component-dsl/src/main/java/org/apache/aries/component/dsl/internal/OnlyLastPublisher.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.aries.component.dsl.internal;

import org.apache.aries.component.dsl.OSGi;
import org.apache.aries.component.dsl.Publisher;
import org.apache.aries.component.dsl.OSGiResult;

import java.util.concurrent.atomic.AtomicLong;
import java.util.function.Supplier;

/**
 * @author Carlos Sierra Andrés
 */
public class OnlyLastPublisher<T> implements Publisher<T> {

    public OnlyLastPublisher(Publisher<? super T> op) {
       this(op, null);
    }

    public OnlyLastPublisher(
        Publisher<? super T> op, Supplier<T> injectOnLeave) {

        _op = op;
        _injectOnLeave = injectOnLeave;
        _terminator = OSGi.NOOP;
    }

    private final Publisher<? super T> _op;
    private AtomicLong _counter = new AtomicLong();
    private Supplier<T> _injectOnLeave;
    private OSGiResult _terminator;

    @Override
    public synchronized OSGiResult publish(T t) {
        _terminator.run();

        _terminator = _op.publish(t);

        if (_injectOnLeave == null) {
            return OSGi.NOOP;
        }
        else {
            _counter.incrementAndGet();

            return new OSGiResultImpl(
                () -> {
                    synchronized (this) {
                        _terminator.run();

                        if (_counter.decrementAndGet() > 0) {
                            _terminator = _op.publish(_injectOnLeave.get());
                        }
                    }
                },
                us -> _terminator.update(us)
            );
        }
    }

    @Override
    public <E extends Exception> OSGiResult error(T t, Exception e) throws E {
        return _op.error(t, e);
    }

}