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.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.core.SingleObserver;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.internal.util.EndConsumerHelper;
  
  /**
   * An abstract {@link SingleObserver} that allows asynchronous cancellation by implementing {@link Disposable}.
   *
   * <p>All pre-implemented final methods are thread-safe.
   *
   * <p>Like all other consumers, {@code DisposableSingleObserver} 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 #onSuccess(Object)} and {@link #onError(Throwable)}
   * are not allowed to throw any unchecked exceptions.
   *
   * <p>Example<pre><code>
   * Disposable d =
   *     Single.just(1).delay(1, TimeUnit.SECONDS)
   *     .subscribeWith(new DisposableSingleObserver&lt;Integer&gt;() {
   *         &#64;Override public void onStart() {
   *             System.out.println("Start!");
   *         }
   *         &#64;Override public void onSuccess(Integer t) {
   *             System.out.println(t);
   *         }
   *         &#64;Override public void onError(Throwable t) {
   *             t.printStackTrace();
   *         }
   *     });
   * // ...
   * d.dispose();
   * </code></pre>
   *
   * @param <T> the received value type
   */
  public abstract class DisposableSingleObserver<T> implements SingleObserver<T>, Disposable {
  
      final AtomicReference<Disposable> upstream = new AtomicReference<>();
  
      @Override
      public final void onSubscribe(@NonNull Disposable d) {
          if (EndConsumerHelper.setOnce(this.upstream, d, getClass())) {
              onStart();
          }
      }
  
      /**
       * Called once the single upstream {@link Disposable} is set via {@link #onSubscribe(Disposable)}.
       */
      protected void onStart() {
      }
  
      @Override
      public final boolean isDisposed() {
          return upstream.get() == DisposableHelper.DISPOSED;
      }
  
      @Override
      public final void dispose() {
          DisposableHelper.dispose(upstream);
      }
  }
  