event-bus-core/src/main/java/dev/kske/eventbus/core/Event.java

@@ -22,7 +22,7 @@ import java.lang.annotation.*;
 public @interface Event {
 
 	/**
-	 * Defines the event type the handler listens to. If this value is set, the handler is not
+	 * Defines the event type the handler listens for. If this value is set, the handler is not
 	 * allowed to declare parameters.
 	 * <p>
 	 * This is useful when the event handler does not utilize the event instance.

event-bus-core/src/main/java/dev/kske/eventbus/core/EventBus.java

@@ -5,6 +5,9 @@ import java.lang.System.Logger.Level;
 import java.lang.reflect.InvocationTargetException;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Consumer;
+
+import dev.kske.eventbus.core.handler.*;
 
 /**
  * Event listeners can be registered at an event bus to be notified when an event is dispatched.
@@ -56,6 +59,19 @@ public final class EventBus {
 
 	private static final Logger logger = System.getLogger(EventBus.class.getName());
 
+	/**
+	 * Compares event handlers based on priority, but uses hash codes for equal priorities.
+	 *
+	 * @implNote As the priority comparator by itself is not consistent with equals (two handlers
+	 *           with the same priority are not necessarily equal, but would have a comparison
+	 *           result of 0), the hash code is used for the fallback comparison. This way,
+	 *           consistency with equals is restored.
+	 * @since 1.2.0
+	 */
+	private static final Comparator<EventHandler> byPriority =
+		Comparator.comparingInt(EventHandler::getPriority).reversed()
+			.thenComparingInt(EventHandler::hashCode);
+
 	/**
 	 * Returns the default event bus, which is a statically initialized singleton instance.
 	 *
@@ -154,18 +170,19 @@ public final class EventBus {
 	 * Searches for the event handlers bound to an event class. This includes polymorphic handlers
 	 * that are bound to a supertype of the event class.
 	 *
-	 * @param eventClass the event class to use for the search
+	 * @param eventType the event type to use for the search
 	 * @return a navigable set containing the applicable handlers in descending order of priority
 	 * @since 1.2.0
 	 */
-	private NavigableSet<EventHandler> getHandlersFor(Class<?> eventClass) {
+	private NavigableSet<EventHandler> getHandlersFor(Class<?> eventType) {
 
 		// Get handlers defined for the event class
-		TreeSet<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
+		TreeSet<EventHandler> handlers =
+			bindings.getOrDefault(eventType, new TreeSet<>(byPriority));
 
 		// Get polymorphic handlers
 		for (var binding : bindings.entrySet())
-			if (binding.getKey().isAssignableFrom(eventClass))
+			if (binding.getKey().isAssignableFrom(eventType))
 				for (var handler : binding.getValue())
 					if (handler.isPolymorphic())
 						handlers.add(handler);
@@ -223,11 +240,8 @@ public final class EventBus {
 				continue;
 
 			// Initialize and bind the handler
-			var handler = new EventHandler(listener, method, annotation, polymorphic, priority);
-			bindings.putIfAbsent(handler.getEventType(), new TreeSet<>());
-			logger.log(Level.DEBUG, "Binding event handler {0}", handler);
-			bindings.get(handler.getEventType())
-				.add(handler);
+			bindHandler(
+				new ReflectiveEventHandler(listener, method, annotation, polymorphic, priority));
 			handlerBound = true;
 		}
 
@@ -238,6 +252,86 @@ public final class EventBus {
 				listener.getClass().getName());
 	}
 
