Replace includeSubtypes with @Polymorphic

The new @Polymorphic annotation serves the exact same purpose as
@Event(includeSubtypes = true), but should be easier to read in complex
handler declarations. It has to be used in conjunction with the @Event
annotation, not instead of it.

README.md

@@ -55,21 +55,23 @@ Note that creating static event handlers like this
 
 ```java
 @Event
-private static void onSimpleEvent(SimpleEvent event) ...
+private static void onSimpleEvent(SimpleEvent event) { ... }
 ```
 
 is technically possible, however you would still have to create an instance of the event listener to register it at an event bus.
 
-## Event handlers for subtypes
+## Polymorphic Event Handlers
 
 On certain occasions it's practical for an event handler to accept both events of the specified type, as well as subclasses of that event.
-To include subtypes for an event handler, use the `includeSubtypes` parameter as follows:
+To include subtypes for an event handler, use the `@Polymorphic` annotation in addition to `@Event`:
 
 ```java
-@Event(includeSubtypes = true)
+@Event
+@Polymorphic
+private void onSimpleEvent(SimpleEvent event) { ... }
 ```
 
-## Event handler execution order
+## Event Handler Execution Order
 
 Sometimes when using multiple handlers for one event, it might be useful to know in which order they will be executed.
 Event Bus provides a mechanism to ensure the correct propagation of events: the `priority`.
@@ -86,7 +88,7 @@ Events are dispatched top-down, meaning the event handler with the highest prior
 
 If no priority is set or multiple handlers have the same priority, the order of execution is undefined.
 
-## Parameter-less event handlers
+## Parameter-Less Event Handlers
 
 In some cases an event handler is not interested in the dispatched event instance.
 To avoid declaring a useless parameter just to specify the event type of the handler, there is an alternative:
@@ -100,7 +102,7 @@ private void onSimpleEvent() {
 
 Make sure that you **do not** declare both a parameter and the `eventType` value of the annotation, as this would be ambiguous.
 
-## Event consumption
+## Event Consumption
 
 In some cases it might be useful to stop the propagation of an event.
 Event Bus makes this possible with event consumption:
@@ -152,7 +154,7 @@ Then, require the Event Bus Core module in your `module-info.java`:
 requires dev.kske.eventbus.core;
 ```
 
-# Compile-Time Error Checking with Event Bus AP
+## Compile-Time Error Checking with Event Bus AP
 
 To assist you with writing event listeners, the Event Bus AP (Annotation Processor) module enforces correct usage of the `@Event` annotation during compile time.
 This reduces difficult-to-debug bugs that occur during runtime to compile-time errors which can be easily fixed.

event-bus-ap/src/main/java/dev/kske/eventbus/ap/EventProcessor.java

@@ -73,8 +73,9 @@ public class EventProcessor extends AbstractProcessor {
 				getTypeMirror(IEvent.class)))
 				error(paramElement, "Parameter must implement IEvent");
 
-			// Check for handlers for abstract types that don't include subtypes
-			if (!eventAnnotation.includeSubtypes() && paramType.getKind() == TypeKind.DECLARED) {
+			// Check for handlers for abstract types that aren't polymorphic
+			if (eventHandler.getAnnotation(Polymorphic.class) == null
+				&& paramType.getKind() == TypeKind.DECLARED) {
 				var declaredElement = ((DeclaredType) paramType).asElement();
 				if (declaredElement.getKind() == ElementKind.INTERFACE
 					|| declaredElement.getModifiers().contains(Modifier.ABSTRACT))

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

@@ -21,6 +21,7 @@ import java.lang.annotation.*;
  *
  * @author Kai S. K. Engelbart
  * @since 0.0.1
+ * @see Polymorphic
  */
 @Documented
 @Retention(RUNTIME)
@@ -38,14 +39,6 @@ public @interface Event {
 	 */
 	int priority() default 100;
 
-	/**
-	 * Defines whether instances of subtypes of the event type are dispatched to the event handler.
-	 *
-	 * @return whether the event handler includes subtypes
-	 * @since 0.0.4
-	 */
-	boolean includeSubtypes() default false;
-
 	/**
 	 * Defines the event type the handler listens to. If this value is set, the handler is not
 	 * allowed to declare parameters.

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

@@ -104,7 +104,7 @@ public final class EventBus {
 		for (var binding : bindings.entrySet())
 			if (binding.getKey().isAssignableFrom(eventClass))
 				for (var handler : binding.getValue())
-					if (handler.includeSubtypes())
+					if (handler.isPolymorphic())
 						handlers.add(handler);
 
 		return new ArrayList<>(handlers);

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

@@ -17,6 +17,7 @@ final class EventHandler implements Comparable<EventHandler> {
 	private final Method					method;
 	private final Event						annotation;
 	private final Class<? extends IEvent>	eventType;
+	private final boolean					polymorphic;
 
 	/**
 	 * Constructs an event handler.
@@ -56,6 +57,7 @@ final class EventHandler implements Comparable<EventHandler> {
 			eventType = (Class<? extends IEvent>) param;
 		}
 		this.eventType	= eventType;
+		polymorphic		= method.isAnnotationPresent(Polymorphic.class);
 
 		// Allow access if the method is non-public
 		method.setAccessible(true);
@@ -65,7 +67,7 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * Compares this to another event handler based on {@link Event#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 the correct order from a tree set.
+	 * This is used to retrieve event handlers in order of descending priority from a tree set.
 	 *
 	 * @since 0.0.1
 	 */
@@ -91,15 +93,11 @@ final class EventHandler implements Comparable<EventHandler> {
 	 */
 	void execute(IEvent event) throws EventBusException {
 		try {
-			if (annotation.eventType().equals(USE_PARAMETER.class))
+			if (annotation.eventType() == USE_PARAMETER.class)
 				method.invoke(listener, event);
 			else
 				method.invoke(listener);
-		} catch (
-			IllegalAccessException
-			| IllegalArgumentException
-			| InvocationTargetException e
-		) {
+		} catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
 			throw new EventBusException("Failed to invoke event handler!", e);
 		}
 	}
@@ -123,10 +121,11 @@ final class EventHandler implements Comparable<EventHandler> {
 	int getPriority() { return annotation.priority(); }
 
 	/**
-	 * @return whether this handler includes subtypes
-	 * @since 0.0.4
+	 * @return whether this handler is polymorphic
+	 * @since 1.0.0
+	 * @see Polymorphic
 	 */
-	boolean includeSubtypes() { return annotation.includeSubtypes(); }
+	boolean isPolymorphic() { return polymorphic; }
 
 	/**
 	 * @return the event type this handler listens to

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

@@ -0,0 +1,20 @@
+package dev.kske.eventbus.core;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.*;
+
+/**
+ * Allows an event handler to receive events that are subtypes of the declared event type.
+ * <p>
+ * This is useful when defining an event handler for an interface or an abstract class.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ * @see Event
+ */
+@Documented
+@Retention(RUNTIME)
+@Target(METHOD)
+public @interface Polymorphic {}

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

@@ -38,7 +38,8 @@ class DispatchTest implements EventListener {
 		bus.dispatch(new SimpleEvent());
 	}
 
-	@Event(eventType = SimpleEvent.class, includeSubtypes = true, priority = 200)
+	@Event(eventType = SimpleEvent.class, priority = 200)
+	@Polymorphic
 	void onSimpleEventFirst() {
 		++hits;
 		assertTrue(hits == 1 || hits == 2);