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.subscribers;
  
  import org.reactivestreams.*;
  
  import io.reactivex.rxjava3.core.FlowableSubscriber;
  import io.reactivex.rxjava3.internal.subscriptions.*;
  
  /**
   * A subscriber, extending a DeferredScalarSubscription,
   *  that is unbounded-in and can generate 0 or 1 resulting value.
   * @param <T> the input value type
   * @param <R> the output value type
   */
  public abstract class DeferredScalarSubscriber<T, R> extends DeferredScalarSubscription<R>
  implements FlowableSubscriber<T> {
  
      private static final long serialVersionUID = 2984505488220891551L;
  
      /** The upstream subscription. */
      protected Subscription upstream;
  
      /** Can indicate if there was at least on onNext call. */
      protected boolean hasValue;
  
      /**
       * Creates a DeferredScalarSubscriber instance and wraps a downstream Subscriber.
       * @param downstream the downstream subscriber, not null (not verified)
       */
      public DeferredScalarSubscriber(Subscriber<? super R> downstream) {
          super(downstream);
      }
  
      @Override
      public void onSubscribe(Subscription s) {
          if (SubscriptionHelper.validate(this.upstream, s)) {
              this.upstream = s;
  
              downstream.onSubscribe(this);
  
              s.request(Long.MAX_VALUE);
          }
      }
  
      @Override
      public void onError(Throwable t) {
          value = null;
          downstream.onError(t);
      }
  
      @Override
      public void onComplete() {
          if (hasValue) {
              complete(value);
          } else {
              downstream.onComplete();
          }
      }
  
      @Override
      public void cancel() {
          super.cancel();
          upstream.cancel();
      }
  }
  