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.subjects;
  
  import java.util.concurrent.atomic.AtomicReference;
  
  import io.reactivex.rxjava3.annotations.*;
  import io.reactivex.rxjava3.core.Observer;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.observers.DeferredScalarDisposable;
  import io.reactivex.rxjava3.internal.util.ExceptionHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * A Subject that emits the very last value followed by a completion event or the received error to Observers.
   * <p>
   * <img width="640" height="239" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/AsyncSubject.png" alt="">
   * <p>
   * This subject does not have a public constructor by design; a new empty instance of this
   * {@code AsyncSubject} can be created via the {@link #create()} method.
   * <p>
   * Since a {@code Subject} is conceptionally derived from the {@code Processor} type in the Reactive Streams specification,
   * {@code null}s are not allowed (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.13">Rule 2.13</a>)
   * as parameters to {@link #onNext(Object)} and {@link #onError(Throwable)}. Such calls will result in a
   * {@link NullPointerException} being thrown and the subject's state is not changed.
   * <p>
   * Since an {@code AsyncSubject} is an {@link io.reactivex.rxjava3.core.Observable}, it does not support backpressure.
   * <p>
   * When this {@code AsyncSubject} is terminated via {@link #onError(Throwable)}, the
   * last observed item (if any) is cleared and late {@link io.reactivex.rxjava3.core.Observer}s only receive
   * the {@code onError} event.
   * <p>
   * The {@code AsyncSubject} caches the latest item internally and it emits this item only when {@code onComplete} is called.
   * Therefore, it is not recommended to use this {@code Subject} with infinite or never-completing sources.
   * <p>
   * Even though {@code AsyncSubject} implements the {@code Observer} interface, calling
   * {@code onSubscribe} is not required (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.12">Rule 2.12</a>)
   * if the subject is used as a standalone source. However, calling {@code onSubscribe}
   * after the {@code AsyncSubject} reached its terminal state will result in the
   * given {@code Disposable} being disposed immediately.
   * <p>
   * Calling {@link #onNext(Object)}, {@link #onError(Throwable)} and {@link #onComplete()}
   * is required to be serialized (called from the same thread or called non-overlappingly from different threads
   * through external means of serialization). The {@link #toSerialized()} method available to all {@code Subject}s
   * provides such serialization and also protects against reentrance (i.e., when a downstream {@code Observer}
   * consuming this subject also wants to call {@link #onNext(Object)} on this subject recursively).
   * The implementation of onXXX methods are technically thread-safe but non-serialized calls
   * to them may lead to undefined state in the currently subscribed Observers.
   * <p>
   * This {@code AsyncSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
   * {@link #getThrowable()} and {@link #hasObservers()} as well as means to read the very last observed value -
   * after this {@code AsyncSubject} has been completed - in a non-blocking and thread-safe
   * manner via {@link #hasValue()} or {@link #getValue()}.
   * <dl>
   *  <dt><b>Scheduler:</b></dt>
   *  <dd>{@code AsyncSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
   *  the {@code Observer}s get notified on the thread where the terminating {@code onError} or {@code onComplete}
   *  methods were invoked.</dd>
   *  <dt><b>Error handling:</b></dt>
   *  <dd>When the {@link #onError(Throwable)} is called, the {@code AsyncSubject} enters into a terminal state
   *  and emits the same {@code Throwable} instance to the last set of {@code Observer}s. During this emission,
   *  if one or more {@code Observer}s dispose their respective {@code Disposable}s, the
   *  {@code Throwable} is delivered to the global error handler via
   *  {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code Observer}s
   *  cancel at once).
   *  If there were no {@code Observer}s subscribed to this {@code AsyncSubject} when the {@code onError()}
   *  was called, the global error handler is not invoked.
   *  </dd>
   * </dl>
   * <p>
   * Example usage:
   * <pre><code>
   * AsyncSubject&lt;Object&gt; subject = AsyncSubject.create();
   * 
   * TestObserver&lt;Object&gt; to1 = subject.test();
   *
   * to1.assertEmpty();
   *
   * subject.onNext(1);
   *
   * // AsyncSubject only emits when onComplete was called.
   * to1.assertEmpty();
   *
   * subject.onNext(2);
   * subject.onComplete();
   *
   * // onComplete triggers the emission of the last cached item and the onComplete event.
   * to1.assertResult(2);
   *
   * TestObserver&lt;Object&gt; to2 = subject.test();
   *
   * // late Observers receive the last cached item too
   * to2.assertResult(2);
   * </code></pre>
   * @param <T> the value type
   */
  public final class AsyncSubject<T> extends Subject<T> {
  
      @SuppressWarnings("rawtypes")
      static final AsyncDisposable[] EMPTY = new AsyncDisposable[0];
  
      @SuppressWarnings("rawtypes")
      static final AsyncDisposable[] TERMINATED = new AsyncDisposable[0];
  
      final AtomicReference<AsyncDisposable<T>[]> subscribers;
  
      /** Write before updating subscribers, read after reading subscribers as TERMINATED. */
      Throwable error;
  
      /** Write before updating subscribers, read after reading subscribers as TERMINATED. */
      T value;
  
      /**
       * Creates a new AsyncProcessor.
       * @param <T> the value type to be received and emitted
       * @return the new AsyncProcessor instance
       */
      @CheckReturnValue
      @NonNull
      public static <T> AsyncSubject<T> create() {
          return new AsyncSubject<>();
      }
  
