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.observers;
  
  import io.reactivex.rxjava3.core.Observer;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  
  /**
   * A fuseable Observer that can generate 0 or 1 resulting value.
   * @param <T> the input value type
   * @param <R> the output value type
   */
  public abstract class DeferredScalarObserver<T, R>
  extends DeferredScalarDisposable<R>
  implements Observer<T> {
  
      private static final long serialVersionUID = -266195175408988651L;
  
      /** The upstream disposable. */
      protected Disposable upstream;
  
      /**
       * Creates a DeferredScalarObserver instance and wraps a downstream Observer.
       * @param downstream the downstream subscriber, not null (not verified)
       */
      public DeferredScalarObserver(Observer<? super R> downstream) {
          super(downstream);
      }
  
      @Override
      public void onSubscribe(Disposable d) {
          if (DisposableHelper.validate(this.upstream, d)) {
              this.upstream = d;
  
              downstream.onSubscribe(this);
          }
      }
  
      @Override
      public void onError(Throwable t) {
          value = null;
          error(t);
      }
  
      @Override
      public void onComplete() {
          R v = value;
          if (v != null) {
              value = null;
              complete(v);
          } else {
              complete();
          }
      }
  
      @Override
      public void dispose() {
          super.dispose();
          upstream.dispose();
      }
  }
  