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Reactive Extensions for Java
There is a newer version: 3.1.9
/*
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
 * compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License is
 * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
 * the License for the specific language governing permissions and limitations under the License.
 */

package io.reactivex.rxjava3.internal.operators.maybe;

import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.rxjava3.core.*;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
import io.reactivex.rxjava3.internal.fuseable.HasUpstreamMaybeSource;

/**
 * Subscribes to the other source if the main source is empty.
 *
 * @param  the value type
 */
public final class MaybeSwitchIfEmptySingle extends Single implements HasUpstreamMaybeSource {

    final MaybeSource source;
    final SingleSource other;

    public MaybeSwitchIfEmptySingle(MaybeSource source, SingleSource other) {
        this.source = source;
        this.other = other;
    }

    @Override
    public MaybeSource source() {
        return source;
    }

    @Override
    protected void subscribeActual(SingleObserver observer) {
        source.subscribe(new SwitchIfEmptyMaybeObserver<>(observer, other));
    }

    static final class SwitchIfEmptyMaybeObserver
    extends AtomicReference
    implements MaybeObserver, Disposable {

        private static final long serialVersionUID = 4603919676453758899L;

        final SingleObserver downstream;

        final SingleSource other;

        SwitchIfEmptyMaybeObserver(SingleObserver actual, SingleSource other) {
            this.downstream = actual;
            this.other = other;
        }

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

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

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

        @Override
        public void onSuccess(T value) {
            downstream.onSuccess(value);
        }

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

        @Override
        public void onComplete() {
            Disposable d = get();
            if (d != DisposableHelper.DISPOSED) {
                if (compareAndSet(d, null)) {
                    other.subscribe(new OtherSingleObserver(downstream, this));
                }
            }
        }

        static final class OtherSingleObserver implements SingleObserver {

            final SingleObserver downstream;

            final AtomicReference parent;
            OtherSingleObserver(SingleObserver actual, AtomicReference parent) {
                this.downstream = actual;
                this.parent = parent;
            }

            @Override
            public void onSubscribe(Disposable d) {
                DisposableHelper.setOnce(parent, d);
            }

            @Override
            public void onSuccess(T value) {
                downstream.onSuccess(value);
            }

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

    }
}