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.
/**
 * 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.observers;

import java.util.concurrent.atomic.AtomicReference;

import io.reactivex.SingleObserver;
import io.reactivex.annotations.NonNull;
import io.reactivex.disposables.Disposable;
import io.reactivex.internal.disposables.DisposableHelper;
import io.reactivex.internal.util.EndConsumerHelper;

An abstract SingleObserver that allows asynchronous cancellation by implementing Disposable. 
All pre-implemented final methods are thread-safe.
Like all other consumers, DisposableSingleObserver can be subscribed only once. Any subsequent attempt to subscribe it to a new source will yield an IllegalStateException with message "It is not allowed to subscribe with a(n) <class name> multiple times.". 
Implementation of onStart(), SingleObserver.onSuccess(Object) and SingleObserver.onError(Throwable) are not allowed to throw any unchecked exceptions. 
Example

Disposable d =
    Single.just(1).delay(1, TimeUnit.SECONDS)
    .subscribeWith(new DisposableSingleObserver<Integer>() {
        @Override public void onStart() {
            System.out.println("Start!");
        }
        @Override public void onSuccess(Integer t) {
            System.out.println(t);
        }
        @Override public void onError(Throwable t) {
            t.printStackTrace();
        }
    });
// ...
d.dispose();

Type parameters:
<T> – the received value type/**
 * An abstract {@link SingleObserver} that allows asynchronous cancellation by implementing Disposable.
 *
 * <p>All pre-implemented final methods are thread-safe.
 *
 * <p>Like all other consumers, {@code DisposableSingleObserver} can be subscribed only once.
 * Any subsequent attempt to subscribe it to a new source will yield an
 * {@link IllegalStateException} with message {@code "It is not allowed to subscribe with a(n) <class name> multiple times."}.
 *
 * <p>Implementation of {@link #onStart()}, {@link #onSuccess(Object)} and {@link #onError(Throwable)}
 * are not allowed to throw any unchecked exceptions.
 *
 * <p>Example<pre><code>
 * Disposable d =
 *     Single.just(1).delay(1, TimeUnit.SECONDS)
 *     .subscribeWith(new DisposableSingleObserver&lt;Integer&gt;() {
 *         &#64;Override public void onStart() {
 *             System.out.println("Start!");
 *         }
 *         &#64;Override public void onSuccess(Integer t) {
 *             System.out.println(t);
 *         }
 *         &#64;Override public void onError(Throwable t) {
 *             t.printStackTrace();
 *         }
 *     });
 * // ...
 * d.dispose();
 * </code></pre>
 *
 * @param <T> the received value type
 */
public abstract class DisposableSingleObserver<T> implements SingleObserver<T>, Disposable {

    final AtomicReference<Disposable> upstream = new AtomicReference<Disposable>();

    @Override
    public final void onSubscribe(@NonNull Disposable d) {
        if (EndConsumerHelper.setOnce(this.upstream, d, getClass())) {
            onStart();
        }
    }

    
Called once the single upstream Disposable is set via onSubscribe.
/**
     * Called once the single upstream Disposable is set via onSubscribe.
     */
    protected void onStart() {
    }

    @Override
    public final boolean isDisposed() {
        return upstream.get() == DisposableHelper.DISPOSED;
    }

    @Override
    public final void dispose() {
        DisposableHelper.dispose(upstream);
    }
}