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.flowable;
  
  import java.util.concurrent.atomic.*;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
  import io.reactivex.rxjava3.internal.util.BackpressureHelper;
  
  /**
   * Subscribes to the source Flowable on the specified Scheduler and makes
   * sure downstream requests are scheduled there as well.
   *
   * @param <T> the value type emitted
   */
  public final class FlowableSubscribeOn<T> extends AbstractFlowableWithUpstream<T , T> {
  
      final Scheduler scheduler;
  
      final boolean nonScheduledRequests;
  
      public FlowableSubscribeOn(Flowable<T> source, Scheduler scheduler, boolean nonScheduledRequests) {
          super(source);
          this.scheduler = scheduler;
          this.nonScheduledRequests = nonScheduledRequests;
      }
  
      @Override
      public void subscribeActual(final Subscriber<? super T> s) {
          Scheduler.Worker w = scheduler.createWorker();
          final SubscribeOnSubscriber<T> sos = new SubscribeOnSubscriber<>(s, w, source, nonScheduledRequests);
          s.onSubscribe(sos);
  
          w.schedule(sos);
      }
  
      static final class SubscribeOnSubscriber<T> extends AtomicReference<Thread>
      implements FlowableSubscriber<T>, Subscription, Runnable {
  
          private static final long serialVersionUID = 8094547886072529208L;
  
          final Subscriber<? super T> downstream;
  
          final Scheduler.Worker worker;
  
          final AtomicReference<Subscription> upstream;
  
          final AtomicLong requested;
  
          final boolean nonScheduledRequests;
  
          Publisher<T> source;
  
          SubscribeOnSubscriber(Subscriber<? super T> actual, Scheduler.Worker worker, Publisher<T> source, boolean requestOn) {
              this.downstream = actual;
              this.worker = worker;
              this.source = source;
              this.upstream = new AtomicReference<>();
              this.requested = new AtomicLong();
              this.nonScheduledRequests = !requestOn;
          }
  
          @Override
          public void run() {
              lazySet(Thread.currentThread());
              Publisher<T> src = source;
              source = null;
              src.subscribe(this);
          }
  
          @Override
          public void onSubscribe(Subscription s) {
              if (SubscriptionHelper.setOnce(this.upstream, s)) {
                  long r = requested.getAndSet(0L);
                  if (r != 0L) {
                      requestUpstream(r, s);
                  }
              }
          }
  
          @Override
          public void onNext(T t) {
              downstream.onNext(t);
          }
  
          @Override
          public void onError(Throwable t) {
              downstream.onError(t);
              worker.dispose();
          }
  
          @Override
          public void onComplete() {
              downstream.onComplete();
              worker.dispose();
          }
  
          @Override
          public void request(final long n) {
              if (SubscriptionHelper.validate(n)) {
                  Subscription s = this.upstream.get();
                  if (s != null) {
                      requestUpstream(n, s);
                  } else {
                      BackpressureHelper.add(requested, n);
                      s = this.upstream.get();
                      if (s != null) {
                          long r = requested.getAndSet(0L);
                          if (r != 0L) {
                              requestUpstream(r, s);
                          }
                      }
                  }
              }
          }
  
          void requestUpstream(final long n, final Subscription s) {
              if (nonScheduledRequests || Thread.currentThread() == get()) {
                  s.request(n);
              } else {
                  worker.schedule(new Request(s, n));
              }
          }
  
          @Override
          public void cancel() {
              SubscriptionHelper.cancel(upstream);
              worker.dispose();
          }
  
          static final class Request implements Runnable {
              final Subscription upstream;
              final long n;
  
              Request(Subscription s, long n) {
                  this.upstream = s;
                  this.n = n;
              }
  
              @Override
              public void run() {
                  upstream.request(n);
              }
          }
      }
  }
  