RxJava源码解析(二)

前言

本篇主要解析RxJava的线程切换的原理实现

subscribeOn

首先, 我们先看下subscribeOn()方法, 老样子, 先上Demo

Observable observable =
                Observable
                .create(new ObservableOnSubscribe() {
                    @Override
                    public void subscribe(ObservableEmitter emitter) throws Exception {
                        emitter.onNext(123);
                        emitter.onComplete();
                    }
                });

observable
        .subscribeOn(Schedulers.io())
        .subscribe(getObserver());

subscribeOn操作符源码里其实是返回了一个ObservableSubscribeOn对象, 而从上篇我们已经知道, 订阅的动作其实在每个Observable的subscribeActual(observer)中执行, 所以我们直接去看ObservableSubscribeOn中的对应重载方法就行了.

@Override
    public void subscribeActual(final Observer s) {
        final SubscribeOnObserver parent = new SubscribeOnObserver(s);

        s.onSubscribe(parent);

        parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
    }

final class SubscribeTask implements Runnable {
        private final SubscribeOnObserver parent;

        SubscribeTask(SubscribeOnObserver parent) {
            this.parent = parent;
        }

        @Override
        public void run() {
            source.subscribe(parent);
        }
    }

SubscribeTask是一个Runnable的实现类, 执行内容就是修饰后的Observer订阅上游的动作, 我们先看scheduler.scheduleDirect(runable)方法

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

@NonNull
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;
}

这里createWorker是个抽象方法, 我们需要找到对应的修饰类, 我们返回去看Schedulers.io(), IO是IoScheduler的实例, 它的重载方法代码如下

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

可以看到IO线程实际使用的是一个有线程缓存的线程调度器.它内部通过ScheduledExecutorService实例来尝试重用之前worker开始使用的实例, 由于本篇着重在流程实现原理, 所以略过细节处.
在EventLoopWorker中, 我们看下对应的重载方法

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);
        }

继续往下, 其实这个时候已经是在线程池目标线程执行相关的工作了. 再深入就是线程池的操作了, 所以这里我们不再赘述

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