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SciJava Common is a shared library for SciJava software. It provides a plugin framework, with an extensible mechanism for service discovery, backed by its own annotation processor, so that plugins can be loaded dynamically. It is used by both ImageJ and SCIFIO.
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/*
 * #%L
 * SciJava Common shared library for SciJava software.
 * %%
 * Copyright (C) 2009 - 2016 Board of Regents of the University of
 * Wisconsin-Madison, Broad Institute of MIT and Harvard, and Max Planck
 * Institute of Molecular Cell Biology and Genetics.
 * %%
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * #L%
 */

package org.scijava.event;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.WeakHashMap;

import org.bushe.swing.event.annotation.AbstractProxySubscriber;
import org.bushe.swing.event.annotation.BaseProxySubscriber;
import org.bushe.swing.event.annotation.ReferenceStrength;
import org.scijava.Priority;
import org.scijava.log.LogService;
import org.scijava.plugin.Parameter;
import org.scijava.plugin.Plugin;
import org.scijava.service.AbstractService;
import org.scijava.service.Service;
import org.scijava.thread.ThreadService;
import org.scijava.util.ClassUtils;

/**
 * Default service for publishing and subscribing to SciJava events.
 * 
 * @author Curtis Rueden
 * @author Grant Harris
 */
@Plugin(type = Service.class, priority = DefaultEventService.PRIORITY)
public class DefaultEventService extends AbstractService implements
	EventService
{

	/**
	 * The default event service's priority.
	 * 
	 * Alternative event service implementations that wish to prioritize
	 * themselves above this one can still ensure preferential usage via
	 * {@code priority = DefaultEventService.PRIORITY + 1} or similar.
	 * 
	 */
	public static final double PRIORITY = 10 * Priority.VERY_HIGH_PRIORITY;

	@Parameter
	private LogService log;

	@Parameter
	private ThreadService threadService;

	private DefaultEventBus eventBus;

	/**
	 * A cache for mapping {@link Method}s to the {@link SciJavaEvent} class taken
	 * as parameters. Only methods with event parameters will cached here.
	 */
	private final Map> eventClasses = new HashMap>();

	/**
	 * Set of claimed {@link EventHandler#key()}s. Additional event handlers
	 * specifying the same key will be ignored rather than subscribed.
	 */
	private final HashSet keys = new HashSet();

	// -- EventService methods --

	@Override
	public  void publish(final E e) {
		e.setContext(getContext());
		e.setCallingThread(Thread.currentThread());
		eventBus.publishNow(e);
	}

	@Override
	public  void publishLater(final E e) {
		e.setContext(getContext());
		e.setCallingThread(Thread.currentThread());
		eventBus.publishLater(e);
	}

	@Override
	public List> subscribe(final Object o) {
		List> subscribers = Collections.emptyList();
		final List eventHandlers =
			ClassUtils.getAnnotatedMethods(o.getClass(), EventHandler.class);

		if (!eventHandlers.isEmpty()) {
			subscribers = new ArrayList>();
			for (final Method m : eventHandlers) {
				// verify that the event handler method is valid
				final Class eventClass = getEventClass(m);
				if (eventClass == null) {
					log.warn("Invalid EventHandler method: " + m);
					continue;
				}

				// verify that the event handler key isn't already claimed
				final String key = m.getAnnotation(EventHandler.class).key();
				if (!key.isEmpty()) {
					synchronized (keys) {
						if (keys.contains(key)) continue;
						keys.add(key);
					}
				}

				// subscribe the event handler
				subscribers.add(subscribe(eventClass, o, m));
			}
		}
		return subscribers;
	}

	@Override
	public void unsubscribe(final Collection> subscribers) {
		for (final EventSubscriber subscriber : subscribers) {
			unsubscribe(subscriber);
		}
	}

	@Override
	public  List> getSubscribers(
		final Class c)
	{
		// HACK - It appears that EventBus API is incorrect, in that
		// EventBus#getSubscribers(Class) returns a List when it should
		// actually be a List>. This method works around the
		// problem with casts.
		@SuppressWarnings("rawtypes")
		final List list = eventBus.getSubscribers(c);
		@SuppressWarnings("unchecked")
		final List> typedList = list;
		return typedList;
	}

	// -- Service methods --

	@Override
	public void initialize() {
		eventBus = new DefaultEventBus(threadService, log);
		super.initialize();
	}

