/*
 * #%L
 * SciJava Common shared library for SciJava software.
 * %%
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 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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package org.scijava.event;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Annotation identifying an event handler method. This annotation allows a
 * class to subscribe to multiple types of events by implementing multiple event
 * handling methods and annotating each with @{@link EventHandler}.
 * <p>
 * Note to developers: This annotation serves exactly the same purpose as
 * EventBus's {@code org.scijava.event.bushe.EventSubscriber} annotation,
 * recapitulating a subset of the same functionality. We do this to avoid third
 * party code depending directly on EventBus. That is, we do not wish to require
 * SciJava developers to {@code import org.scijava.event.bushe.*} or similar. In
 * this way, EventBus is isolated as only a transitive dependency of downstream
 * code, rather than a direct dependency. Unfortunately, because Java annotation
 * interfaces cannot utilize inheritance, we have to recapitulate the
 * functionality rather than extend it (as we are able to do with
 * {@link EventSubscriber}).
 * </p>
 * 
 * @author Curtis Rueden
 * @see EventService
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface EventHandler {

	/**
	 * Unique subscription key. If multiple {@code @EventHandler} annotations
	 * exist with the same key, only the first to be subscribed will be respected;
	 * the others will be silently ignored. If no key is specified, the event
	 * handler is always subscribed.
	 * <p>
	 * This feature exists to enable better extensibility of event handling: if
	 * code exists that handles an event in an undesirable way, that logic can be
	 * completely intercepted and overridden by writing more code that handles the
	 * event in a better way, specifying the same key as the original.
	 * </p>
	 */
	String key() default "";

}
