Merge pull request 'Split @Event Parameters Into @Polymorphic and @Property, Remove Marker Interfaces' (#5) from f/new-annotations into develop

Reviewed-on: https://git.kske.dev/kske/event-bus/pulls/5
Reviewed-by: delvh <leon@kske.dev>

README.md

@@ -3,7 +3,7 @@
 ## Introduction
 
 This library allows passing events between different objects without them having a direct reference to each other.
-Any class can be made an event by implementing the `IEvent` interface.
+Any object can serve as an event.
 
 Using an instance of the `EventBus` class, an instant of the event class can be dispatched.
 This means that it will be forwarded to all listeners registered for it at the event bus.
@@ -13,16 +13,13 @@ In addition, a singleton instance of the event bus is provided by the `EventBus#
 To listen to events, register event handling methods using the `Event` annotation.
 For this to work, the method must have a return type of `void` and declare a single parameter of the desired event type.
 Alternatively, a parameter-less event handler can be declared as shown [below](#parameter-less-event-handlers).
-Additionally, the class containing the method must implement the `EventListener` interface.
 
 ## A Simple Example
 
-Lets look at a simple example: we declare the empty class `SimpleEvent` that implements `IEvent` and can thus be used as an event.
+Lets look at a simple example: we declare the empty class `SimpleEvent` whose objects can be used as events.
 
 ```java
-import dev.kske.eventbus.core.IEvent;
+public class SimpleEvent {}
-
-public class SimpleEvent implements IEvent {}
 ```
 
 Next, an event listener for the `SimpleEvent` is declared:
@@ -30,7 +27,7 @@ Next, an event listener for the `SimpleEvent` is declared:
 ```java
 import dev.kske.eventbus.core.*;
 
-public class SimpleEventListener implements EventListener {
+public class SimpleEventListener {
 
     public SimpleEventListener() {
 
@@ -55,63 +52,65 @@ 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.
+Sometimes when using multiple handlers for one event, it might be useful to define in which order they will be executed.
-Event Bus provides a mechanism to ensure the correct propagation of events: the `priority`.
+Event Bus assigns a priority to every handler, which is `100` by default, but can be explicitly set using the `@Priority` annotation in addition to `@Event`:
 
-Priority can be set on the `@Event` annotation like that:
 ```java
-@Event(priority=100)
+@Event
+@Priority(250)
+private void onSimpleEvent(SimpleEvent event) { ... }
 ```
 
-The default priority for events is `100`.
-
 **Important:**
-Events are dispatched top-down, meaning the event handler with the highest priority will be executed first.
+Events are dispatched to handlers in descending order of their priority.
+The execution order is undefined for handlers with the same priority.
 
-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:
 
 ```java
-@Event(eventType = SimpleEvent.class)
+@Event(SimpleEvent.class)
 private void onSimpleEvent() {
 	System.out.println("SimpleEvent received!");
 }
 ```
 
-Make sure that you **do not** declare both a parameter and the `eventType` value of the annotation, as this would be ambiguous.
+Make sure that you **do not** both declare a parameter and specify the event type in 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:
 
 ```java
-@Event(eventType = SimpleEvent.class, priority=100)
+@Event(SimpleEvent.class)
+@Priority(100)
 private void onSimpleEvent() {
 	EventBus.getInstance().cancel();
 }
 
-@Event(eventType = SimpleEvent.class, priority=50)
+@Event(SimpleEvent.class)
+@Priority(50)
 private void onSimpleEvent2() {
 	System.out.println("Will not be printed!");
 }
@@ -152,12 +151,18 @@ 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
+If you intend to use event handlers that are inaccessible to Event Bus by means of Java language access control, make sure to allow reflective access from your module:
+
+```java
+opens my.module to dev.kske.eventbus.core;
+```
+
+## 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.
 
-The event annotation processor detects invalid event handlers, missing `EventListener` implementations, event type issues with more to come in future versions.
+The event annotation processor detects invalid event handlers and event type issues with more to come in future versions.
 
