响应式编程之OnSubscribeDoOnEach

该类型的被观察者包括：doOnCompleted、doOnEach、doOnError、doOnNext、doOnTerminate。

public class OnSubscribeDoOnEach<T> implements OnSubscribe<T> {
    private final Observer<? super T> doOnEachObserver;
    private final Observable<T> source;

    public OnSubscribeDoOnEach(Observable<T> source, Observer<? super T> doOnEachObserver) {
        this.source = source;//创建当前OnSubscribeDoOnEach的Observable，即紧邻的UpStream
        this.doOnEachObserver = doOnEachObserver;
    }

    @Override
    public void call(final Subscriber<? super T> subscriber) {
        //触发call回调时，优先处理其上游UpStream的回调
        source.unsafeSubscribe(new DoOnEachSubscriber<T>(subscriber, doOnEachObserver));
    }

    private static final class DoOnEachSubscriber<T> extends Subscriber<T> {

        private final Subscriber<? super T> subscriber;
        private final Observer<? super T> doOnEachObserver;

        private boolean done;

        DoOnEachSubscriber(Subscriber<? super T> subscriber, Observer<? super T> doOnEachObserver) {
            super(subscriber);
            this.subscriber = subscriber;
            this.doOnEachObserver = doOnEachObserver;
        }

        @Override
        public void onCompleted() {
            if (done) {
                return;
            }
            try {
                doOnEachObserver.onCompleted();
            } catch (Throwable e) {
                Exceptions.throwOrReport(e, this);
                return;
            }
            // Set `done` here so that the error in `doOnEachObserver.onCompleted()` can be noticed by observer
            done = true;
            subscriber.onCompleted();
        }

        @Override
        public void onError(Throwable e) {
            if (done) {
                RxJavaHooks.onError(e);
                return;
            }
            done = true;
            try {
                doOnEachObserver.onError(e);
            } catch (Throwable e2) {
                Exceptions.throwIfFatal(e2);
                subscriber.onError(new CompositeException(Arrays.asList(e, e2)));
                return;
            }
            subscriber.onError(e);
        }

        @Override
        public void onNext(T value) {
            if (done) {
                return;
            }
            try {
                doOnEachObserver.onNext(value);
            } catch (Throwable e) {
                Exceptions.throwOrReport(e, this, value);
                return;
            }
            subscriber.onNext(value);
        }
    }
}