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
+	 * listener is not polymorphic and has the {@link #DEFAULT_PRIORITY}.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener) {
+		registerListener(eventType, eventListener, false, DEFAULT_PRIORITY);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
+	 * listener has the {@link #DEFAULT_PRIORITY}.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param polymorphic   whether the listener is also invoked for subtypes of the event type
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener,
+		boolean polymorphic) {
+		registerListener(eventType, eventListener, polymorphic, DEFAULT_PRIORITY);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
+	 * listener is not polymorphic.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param priority      the priority to assign to the listener
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener, int priority) {
+		registerListener(eventType, eventListener, false, priority);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param polymorphic   whether the listener is also invoked for subtypes of the event type
+	 * @param priority      the priority to assign to the listener
+	 * @since 1.2.0
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener,
+		boolean polymorphic,
+		int priority) {
+		Objects.requireNonNull(eventListener);
+		if (registeredListeners.contains(eventListener))
+			throw new EventBusException(eventListener + " already registered!");
+		logger.log(Level.INFO, "Registering callback event listener {0}",
+			eventListener.getClass().getName());
+
+		registeredListeners.add(eventListener);
+		bindHandler(new CallbackEventHandler(eventType, eventListener, polymorphic, priority));
+	}
+
+	/**
+	 * Inserts a new handler into the {@link #bindings} map.
+	 *
+	 * @param handler the handler to bind
+	 * @since 1.2.0
+	 */
+	private void bindHandler(EventHandler handler) {
+		bindings.putIfAbsent(handler.getEventType(), new TreeSet<>(byPriority));
+		logger.log(Level.DEBUG, "Binding event handler {0}", handler);
+		bindings.get(handler.getEventType()).add(handler);
+	}
+
 	/**
 	 * Removes a specific listener from this event bus.
 	 *

event-bus-core/src/main/java/dev/kske/eventbus/core/handler/CallbackEventHandler.java

@@ -0,0 +1,73 @@
+package dev.kske.eventbus.core.handler;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.function.Consumer;
+
+/**
+ * An event handler wrapping a callback method.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ */
+public final class CallbackEventHandler implements EventHandler {
+
+	private final Class<?>			eventType;
+	private final Consumer<Object>	callback;
+	private final boolean			polymorphic;
+	private final int				priority;
+
+	/**
+	 * Constructs a callback event handler.
+	 *
+	 * @param <E>         the event type of the handler
+	 * @param eventType   the event type of the handler
+	 * @param callback    the callback method to execute when the handler is invoked
+	 * @param polymorphic whether the handler is polymorphic
+	 * @param priority    the priority of the handler
+	 * @since 1.2.0
+	 */
+	@SuppressWarnings("unchecked")
+	public <E> CallbackEventHandler(Class<E> eventType, Consumer<E> callback, boolean polymorphic,
+		int priority) {
+		this.eventType		= eventType;
+		this.callback		= (Consumer<Object>) callback;
+		this.polymorphic	= polymorphic;
+		this.priority		= priority;
+	}
+
+	@Override
+	public void execute(Object event) throws InvocationTargetException {
+		try {
+			callback.accept(event);
+		} catch (RuntimeException e) {
+			throw new InvocationTargetException(e, "Callback event handler failed!");
+		}
+	}
+
+	@Override
+	public String toString() {
+		return String.format(
+			"CallbackEventHandler[eventType=%s, polymorphic=%b, priority=%d]",
+			eventType, polymorphic, priority);
+	}
+
+	@Override
+	public Consumer<?> getListener() {
+		return callback;
+	}
+
+	@Override
+	public Class<?> getEventType() {
+		return eventType;
+	}
+
+	@Override
+	public int getPriority() {
+		return priority;
+	}
+
+	@Override
+	public boolean isPolymorphic() {
+		return polymorphic;
+	}
+}

event-bus-core/src/main/java/dev/kske/eventbus/core/handler/EventHandler.java

@@ -0,0 +1,53 @@
+package dev.kske.eventbus.core.handler;
+
+import java.lang.reflect.InvocationTargetException;
+
+import dev.kske.eventbus.core.*;
+
+/**
+ * Internal representation of an event handling method.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ * @see EventBus
+ */
+public interface EventHandler {
+
+	/**
+	 * Executes the event handler.
+	 *
+	 * @param event the event used as the method parameter
+	 * @throws EventBusException         if the event handler isn't accessible or has an invalid
+	 *                                   signature
+	 * @throws InvocationTargetException if the handler throws an exception
+	 * @throws EventBusException         if the handler has the wrong signature or is inaccessible
+	 * @since 1.2.0
+	 */
+	void execute(Object event) throws EventBusException, InvocationTargetException;
+
+	/**
+	 * @return the listener containing this handler
+	 * @since 1.2.0
+	 */
+	Object getListener();
+
+	/**
+	 * @return the event type this handler listens for
+	 * @since 1.2.0
+	 */
+	Class<?> getEventType();
+
+	/**
+	 * @return the priority of this handler
+	 * @since 1.2.0
+	 * @see Priority
+	 */
+	int getPriority();
+
+	/**
+	 * @return whether this handler also accepts subtypes of the event type
+	 * @since 1.2.0
+	 * @see Polymorphic
+	 */
+	boolean isPolymorphic();
+}

