RxJava线程切换原理

RxJava通过责任链的方式，将各个 操作符 节点串连起来。当调用订阅subscribe方法时，链上节点都会依赖订阅上一个节点。那线程切换是如何实现的？

subscribeOn操作符是如何让上游节点工作在指定线程上？
observeOn操作符是如何让下游节点工作在指定线程上？

subscribeOn操作符关键代码：

public final class ObservableSubscribeOn extends AbstractObservableWithUpstream {
final Scheduler scheduler;
    ///scheduler 指定工作线程
    public ObservableSubscribeOn(ObservableSource source, Scheduler scheduler) {
        super(source);
        this.scheduler = scheduler;
    }

    .......
    @Override
    public void subscribeActual(final Observer observer) {
        final SubscribeOnObserver parent = new SubscribeOnObserver<>(observer);

        observer.onSubscribe(parent);
        ///scheduler.scheduleDirect 线程启动
        parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
    }
     ........
}

///关键在SubscribeTask中的`run`方法
final class SubscribeTask implements Runnable {

        private final SubscribeOnObserver parent;

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

        @Override
        public void run() {
            ///在线程运行的代码块中进行了**订阅**
            source.subscribe(parent);
        }
    }

subscribeOn分析

Observable.just("goods")
        .subscribeOn(Schedulers.io())
        .subscribe()

等价于

Schedulers.io().createWorker().schedule {
        Observable.just("goods").subscribe()    
    }

observeOn分析

Observable.just("goods")
        .observeOn(Schedulers.io())
        .subscribe()

等价于

Observable.just("goods").subscribe {
        Schedulers.io().createWorker().schedule {
            ///do somethind
        }
    }