Add callback event handling

The EventHandler class has been converted to an interface, with the
reflection specific part being moved to the new ReflectiveEventHandler
class. Callback event handlers implement the same interface through the
CallbackEventHandler class.

The event handlers are defined in the new handler package, which is not
exported by the eventbus.core module.

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 +252,85 @@ 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 callback  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> callback) {
+		registerListener(eventType, callback, 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 callback    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> callback,
+		boolean polymorphic) {
+		registerListener(eventType, callback, 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 callback  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> callback, int priority) {
+		registerListener(eventType, callback, 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 callback    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> callback, boolean polymorphic,
+		int priority) {
+		Objects.requireNonNull(callback);
+		if (registeredListeners.contains(callback))
+			throw new EventBusException(callback + " already registered!");
+		logger.log(Level.INFO, "Registering callback event listener {0}",
+			callback.getClass().getName());
+
+		registeredListeners.add(callback);
+		bindHandler(new CallbackEventHandler(eventType, callback, 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 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 is polymorphic
+	 * @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);
-	}
 }