 When using Maven, it can be registered using the Maven Compiler Plugin:
 

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

@@ -34,13 +34,12 @@ public class EventProcessor extends AbstractProcessor {
 
 	private void processRound(Set<ExecutableElement> eventHandlers) {
 		for (ExecutableElement eventHandler : eventHandlers) {
-			TypeElement	eventListener	= (TypeElement) eventHandler.getEnclosingElement();
 			Event eventAnnotation = eventHandler.getAnnotation(Event.class);
 
 			// Determine how the event type is defined
 			boolean useParameter;
 			try {
-				eventAnnotation.eventType();
+				eventAnnotation.value();
 				throw new EventBusException(
 					"Could not determine event type of handler " + eventHandler);
 			} catch (MirroredTypeException e) {
@@ -68,24 +67,15 @@ public class EventProcessor extends AbstractProcessor {
 			var	paramElement	= eventHandler.getParameters().get(0);
 			var	paramType		= paramElement.asType();
 
-			// Check for valid event type
+			// Check for handlers for abstract types that aren't polymorphic
-			if (useParameter && !processingEnv.getTypeUtils().isAssignable(paramType,
+			if (eventHandler.getAnnotation(Polymorphic.class) == null
-				getTypeMirror(IEvent.class)))
+				&& paramType.getKind() == TypeKind.DECLARED) {
-				error(paramElement, "Parameter must implement IEvent");
-
-			// Check for handlers for abstract types that don't include subtypes
-			if (!eventAnnotation.includeSubtypes() && paramType.getKind() == TypeKind.DECLARED) {
 				var declaredElement = ((DeclaredType) paramType).asElement();
 				if (declaredElement.getKind() == ElementKind.INTERFACE
 					|| declaredElement.getModifiers().contains(Modifier.ABSTRACT))
 					warning(paramElement,
-						"Parameter should be instantiable or handler should include subtypes");
+						"Parameter should be instantiable or handler should use @Polymorphic");
 			}
-
-			// Check listener for interface implementation
-			if (!eventListener.getInterfaces().contains(getTypeMirror(EventListener.class)))
-				warning(eventHandler.getEnclosingElement(),
-					"Class should implement EventListener interface");
 		}
 	}
 

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

@@ -9,11 +9,10 @@ import java.lang.annotation.*;
  * Indicates that a method is an event handler. To be successfully used as such, the method has to
  * comply with the following specifications:
  * <ul>
- * <li>Declared inside a class that implements {@link EventListener}</li>
  * <li>Specifying an event type by either
  * <ul>
- * <li>Declaring one parameter of a type that implements {@link IEvent}</li>
+ * <li>Declaring one object parameter</li>
- * <li>Defining the class of the event using the {@link Event#eventType()} value</li>
+ * <li>Defining the class of the event using the annotation value</li>
  * </ul>
  * </li>
  * <li>Return type of {@code void}</li>
@@ -21,31 +20,14 @@ import java.lang.annotation.*;
  *
  * @author Kai S. K. Engelbart
  * @since 0.0.1
+ * @see Polymorphic
+ * @see Priority
  */
 @Documented
 @Retention(RUNTIME)
 @Target(METHOD)
 public @interface Event {
 
-	/**
-	 * Defines the priority of the event handler. Handlers are executed in descending order of their
-	 * priority.
-	 * <p>
-	 * The execution order of handlers with the same priority is undefined.
-	 *
-	 * @return the priority of the event handler
-	 * @since 0.0.1
-	 */
-	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.
@@ -53,9 +35,9 @@ public @interface Event {
 	 * This is useful when the event handler does not utilize the event instance.
 	 *
 	 * @return the event type accepted by the handler
-	 * @since 0.0.3
+	 * @since 1.0.0
 	 */
-	Class<? extends IEvent> eventType() default USE_PARAMETER.class;
+	Class<?> value() default USE_PARAMETER.class;
 
 	/**
 	 * Signifies that the event type the handler listens to is determined by the type of its only
@@ -63,5 +45,5 @@ public @interface Event {
 	 *
 	 * @since 0.0.3
 	 */
-	static final class USE_PARAMETER implements IEvent {}
+	static final class USE_PARAMETER {}
 }

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

@@ -51,9 +51,9 @@ public final class EventBus {
 		return instance;
 	}
 
-	private final Map<Class<? extends IEvent>, TreeSet<EventHandler>>	bindings			=
+	private final Map<Class<?>, TreeSet<EventHandler>>	bindings			=
 		new ConcurrentHashMap<>();
-	private final Set<EventListener>									registeredListeners	=
+	private final Set<Object>							registeredListeners	=
 		ConcurrentHashMap.newKeySet();
 	private final ThreadLocal<DispatchState>			dispatchState		=
 		ThreadLocal.withInitial(DispatchState::new);
@@ -65,7 +65,7 @@ public final class EventBus {
 	 * @param event the event to dispatch
 	 * @since 0.0.1
 	 */
-	public void dispatch(IEvent event) {
+	public void dispatch(Object event) {
 		Objects.requireNonNull(event);
 		logger.log(Level.INFO, "Dispatching event {0}", event);
 
@@ -95,7 +95,7 @@ public final class EventBus {
 	 * @return all event handlers registered for the event class
 	 * @since 0.0.1
 	 */
-	private List<EventHandler> getHandlersFor(Class<? extends IEvent> eventClass) {
+	private List<EventHandler> getHandlersFor(Class<?> eventClass) {
 
