose();
                    return true;
                }
            } else {
                if (e != null) {
                    queue.clear();
                    a.onError(e);
                    worker.dispose();
                    return true;
                } else
                if (empty) {
                    a.onComplete();
                    worker.dispose();
                    return true;
                }
            }
        }
        return false;
    }

    @Override
    public int requestFusion(int mode) {
        if ((mode & ASYNC) != 0) {
            outputFused = true;
            return ASYNC;
        }
        return NONE;
    }

    @Override
    public T poll() throws Exception {
        return queue.poll();
    }

    @Override
    public void clear() {
        queue.clear();
    }

    @Override
    public boolean isEmpty() {
        return queue.isEmpty();
    }
}

包装完ObserveOnObserver后，调用了source.subscribe 这里的source 即ObservableSubscribeOn.subscribe，进而调用ObservableSubscribeOn.subscribeActual

//ObservableSubscribeOn.java
@Override
public void subscribeActual(final Observer<? super T> s) {
    final SubscribeOnObserver<T> parent = new scheduler<T>(s);

    s.onSubscribe(parent);

    parent.setDisposable(scheduler.scheduleDirect(new Runnable() {
        @Override
        public void run() {
            source.subscribe(parent);
        }
    }));
}

static final class SubscribeOnObserver<T> extends AtomicReference<Disposable> implements Observer<T>, Disposable {

    private static final long serialVersionUID = 8094547886072529208L;
    final Observer<? super T> actual;

    final AtomicReference<Disposable> s;

    SubscribeOnObserver(Observer<? super T> actual) {
        this.actual = actual;
        this.s = new AtomicReference<Disposable>();
    }

    @Override
    public void onSubscribe(Disposable s) {
        DisposableHelper.setOnce(this.s, s);
    }

    @Override
    public void onNext(T t) {
        actual.onNext(t);
    }

    @Override
    public void onError(Throwable t) {
        actual.onError(t);
    }

    @Override
    public void onComplete() {
        actual.onComplete();
    }

    @Override
    public void dispose() {
        DisposableHelper.dispose(s);
        DisposableHelper.dispose(this);
    }

    @Override
    public boolean isDisposed() {
        return DisposableHelper.isDisposed(get());
    }

    void setDisposable(Disposable d) {
        DisposableHelper.setOnce(this, d);
    }
}

ObservableSubscribeOn.subscribeActual

首先将传入的观察者封装成 SubscribeOnObserver

然后触发了 onSubscribe,接着调用 scheduler.scheduleDirect(new Runnable() 这里的scheduler 是 subscribeOn 传入的

最后调用了 scheduler.setsetDisposable方法。

我们看到 run 的方法体即source.subscribe(parent);这里的source 即 ObservableCreate（ObservableOnSubscribe），传入了observer,然后调用 observer的OnNext 和 OnComplete 方法。

5.小结：

a. 调用Observer.OnSubscribe 方法是 不受线程调度影响的

b.subscribeOn 影响的是发送事件的线程

c.observerOn 影响的是观察者处理接受数据的线程，如果没有调用observeOn 则不会进行包装成 ObserveOnObserver，也就是说不会执行观察者的线程切换，和 发送者的线程一致

d.多次调用subscribeOn切换线程，每次都会new ObservableSubscribeOn，触发事件发送时会往上调用，也就是第一次调用的subscribeOn传入的线程 会执行发送事件，后面的线程切换无效

e.Observer.OnSubscribe 只会执行一次，因为调用DisposableHelper.setOnce(this.s, s)

f.处理完onComplete 或者onError 后就不会再发出事件，因为被观察者发送完这两个事件后 就会调用disposed

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