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.disposables;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.Nullable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  
  /**
   * A Disposable container that allows atomically updating/replacing the contained
   * Disposable with another Disposable, disposing the old one when updating plus
   * handling the disposition when the container itself is disposed.
   */
  public final class SerialDisposable implements Disposable {
      final AtomicReference<Disposable> resource;
  
      /**
       * Constructs an empty SerialDisposable.
       */
      public SerialDisposable() {
          this.resource = new AtomicReference<>();
      }
  
      /**
       * Constructs a SerialDisposable with the given initial Disposable instance.
       * @param initialDisposable the initial Disposable instance to use, null allowed
       */
      public SerialDisposable(@Nullable Disposable initialDisposable) {
          this.resource = new AtomicReference<>(initialDisposable);
      }
  
      /**
       * Atomically: set the next disposable on this container and dispose the previous
       * one (if any) or dispose next if the container has been disposed.
       * @param next the Disposable to set, may be null
       * @return true if the operation succeeded, false if the container has been disposed
       * @see #replace(Disposable)
       */
      public boolean set(@Nullable Disposable next) {
          return DisposableHelper.set(resource, next);
      }
  
      /**
       * Atomically: set the next disposable on this container but don't dispose the previous
       * one (if any) or dispose next if the container has been disposed.
       * @param next the Disposable to set, may be null
       * @return true if the operation succeeded, false if the container has been disposed
       * @see #set(Disposable)
       */
      public boolean replace(@Nullable Disposable next) {
          return DisposableHelper.replace(resource, next);
      }
  
      /**
       * Returns the currently contained Disposable or null if this container is empty.
       * @return the current Disposable, may be null
       */
      @Nullable
      public Disposable get() {
          Disposable d = resource.get();
          if (d == DisposableHelper.DISPOSED) {
              return Disposable.disposed();
          }
          return d;
      }
  
      @Override
      public void dispose() {
          DisposableHelper.dispose(resource);
      }
  
      @Override
      public boolean isDisposed() {
          return DisposableHelper.isDisposed(resource.get());
      }
  }
  