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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 downstream projects in the SciJava ecosystem, such as ImageJ and SCIFIO.
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/*
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 * SciJava Common shared library for SciJava software.
 * %%
 * Copyright (C) 2009 - 2017 Board of Regents of the University of
 * Wisconsin-Madison, Broad Institute of MIT and Harvard, Max Planck
 * Institute of Molecular Cell Biology and Genetics, University of
 * Konstanz, and KNIME GmbH.
 * %%
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 * 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.
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package org.scijava.event;


import java.util.Collection;
import java.util.List;

import org.scijava.service.SciJavaService;

/**
 * Interface for the event handling service.
 * 
 * @author Curtis Rueden
 * @author Grant Harris
 */
public interface EventService extends SciJavaService {

	/**
	 * Publishes the given event immediately, reporting it to all subscribers.
	 * Does not return until all subscribers have handled the event.
	 * 
	 * Note that with {@link #publish}, in the case of multiple events published
	 * in a chain to multiple subscribers, the delivery order will resemble that
	 * of a stack. For example:
	 * 
	 * 
	 * {@link org.scijava.module.event.ModulesUpdatedEvent} is published with
	 * {@link #publish}.
	 * {@link org.scijava.menu.DefaultMenuService} receives the event and
	 * handles it, publishing {@link org.scijava.menu.event.MenusUpdatedEvent} in
	 * response.
	 * A third party that subscribes to both
	 * {@link org.scijava.module.event.ModulesUpdatedEvent} and
	 * {@link org.scijava.menu.event.MenusUpdatedEvent} will receive the latter
	 * before the former.
	 * 
	 * That said, the behavior of {@link #publish} depends on the thread from
	 * which it is called: if called from a thread identified as a dispatch thread
	 * by {@link org.scijava.thread.ThreadService#isDispatchThread()}, it will
	 * publish immediately; otherwise, it will be queued for publication on a
	 * dispatch thread, and block the calling thread until publication is
	 * complete. This means that a chain of events published with a mixture of
	 * {@link #publish} and {@link #publishLater} may result in event delivery in
	 * an unintuitive order.
	 */
	 void publish(E e);

	/**
	 * Queues the given event for publication, typically on a separate thread
	 * (called the "event dispatch thread"). This method returns immediately,
	 * before subscribers have fully received the event.
	 * 
	 * Note that with {@link #publishLater}, in the case of multiple events
	 * published in a chain to multiple subscribers, the delivery order will
	 * resemble that of a queue. For example:
	 * 
	 * 
	 * {@link org.scijava.module.event.ModulesUpdatedEvent} is published with
	 * {@link #publishLater}.
	 * {@link org.scijava.menu.DefaultMenuService} receives the event and
	 * handles it, publishing {@link org.scijava.menu.event.MenusUpdatedEvent} in
	 * response.
	 * A third party that subscribes to both
	 * {@link org.scijava.module.event.ModulesUpdatedEvent} and
	 * {@link org.scijava.menu.event.MenusUpdatedEvent} will receive the former
	 * first, since it was already queued by the time the latter was published.
	 * 
	 */
	 void publishLater(E e);

	/**
	 * Subscribes all of the given object's @{@link EventHandler} annotated
	 * methods.
	 * 
	 * This allows a single class to subscribe to multiple types of events by
	 * implementing multiple event handling methods and annotating each one with
	 * the {@link EventHandler} annotation.
	 * 
	 * 
	 * Note that it is not necessary to store a copy of the event
	 * subscribers (because the event service is expected to hold a weak mapping
	 * between the event handler object and the subscribers) unless the
	 * subscribers need to be unsubscribed explicitly.
	 * 
	 * 
	 * Most users will want to extend {@link org.scijava.AbstractContextual}, or
	 * call {@link org.scijava.Context#inject(Object)}, instead of subscribing to
	 * the event service explicitly.
	 * 
	 * 
	 * @param o the event handler object containing the {@link EventHandler}
	 *          annotated methods
	 * @return The list of newly created {@link EventSubscriber}s, weakly
	 *         subscribed to the event service.
	 * @see org.scijava.AbstractContextual
	 * @see org.scijava.Context#inject(Object)
	 */
	List> subscribe(Object o);

	/**
	 * Removes all the given subscribers; they will no longer be notified when
	 * events are published.
	 */
	void unsubscribe(Collection> subscribers);

	/**
	 * Gets a list of all subscribers to the given event class (and subclasses
	 * thereof).
	 */
	 List> getSubscribers(Class c);

}