      /**
       * Constructs an AsyncSubject.
       * @since 2.0
       */
      @SuppressWarnings("unchecked")
      AsyncSubject() {
          this.subscribers = new AtomicReference<>(EMPTY);
      }
  
      @Override
      public void onSubscribe(Disposable d) {
          if (subscribers.get() == TERMINATED) {
              d.dispose();
          }
      }
  
      @Override
      public void onNext(T t) {
          ExceptionHelper.nullCheck(t, "onNext called with a null value.");
          if (subscribers.get() == TERMINATED) {
              return;
          }
          value = t;
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onError(Throwable t) {
          ExceptionHelper.nullCheck(t, "onError called with a null Throwable.");
          if (subscribers.get() == TERMINATED) {
              RxJavaPlugins.onError(t);
              return;
          }
          value = null;
          error = t;
          for (AsyncDisposable<T> as : subscribers.getAndSet(TERMINATED)) {
              as.onError(t);
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public void onComplete() {
          if (subscribers.get() == TERMINATED) {
              return;
          }
          T v = value;
          AsyncDisposable<T>[] array = subscribers.getAndSet(TERMINATED);
          if (v == null) {
              for (AsyncDisposable<T> as : array) {
                  as.onComplete();
              }
          } else {
              for (AsyncDisposable<T> as : array) {
                  as.complete(v);
              }
          }
      }
  
      @Override
      @CheckReturnValue
      public boolean hasObservers() {
          return subscribers.get().length != 0;
      }
  
      @Override
      @CheckReturnValue
      public boolean hasThrowable() {
          return subscribers.get() == TERMINATED && error != null;
      }
  
      @Override
      @CheckReturnValue
      public boolean hasComplete() {
          return subscribers.get() == TERMINATED && error == null;
      }
  
      @Override
      @CheckReturnValue
      public Throwable getThrowable() {
          return subscribers.get() == TERMINATED ? error : null;
      }
  
      @Override
      protected void subscribeActual(Observer<? super T> observer) {
          AsyncDisposable<T> as = new AsyncDisposable<>(observer, this);
          observer.onSubscribe(as);
          if (add(as)) {
              if (as.isDisposed()) {
                  remove(as);
              }
          } else {
              Throwable ex = error;
              if (ex != null) {
                  observer.onError(ex);
              } else {
                  T v = value;
                  if (v != null) {
                      as.complete(v);
                  } else {
                      as.onComplete();
                  }
              }
          }
      }
  
      /**
       * Tries to add the given subscriber to the subscribers array atomically
       * or returns false if the subject has terminated.
       * @param ps the subscriber to add
       * @return true if successful, false if the subject has terminated
       */
      boolean add(AsyncDisposable<T> ps) {
          for (;;) {
              AsyncDisposable<T>[] a = subscribers.get();
              if (a == TERMINATED) {
                  return false;
              }
  
              int n = a.length;
              @SuppressWarnings("unchecked")
              AsyncDisposable<T>[] b = new AsyncDisposable[n + 1];
              System.arraycopy(a, 0, b, 0, n);
              b[n] = ps;
  
              if (subscribers.compareAndSet(a, b)) {
                  return true;
              }
          }
      }
  
      /**
       * Atomically removes the given subscriber if it is subscribed to the subject.
       * @param ps the subject to remove
       */
      @SuppressWarnings("unchecked")
      void remove(AsyncDisposable<T> ps) {
          for (;;) {
              AsyncDisposable<T>[] a = subscribers.get();
              int n = a.length;
              if (n == 0) {
                  return;
              }
  
              int j = -1;
              for (int i = 0; i < n; i++) {
                  if (a[i] == ps) {
                      j = i;
                      break;
                  }
              }
  
              if (j < 0) {
                  return;
              }
  
              AsyncDisposable<T>[] b;
  
              if (n == 1) {
                  b = EMPTY;
              } else {
                  b = new AsyncDisposable[n - 1];
                  System.arraycopy(a, 0, b, 0, j);
                  System.arraycopy(a, j + 1, b, j, n - j - 1);
              }
              if (subscribers.compareAndSet(a, b)) {
                  return;
              }
          }
      }
  
      /**
       * Returns true if the subject has any value.
       * <p>The method is thread-safe.
       * @return true if the subject has any value
       */
      @CheckReturnValue
      public boolean hasValue() {
          return subscribers.get() == TERMINATED && value != null;
      }
  
      /**
       * Returns a single value the Subject currently has or null if no such value exists.
       * <p>The method is thread-safe.
       * @return a single value the Subject currently has or null if no such value exists
       */
      @Nullable
      @CheckReturnValue
      public T getValue() {
          return subscribers.get() == TERMINATED ? value : null;
      }
  
      static final class AsyncDisposable<T> extends DeferredScalarDisposable<T> {
          private static final long serialVersionUID = 5629876084736248016L;
  
          final AsyncSubject<T> parent;
  
          AsyncDisposable(Observer<? super T> actual, AsyncSubject<T> parent) {
              super(actual);
              this.parent = parent;
          }
  
          @Override
          public void dispose() {
              if (super.tryDispose()) {
                  parent.remove(this);
              }
          }
  
          void onComplete() {
              if (!isDisposed()) {
                  downstream.onComplete();
              }
          }
  
          void onError(Throwable t) {
              if (isDisposed()) {
                  RxJavaPlugins.onError(t);
              } else {
                  downstream.onError(t);
              }
          }
      }
  }
  