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.operators.single;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.exceptions.Exceptions;
  import io.reactivex.rxjava3.functions.Consumer;
  import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Calls a callback when the upstream calls onSubscribe with a disposable.
   *
   * @param <T> the value type
   */
  public final class SingleDoOnSubscribe<T> extends Single<T> {
  
      final SingleSource<T> source;
  
      final Consumer<? super Disposable> onSubscribe;
  
      public SingleDoOnSubscribe(SingleSource<T> source, Consumer<? super Disposable> onSubscribe) {
          this.source = source;
          this.onSubscribe = onSubscribe;
      }
  
      @Override
      protected void subscribeActual(final SingleObserver<? super T> observer) {
          source.subscribe(new DoOnSubscribeSingleObserver<>(observer, onSubscribe));
      }
  
      static final class DoOnSubscribeSingleObserver<T> implements SingleObserver<T> {
  
          final SingleObserver<? super T> downstream;
  
          final Consumer<? super Disposable> onSubscribe;
  
          boolean done;
  
          DoOnSubscribeSingleObserver(SingleObserver<? super T> actual, Consumer<? super Disposable> onSubscribe) {
              this.downstream = actual;
              this.onSubscribe = onSubscribe;
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              try {
                  onSubscribe.accept(d);
              } catch (Throwable ex) {
                  Exceptions.throwIfFatal(ex);
                  done = true;
                  d.dispose();
                  EmptyDisposable.error(ex, downstream);
                  return;
              }
  
              downstream.onSubscribe(d);
          }
  
          @Override
          public void onSuccess(T value) {
              if (done) {
                  return;
              }
              downstream.onSuccess(value);
          }
  
          @Override
          public void onError(Throwable e) {
              if (done) {
                  RxJavaPlugins.onError(e);
                  return;
              }
              downstream.onError(e);
          }
      }
  
  }
  