org.scijava.event.EventHandler Maven / Gradle / Ivy
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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}.
*
* Note to developers: This annotation serves exactly the same purpose as
* EventBus's {@link org.bushe.swing.event.annotation.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.bushe.swing.event.*}
* 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}).
*
*
* @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.
*
* 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.
*
*/
String key() default "";
}