	// -- Disposable methods --

	@Override
	public void dispose() {
		eventBus.clearAllSubscribers();
	}

	// -- Helper methods --

	private  void subscribe(final Class c,
		final EventSubscriber subscriber)
	{
		eventBus.subscribe(c, subscriber);
	}

	private  void unsubscribe(
		final EventSubscriber subscriber)
	{
		unsubscribe(subscriber.getEventClass(), subscriber);
	}

	private  void unsubscribe(final Class c,
		final EventSubscriber subscriber)
	{
		eventBus.unsubscribe(c, subscriber);
	}

	private  EventSubscriber subscribe(
		final Class c, final Object o, final Method m)
	{
		final ProxySubscriber subscriber = new ProxySubscriber(c, o, m);
		subscribe(c, subscriber);
		return subscriber;
	}

	/** Gets the event class parameter of the given method. */
	private Class getEventClass(final Method m) {
		// Check for a cached entry for the given method
		Class eventClass = eventClasses.get(m);

		if (eventClass == null) {
			final Class[] c = m.getParameterTypes();
			if (c == null || c.length != 1) return null; // wrong number of args
			if (!SciJavaEvent.class.isAssignableFrom(c[0])) return null; // wrong class

			// Cache the eventClass
			eventClass = c[0];
			eventClasses.put(m, eventClass);
		}

		@SuppressWarnings("unchecked")
		final Class typedClass =
		(Class) eventClass;

		return typedClass;
	}

	// -- Event handlers garbage collection preventer --

	private WeakHashMap>> keepEm =
			new WeakHashMap>>();

	/**
	 * Prevents {@link ProxySubscriber} instances from being garbage collected
	 * prematurely.
	 * 
	 * We instantiate a {@link ProxySubscriber} for each method with an
	 * {@link EventHandler} annotation. These instances are then passed to the
	 * EventBus. The way the instances are created ensures that the event handlers
	 * will be held only as weak references. But they are weak references to the
	 * {@link ProxySubscriber} instances rather than the object containing the
	 * {@link EventHandler}-annotated methods. Therefore, we have to make sure
	 * that there is a non-GC'able reference to each {@link ProxySubscriber} as
	 * long as there is a reference to the containing event handler object.
	 * 
	 * 
	 * @param o the object containing {@link EventHandler}-annotated methods
	 * @param subscriber a {@link ProxySubscriber} for a particular {@link EventHandler}
	 */
	private synchronized void keepIt(final Object o, final ProxySubscriber subscriber) {
		List> list = keepEm.get(o);
		if (list == null) {
			list = new ArrayList>();
			keepEm.put(o, list);
		}
		list.add(subscriber);
	}

	// -- Helper classes --

	/**
	 * Helper class used by {@link #subscribe(Object)}.
	 * 
	 * Recapitulates some logic from {@link BaseProxySubscriber}, because that
	 * class implements {@link org.bushe.swing.event.EventSubscriber} as a raw
	 * type, which is incompatible with this class implementing SciJava's
	 * {@link EventSubscriber} as a typed interface; it becomes impossible to
	 * implement both {@code onEvent(Object)} and {@code onEvent(E)}.
	 * 
	 */
	private class ProxySubscriber extends
		AbstractProxySubscriber implements EventSubscriber
	{

		private final Class c;

		public ProxySubscriber(final Class c, final Object o, final Method m) {
			super(o, m, ReferenceStrength.WEAK, eventBus, false);
			keepIt(o, this);
			this.c = c;

			// allow calling of non-public methods
			m.setAccessible(true);
		}

		/**
		 * Handles the event publication by pushing it to the real subscriber's
		 * subscription method.
		 * 
		 * @param event The event to publish.
		 */
		@Override
		public void onEvent(final E event) {
			try {
				final Object obj = getProxiedSubscriber();
				if (obj == null) return; // has been garbage collected
				getSubscriptionMethod().invoke(obj, event);
			}
			catch (final IllegalAccessException exc) {
				log.error("Exception during event handling:\n\t[Event] " +
					event.getClass().getName() + ":" + event + "\n\t[Subscriber] " +
					getProxiedSubscriber() + "\n\t[Method] " + getSubscriptionMethod(),
					exc);
			}
			catch (final InvocationTargetException exc) {
				log.error("Exception during event handling:\n\t[Event] " +
					event.getClass().getName() + event + "\n\t[Subscriber] " +
					getProxiedSubscriber() + "\n\t[Method] " + getSubscriptionMethod(),
					exc.getCause());
			}
		}

		@Override
		public Class getEventClass() {
			return c;
		}

	}

}