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.internal.subscribers;

import org.reactivestreams.*;

import io.reactivex.FlowableSubscriber;
import io.reactivex.internal.subscriptions.*;

A subscriber, extending a DeferredScalarSubscription,
 that is unbounded-in and can generate 0 or 1 resulting value.
Type parameters:
<T> – the input value type
<R> – the output value type/**
 * A subscriber, extending a DeferredScalarSubscription,
 *  that is unbounded-in and can generate 0 or 1 resulting value.
 * @param <T> the input value type
 * @param <R> the output value type
 */
public abstract class DeferredScalarSubscriber<T, R> extends DeferredScalarSubscription<R>
implements FlowableSubscriber<T> {

    private static final long serialVersionUID = 2984505488220891551L;

    
The upstream subscription. /** The upstream subscription. */
    protected Subscription upstream;

    
Can indicate if there was at least on onNext call. /** Can indicate if there was at least on onNext call. */
    protected boolean hasValue;

    
Creates a DeferredScalarSubscriber instance and wraps a downstream Subscriber.
Params:
downstream – the downstream subscriber, not null (not verified)/**
     * Creates a DeferredScalarSubscriber instance and wraps a downstream Subscriber.
     * @param downstream the downstream subscriber, not null (not verified)
     */
    public DeferredScalarSubscriber(Subscriber<? super R> downstream) {
        super(downstream);
    }

    @Override
    public void onSubscribe(Subscription s) {
        if (SubscriptionHelper.validate(this.upstream, s)) {
            this.upstream = s;

            downstream.onSubscribe(this);

            s.request(Long.MAX_VALUE);
        }
    }

    @Override
    public void onError(Throwable t) {
        value = null;
        downstream.onError(t);
    }

    @Override
    public void onComplete() {
        if (hasValue) {
            complete(value);
        } else {
            downstream.onComplete();
        }
    }

    @Override
    public void cancel() {
        super.cancel();
        upstream.cancel();
    }
}