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.subscriptions;
  
  import org.reactivestreams.Subscriber;
  
  import io.reactivex.rxjava3.annotations.Nullable;
  import io.reactivex.rxjava3.operators.QueueSubscription;
  
  /**
   * An empty subscription that does nothing other than validates the request amount.
   */
  public enum EmptySubscription implements QueueSubscription<Object> {
      /** A singleton, stateless instance. */
      INSTANCE;
  
      @Override
      public void request(long n) {
          SubscriptionHelper.validate(n);
      }
  
      @Override
      public void cancel() {
          // no-op
      }
  
      @Override
      public String toString() {
          return "EmptySubscription";
      }
  
      /**
       * Sets the empty subscription instance on the subscriber and then
       * calls onError with the supplied error.
       *
       * <p>Make sure this is only called if the subscriber hasn't received a
       * subscription already (there is no way of telling this).
       *
       * @param e the error to deliver to the subscriber
       * @param s the target subscriber
       */
      public static void error(Throwable e, Subscriber<?> s) {
          s.onSubscribe(INSTANCE);
          s.onError(e);
      }
  
      /**
       * Sets the empty subscription instance on the subscriber and then
       * calls onComplete.
       *
       * <p>Make sure this is only called if the subscriber hasn't received a
       * subscription already (there is no way of telling this).
       *
       * @param s the target subscriber
       */
      public static void complete(Subscriber<?> s) {
          s.onSubscribe(INSTANCE);
          s.onComplete();
      }
  
      @Nullable
      @Override
      public Object poll() {
          return null; // always empty
      }
  
      @Override
      public boolean isEmpty() {
          return true;
      }
  
      @Override
      public void clear() {
          // nothing to do
      }
  
      @Override
      public int requestFusion(int mode) {
          return mode & ASYNC; // accept async mode: an onComplete or onError will be signalled after anyway
      }
  
      @Override
      public boolean offer(Object value) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  
      @Override
      public boolean offer(Object v1, Object v2) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  }
  