Rxjava2 defaultIfEmpty

defaultIfEmpty操作符的作用就是如果生产者没有发送数据，就会给出默认数据。

 Observable.empty().defaultIfEmpty("empty").subscribe(new Observer<Object>() {
            @Override
            public void onSubscribe(@NotNull Disposable d) {
                System.out.println("onSubscribe ");
            }

            @Override
            public void onNext(@NotNull Object s) {
                System.out.println("onNext " + s);
            }

            @Override
            public void onError(@NotNull Throwable e) {
                System.out.println("onError " + e.getMessage());
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete ");
            }
        });

看看源码 很简单，empty标识是否是空数据，如果生产者没有生产数据，就会调用onComplete方法，继续就会调用 other.subscribe(this);，这个other就是我们defaultIfEmpty创造出来的新生产者，就会把默认数据发送到下游。

public final class ObservableSwitchIfEmpty<T> extends AbstractObservableWithUpstream<T, T> {
    final ObservableSource<? extends T> other;
    public ObservableSwitchIfEmpty(ObservableSource<T> source, ObservableSource<? extends T> other) {
        super(source);
        this.other = other;
    }

    @Override
    public void subscribeActual(Observer<? super T> t) {
        SwitchIfEmptyObserver<T> parent = new SwitchIfEmptyObserver<T>(t, other);
        t.onSubscribe(parent.arbiter);
        source.subscribe(parent);
    }

    static final class SwitchIfEmptyObserver<T> implements Observer<T> {
        final Observer<? super T> downstream;
        final ObservableSource<? extends T> other;
        final SequentialDisposable arbiter;

        boolean empty;

        SwitchIfEmptyObserver(Observer<? super T> actual, ObservableSource<? extends T> other) {
            this.downstream = actual;
            this.other = other;
            this.empty = true;
            this.arbiter = new SequentialDisposable();
        }

        @Override
        public void onSubscribe(Disposable d) {
            arbiter.update(d);
        }

        @Override
        public void onNext(T t) {
            if (empty) {
                empty = false;
            }
            downstream.onNext(t);
        }

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

        @Override
        public void onComplete() {
            if (empty) {
                empty = false;
                other.subscribe(this);
            } else {
                downstream.onComplete();
            }
        }
    }
}