event-bus-core/src/main/java/dev/kske/eventbus/core/handler/ReflectiveEventHandler.java

@@ -1,17 +1,18 @@
-package dev.kske.eventbus.core;
+package dev.kske.eventbus.core.handler;
 
 import java.lang.reflect.*;
 
+import dev.kske.eventbus.core.*;
 import dev.kske.eventbus.core.Event.USE_PARAMETER;
 
 /**
- * Internal representation of an event handling method.
+ * An event handler wrapping a method annotated with {@link Event} and executing it using
+ * reflection.
  *
  * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see EventBus
+ * @since 1.2.0
  */
-final class EventHandler implements Comparable<EventHandler> {
+public final class ReflectiveEventHandler implements EventHandler {
 
 	private final Object	listener;
 	private final Method	method;
@@ -21,7 +22,7 @@ final class EventHandler implements Comparable<EventHandler> {
 	private final int		priority;
 
 	/**
-	 * Constructs an event handler.
+	 * Constructs a reflective event handler.
 	 *
 	 * @param listener        the listener containing the handler
 	 * @param method          the handler method
@@ -30,10 +31,10 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * @param defPriority     the predefined priority (default or listener-level)
 	 * @throws EventBusException if the method or the annotation do not comply with the
 	 *                           specification
-	 * @since 0.0.1
+	 * @since 1.2.0
 	 */
-	EventHandler(Object listener, Method method, Event annotation, boolean defPolymorphism,
-		int defPriority) throws EventBusException {
+	public ReflectiveEventHandler(Object listener, Method method, Event annotation,
+		boolean defPolymorphism, int defPriority) throws EventBusException {
 		this.listener	= listener;
 		this.method		= method;
 		useParameter	= annotation.value() == USE_PARAMETER.class;
@@ -61,45 +62,13 @@ final class EventHandler implements Comparable<EventHandler> {
 		method.setAccessible(true);
 	}
 
-	/**
-	 * Compares this to another event handler based on priority. In case of equal priority a
-	 * non-zero value based on hash codes is returned.
-	 * <p>
-	 * This is used to retrieve event handlers in descending order of priority from a tree set.
-	 *
-	 * @since 0.0.1
-	 */
 	@Override
-	public int compareTo(EventHandler other) {
-		int priority = other.priority - this.priority;
-		if (priority == 0)
-			priority = listener.hashCode() - other.listener.hashCode();
-		return priority == 0 ? hashCode() - other.hashCode() : priority;
-	}
-
-	@Override
-	public String toString() {
-		return String.format(
-			"EventHandler[method=%s, eventType=%s, useParameter=%b, polymorphic=%b, priority=%d]",
-			method, eventType, useParameter, polymorphic, priority);
-	}
-
-	/**
-	 * Executes the event handler.
-	 *
-	 * @param event the event used as the method parameter
-	 * @throws EventBusException         if the event handler isn't accessible or has an invalid
-	 *                                   signature
-	 * @throws InvocationTargetException if the handler throws an exception
-	 * @throws EventBusException         if the handler has the wrong signature or is inaccessible
-	 * @since 0.0.1
-	 */
-	void execute(Object event) throws EventBusException, InvocationTargetException {
+	public void execute(Object event) throws EventBusException, InvocationTargetException {
 		try {
 			if (useParameter)
-				method.invoke(listener, event);
+				method.invoke(getListener(), event);
 			else
-				method.invoke(listener);
+				method.invoke(getListener());
 		} catch (IllegalArgumentException e) {
 			throw new EventBusException("Event handler rejected target / argument!", e);
 		} catch (IllegalAccessException e) {
@@ -107,29 +76,30 @@ final class EventHandler implements Comparable<EventHandler> {
 		}
 	}
 
