code: /*
   * 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.observable;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  
  public final class ObservableSubscribeOn<T> extends AbstractObservableWithUpstream<T, T> {
      final Scheduler scheduler;
  
      public ObservableSubscribeOn(ObservableSource<T> source, Scheduler scheduler) {
          super(source);
          this.scheduler = scheduler;
      }
  
      @Override
      public void subscribeActual(final Observer<? super T> observer) {
          final SubscribeOnObserver<T> parent = new SubscribeOnObserver<>(observer);
  
          observer.onSubscribe(parent);
  
          parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
      }
  
      static final class SubscribeOnObserver<T> extends AtomicReference<Disposable> implements Observer<T>, Disposable {
  
          private static final long serialVersionUID = 8094547886072529208L;
          final Observer<? super T> downstream;
  
          final AtomicReference<Disposable> upstream;
  
          SubscribeOnObserver(Observer<? super T> downstream) {
              this.downstream = downstream;
              this.upstream = new AtomicReference<>();
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              DisposableHelper.setOnce(this.upstream, d);
          }
  
          @Override
          public void onNext(T t) {
              downstream.onNext(t);
          }
  
          @Override
          public void onError(Throwable t) {
              downstream.onError(t);
          }
  
          @Override
          public void onComplete() {
              downstream.onComplete();
          }
  
          @Override
          public void dispose() {
              DisposableHelper.dispose(upstream);
              DisposableHelper.dispose(this);
          }
  
          @Override
          public boolean isDisposed() {
              return DisposableHelper.isDisposed(get());
          }
  
          void setDisposable(Disposable d) {
              DisposableHelper.setOnce(this, d);
          }
      }
  
      final class SubscribeTask implements Runnable {
          private final SubscribeOnObserver<T> parent;
  
          SubscribeTask(SubscribeOnObserver<T> parent) {
              this.parent = parent;
          }
  
          @Override
          public void run() {
              source.subscribe(parent);
          }
      }
  }
  