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.exceptions.Exceptions;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.operators.QueueDisposable;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Base class for a fuseable intermediate observer.
   * @param <T> the upstream value type
   * @param <R> the downstream value type
   */
  public abstract class BasicFuseableObserver<T, R> implements Observer<T>, QueueDisposable<R> {
  
      /** The downstream subscriber. */
      protected final Observer<? super R> downstream;
  
      /** The upstream subscription. */
      protected Disposable upstream;
  
      /** The upstream's QueueDisposable if not null. */
      protected QueueDisposable<T> qd;
  
      /** Flag indicating no further onXXX event should be accepted. */
      protected boolean done;
  
      /** Holds the established fusion mode of the upstream. */
      protected int sourceMode;
  
      /**
       * Construct a BasicFuseableObserver by wrapping the given subscriber.
       * @param downstream the subscriber, not null (not verified)
       */
      public BasicFuseableObserver(Observer<? super R> downstream) {
          this.downstream = downstream;
      }
  
      // final: fixed protocol steps to support fuseable and non-fuseable upstream
      @SuppressWarnings("unchecked")
      @Override
      public final void onSubscribe(Disposable d) {
          if (DisposableHelper.validate(this.upstream, d)) {
  
              this.upstream = d;
              if (d instanceof QueueDisposable) {
                  this.qd = (QueueDisposable<T>)d;
              }
  
              if (beforeDownstream()) {
  
                  downstream.onSubscribe(this);
  
                  afterDownstream();
              }
  
          }
      }
  
      /**
       * Override this to perform actions before the call {@code actual.onSubscribe(this)} happens.
       * @return true if onSubscribe should continue with the call
       */
      protected boolean beforeDownstream() {
          return true;
      }
  
      /**
       * Override this to perform actions after the call to {@code actual.onSubscribe(this)} happened.
       */
      protected void afterDownstream() {
          // default no-op
      }
  
      // -----------------------------------
      // Convenience and state-aware methods
      // -----------------------------------
  
      @Override
      public void onError(Throwable t) {
          if (done) {
              RxJavaPlugins.onError(t);
              return;
          }
          done = true;
          downstream.onError(t);
      }
  
      /**
       * Rethrows the throwable if it is a fatal exception or calls {@link #onError(Throwable)}.
       * @param t the throwable to rethrow or signal to the actual subscriber
       */
      protected final void fail(Throwable t) {
          Exceptions.throwIfFatal(t);
          upstream.dispose();
          onError(t);
      }
  
      @Override
      public void onComplete() {
          if (done) {
              return;
          }
          done = true;
          downstream.onComplete();
      }
  
      /**
       * Calls the upstream's QueueDisposable.requestFusion with the mode and
       * saves the established mode in {@link #sourceMode} if that mode doesn't
       * have the {@link QueueDisposable#BOUNDARY} flag set.
       * <p>
       * If the upstream doesn't support fusion ({@link #qd} is null), the method
       * returns {@link QueueDisposable#NONE}.
       * @param mode the fusion mode requested
       * @return the established fusion mode
       */
      protected final int transitiveBoundaryFusion(int mode) {
          QueueDisposable<T> qd = this.qd;
          if (qd != null) {
              if ((mode & BOUNDARY) == 0) {
                  int m = qd.requestFusion(mode);
                  if (m != NONE) {
                      sourceMode = m;
                  }
                  return m;
              }
          }
          return NONE;
      }
  
      // --------------------------------------------------------------
      // Default implementation of the RS and QS protocol (can be overridden)
      // --------------------------------------------------------------
  
      @Override
      public void dispose() {
          upstream.dispose();
      }
  
      @Override
      public boolean isDisposed() {
          return upstream.isDisposed();
      }
  
      @Override
      public boolean isEmpty() {
          return qd.isEmpty();
      }
  
      @Override
      public void clear() {
          qd.clear();
      }
  
      // -----------------------------------------------------------
      // The rest of the Queue interface methods shouldn't be called
      // -----------------------------------------------------------
  
      @Override
      public final boolean offer(R e) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  
      @Override
      public final boolean offer(R v1, R v2) {
          throw new UnsupportedOperationException("Should not be called!");
      }
  }
  