-	/**
-	 * @return the listener containing this handler
-	 * @since 0.0.1
-	 */
-	Object getListener() { return listener; }
+	@Override
+	public String toString() {
+		return String.format(
+			"ReflectiveEventHandler[eventType=%s, polymorphic=%b, priority=%d, method=%s, useParameter=%b]",
+			eventType, polymorphic, priority, method, useParameter);
+	}
 
-	/**
-	 * @return the event type this handler listens for
-	 * @since 0.0.3
-	 */
-	Class<?> getEventType() { return eventType; }
+	@Override
+	public Object getListener() {
+		return listener;
+	}
 
-	/**
-	 * @return the priority of this handler
-	 * @since 0.0.1
-	 * @see Priority
-	 */
-	int getPriority() { return priority; }
+	@Override
+	public Class<?> getEventType() {
+		return eventType;
+	}
 
-	/**
-	 * @return whether this handler is polymorphic
-	 * @since 1.0.0
-	 * @see Polymorphic
-	 */
-	boolean isPolymorphic() { return polymorphic; }
+	@Override
+	public int getPriority() {
+		return priority;
+	}
+
+	@Override
+	public boolean isPolymorphic() {
+		return polymorphic;
+	}
 }

event-bus-core/src/main/java/dev/kske/eventbus/core/handler/package-info.java

@@ -0,0 +1,8 @@
+/**
+ * Contains the internal representation of event handling methods.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ * @see dev.kske.eventbus.core.handler.EventHandler
+ */
+package dev.kske.eventbus.core.handler;

event-bus-core/src/test/java/dev/kske/eventbus/core/DispatchTest.java

@@ -26,6 +26,10 @@ class DispatchTest {
 	void registerListener() {
 		bus = new EventBus();
 		bus.registerListener(this);
+		bus.registerListener(SimpleEvent.class, e -> {
+			++hits;
+			assertEquals(4, hits);
+		});
 	}
 
 	/**
@@ -52,9 +56,9 @@ class DispatchTest {
 		assertEquals(
 			"Event handler execution order for class dev.kske.eventbus.core.SimpleEvent (3 handler(s)):\n"
 				+ "==========================================================================================\n"
-				+ "EventHandler[method=void dev.kske.eventbus.core.DispatchTest.onSimpleEventFirst(), eventType=class dev.kske.eventbus.core.SimpleEvent, useParameter=false, polymorphic=true, priority=200]\n"
-				+ "EventHandler[method=static void dev.kske.eventbus.core.DispatchTest.onSimpleEventSecond(), eventType=class dev.kske.eventbus.core.SimpleEvent, useParameter=false, polymorphic=false, priority=150]\n"
-				+ "EventHandler[method=void dev.kske.eventbus.core.DispatchTest.onSimpleEventThird(dev.kske.eventbus.core.SimpleEvent), eventType=class dev.kske.eventbus.core.SimpleEvent, useParameter=true, polymorphic=false, priority=100]\n"
+				+ "ReflectiveEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=true, priority=200, method=void dev.kske.eventbus.core.DispatchTest.onSimpleEventFirst(), useParameter=false]\n"
+				+ "ReflectiveEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=false, priority=150, method=static void dev.kske.eventbus.core.DispatchTest.onSimpleEventSecond(), useParameter=false]\n"
+				+ "CallbackEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=false, priority=100]\n"
 				+ "==========================================================================================",
 			executionOrder);
 	}
@@ -72,12 +76,4 @@ class DispatchTest {
 		++hits;
 		assertEquals(3, hits);
 	}
-
-	@Event
-	@Polymorphic(false)
-	@Priority(100)
-	void onSimpleEventThird(SimpleEvent event) {
-		++hits;
-		assertEquals(4, hits);
-	}
 }