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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

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package org.reactivestreams.example.unicast;

import org.reactivestreams.Subscriber;

import org.reactivestreams.Subscription;

/**

 * SyncSubscriber is an implementation of Reactive Streams `Subscriber`,

 * it runs synchronously (on the Publisher's thread) and requests one element

 * at a time and invokes a user-defined method to process each element.

 *

 * NOTE: The code below uses a lot of try-catches to show the reader where exceptions can be expected, and where they are forbidden.

 */

public abstract class SyncSubscriber<T> implements Subscriber<T> {

  private Subscription subscription; // Obeying rule 3.1, we make this private!

  private boolean done = false;

  @Override public void onSubscribe(final Subscription s) {

    // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Subscription` is `null`

    if (s == null) throw null;

    if (subscription != null) { // If someone has made a mistake and added this Subscriber multiple times, let's handle it gracefully

      try {

        s.cancel(); // Cancel the additional subscription

      } catch(final Throwable t) {

        //Subscription.cancel is not allowed to throw an exception, according to rule 3.15

        (new IllegalStateException(s + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);

      }

    } else {

      // We have to assign it locally before we use it, if we want to be a synchronous `Subscriber`

      // Because according to rule 3.10, the Subscription is allowed to call `onNext` synchronously from within `request`

      subscription = s;

      try {

        // If we want elements, according to rule 2.1 we need to call `request`

        // And, according to rule 3.2 we are allowed to call this synchronously from within the `onSubscribe` method

        s.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time

      } catch(final Throwable t) {

        // Subscription.request is not allowed to throw according to rule 3.16

        (new IllegalStateException(s + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);

      }

    }

  }

  @Override public void onNext(final T element) {

    if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

      (new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onNext prior to onSubscribe.")).printStackTrace(System.err);

    } else {

      // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `element` is `null`

      if (element == null) throw null;

      if (!done) { // If we aren't already done

        try {

          if (whenNext(element)) {

            try {

              subscription.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time

            } catch (final Throwable t) {

              // Subscription.request is not allowed to throw according to rule 3.16

              (new IllegalStateException(subscription + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);

            }

          } else {

            done();

          }

        } catch (final Throwable t) {

          done();

          try {

            onError(t);

          } catch (final Throwable t2) {

            //Subscriber.onError is not allowed to throw an exception, according to rule 2.13

            (new IllegalStateException(this + " violated the Reactive Streams rule 2.13 by throwing an exception from onError.", t2)).printStackTrace(System.err);

          }

        }

      }

    }

  }

  // Showcases a convenience method to idempotently marking the Subscriber as "done", so we don't want to process more elements

  // herefor we also need to cancel our `Subscription`.

  private void done() {

    //On this line we could add a guard against `!done`, but since rule 3.7 says that `Subscription.cancel()` is idempotent, we don't need to.

    done = true; // If we `whenNext` throws an exception, let's consider ourselves done (not accepting more elements)

    try {

      subscription.cancel(); // Cancel the subscription

    } catch(final Throwable t) {

      //Subscription.cancel is not allowed to throw an exception, according to rule 3.15

      (new IllegalStateException(subscription + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);

    }

  }

  // This method is left as an exercise to the reader/extension point

  // Returns whether more elements are desired or not, and if no more elements are desired

  protected abstract boolean whenNext(final T element);

  @Override public void onError(final Throwable t) {

    if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

      (new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onError prior to onSubscribe.")).printStackTrace(System.err);

    } else {

      // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Throwable` is `null`

      if (t == null) throw null;

      // Here we are not allowed to call any methods on the `Subscription` or the `Publisher`, as per rule 2.3

      // And anyway, the `Subscription` is considered to be cancelled if this method gets called, as per rule 2.4

    }

  }

  @Override public void onComplete() {

    if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

      (new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onComplete prior to onSubscribe.")).printStackTrace(System.err);

    } else {

      // Here we are not allowed to call any methods on the `Subscription` or the `Publisher`, as per rule 2.3

      // And anyway, the `Subscription` is considered to be cancelled if this method gets called, as per rule 2.4

    }

  }

}