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.observers;
  
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.CompletableObserver;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.*;
  import io.reactivex.rxjava3.internal.util.EndConsumerHelper;
  
  /**
   * An abstract {@link CompletableObserver} that allows asynchronous cancellation of its subscription and associated resources.
   *
   * <p>All pre-implemented final methods are thread-safe.
   *
   * <p>Override the protected {@link #onStart()} to perform initialization when this
   * {@code ResourceCompletableObserver} is subscribed to a source.
   *
   * <p>Use the public {@link #dispose()} method to dispose the sequence externally and release
   * all resources.
   *
   * <p>To release the associated resources, one has to call {@link #dispose()}
   * in {@code onError()} and {@code onComplete()} explicitly.
   *
   * <p>Use {@link #add(Disposable)} to associate resources (as {@link io.reactivex.rxjava3.disposables.Disposable Disposable}s)
   * with this {@code ResourceCompletableObserver} that will be cleaned up when {@link #dispose()} is called.
   * Removing previously associated resources is not possible but one can create a
   * {@link io.reactivex.rxjava3.disposables.CompositeDisposable CompositeDisposable}, associate it with this
   * {@code ResourceCompletableObserver} and then add/remove resources to/from the {@code CompositeDisposable}
   * freely.
   *
   * <p>Like all other consumers, {@code ResourceCompletableObserver} can be subscribed only once.
   * Any subsequent attempt to subscribe it to a new source will yield an
   * {@link IllegalStateException} with message {@code "It is not allowed to subscribe with a(n) <class name> multiple times."}.
   *
   * <p>Implementation of {@link #onStart()}, {@link #onError(Throwable)}
   * and {@link #onComplete()} are not allowed to throw any unchecked exceptions.
   *
   * <p>Example<pre><code>
   * Disposable d =
   *     Completable.complete().delay(1, TimeUnit.SECONDS)
   *     .subscribeWith(new ResourceCompletableObserver() {
   *         &#64;Override public void onStart() {
   *             add(Schedulers.single()
   *                 .scheduleDirect(() -&gt; System.out.println("Time!"),
   *                     2, TimeUnit.SECONDS));
   *         }
   *         &#64;Override public void onError(Throwable t) {
   *             t.printStackTrace();
   *             dispose();
   *         }
   *         &#64;Override public void onComplete() {
   *             System.out.println("Done!");
   *             dispose();
   *         }
   *     });
   * // ...
   * d.dispose();
   * </code></pre>
   */
  public abstract class ResourceCompletableObserver implements CompletableObserver, Disposable {
      /** The active subscription. */
      private final AtomicReference<Disposable> upstream = new AtomicReference<>();
  
      /** The resource composite, can never be null. */
      private final ListCompositeDisposable resources = new ListCompositeDisposable();
  
      /**
       * Adds a resource to this {@code ResourceCompletableObserver}.
       *
       * @param resource the resource to add
       *
       * @throws NullPointerException if resource is {@code null}
       */
      public final void add(@NonNull Disposable resource) {
          Objects.requireNonNull(resource, "resource is null");
          resources.add(resource);
      }
  
      @Override
      public final void onSubscribe(@NonNull Disposable d) {
          if (EndConsumerHelper.setOnce(this.upstream, d, getClass())) {
              onStart();
          }
      }
  
      /**
       * Called once the upstream sets a {@link Disposable} on this {@code ResourceCompletableObserver}.
       *
       * <p>You can perform initialization at this moment. The default
       * implementation does nothing.
       */
      protected void onStart() {
      }
  
      /**
       * Cancels the main disposable (if any) and disposes the resources associated with
       * this {@code ResourceCompletableObserver} (if any).
       *
       * <p>This method can be called before the upstream calls {@link #onSubscribe(Disposable)} at which
       * case the main {@link Disposable} will be immediately disposed.
       */
      @Override
      public final void dispose() {
          if (DisposableHelper.dispose(upstream)) {
              resources.dispose();
          }
      }
  
      /**
       * Returns true if this {@code ResourceCompletableObserver} has been disposed/cancelled.
       * @return true if this {@code ResourceCompletableObserver} has been disposed/cancelled
       */
      @Override
      public final boolean isDisposed() {
          return DisposableHelper.isDisposed(upstream.get());
      }
  }
  