 		// Get handlers defined for the event class
 		Set<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
@@ -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);
@@ -133,7 +133,7 @@ public final class EventBus {
 	 * @since 0.0.1
 	 * @see Event
 	 */
-	public void registerListener(EventListener listener) throws EventBusException {
+	public void registerListener(Object listener) throws EventBusException {
 		Objects.requireNonNull(listener);
 		if (registeredListeners.contains(listener))
 			throw new EventBusException(listener + " already registered!");
@@ -170,7 +170,7 @@ public final class EventBus {
 	 * @param listener the listener to remove
 	 * @since 0.0.1
 	 */
-	public void removeListener(EventListener listener) {
+	public void removeListener(Object listener) {
 		Objects.requireNonNull(listener);
 		logger.log(Level.INFO, "Removing event listener {0}", listener.getClass().getName());
 
@@ -204,7 +204,7 @@ public final class EventBus {
 	 * @return all registered event listeners
 	 * @since 0.0.1
 	 */
-	public Set<EventListener> getRegisteredListeners() {
+	public Set<Object> getRegisteredListeners() {
 		return Collections.unmodifiableSet(registeredListeners);
 	}
 }

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

@@ -13,10 +13,20 @@ import dev.kske.eventbus.core.Event.USE_PARAMETER;
  */
 final class EventHandler implements Comparable<EventHandler> {
 
-	private final EventListener listener;
+	/**
+	 * The priority assigned to every event handler without an explicitly defined priority.
+	 *
+	 * @since 1.0.0
+	 * @see Priority
+	 */
+	public static final int DEFAULT_PRIORITY = 100;
+
+	private final Object	listener;
 	private final Method	method;
-	private final Event annotation;
+	private final Class<?>	eventType;
-	private final Class<? extends IEvent> eventType;
+	private final boolean	useParameter;
+	private final boolean	polymorphic;
+	private final int		priority;
 
 	/**
 	 * Constructs an event handler.
@@ -28,17 +38,16 @@ final class EventHandler implements Comparable<EventHandler> {
 	 *                           specification
 	 * @since 0.0.1
 	 */
-	@SuppressWarnings("unchecked")
+	EventHandler(Object listener, Method method, Event annotation) throws EventBusException {
-	EventHandler(EventListener listener, Method method, Event annotation) throws EventBusException {
 		this.listener	= listener;
 		this.method		= method;
-		this.annotation = annotation;
+		useParameter	= annotation.value() == USE_PARAMETER.class;
 
 		// Check for correct method signature and return type
-		if (method.getParameterCount() == 0 && annotation.eventType().equals(USE_PARAMETER.class))
+		if (method.getParameterCount() == 0 && useParameter)
 			throw new EventBusException(method + " does not define an event type!");
 
-		if (method.getParameterCount() == 1 && !annotation.eventType().equals(USE_PARAMETER.class))
+		if (method.getParameterCount() == 1 && !useParameter)
 			throw new EventBusException(method + " defines an ambiguous event type!");
 
 		if (method.getParameterCount() > 1)
@@ -47,31 +56,28 @@ final class EventHandler implements Comparable<EventHandler> {
 		if (!method.getReturnType().equals(void.class))
 			throw new EventBusException(method + " does not have a return type of void!");
 
-		// Determine the event type
+		// Determine handler properties
-		Class<? extends IEvent> eventType = annotation.eventType();
+		eventType	= useParameter ? method.getParameterTypes()[0] : annotation.value();
-		if (eventType.equals(USE_PARAMETER.class)) {
+		polymorphic	= method.isAnnotationPresent(Polymorphic.class);
-			var param = method.getParameterTypes()[0];
+		priority	= method.isAnnotationPresent(Priority.class)
-			if (!IEvent.class.isAssignableFrom(param))
+			? method.getAnnotation(Priority.class).value()
-				throw new EventBusException(param + " is not of type IEvent!");
+			: DEFAULT_PRIORITY;
-			eventType = (Class<? extends IEvent>) param;
-		}
-		this.eventType = eventType;
 
 		// Allow access if the method is non-public
 		method.setAccessible(true);
 	}
 
 	/**
-	 * Compares this to another event handler based on {@link Event#priority()}. In case of equal
+	 * Compares this to another event handler based on priority. In case of equal priority a
-	 * priority a non-zero value based on hash codes is returned.
+	 * 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
 	 */
 	@Override
 	public int compareTo(EventHandler other) {
-		int priority = other.annotation.priority() - annotation.priority();
+		int priority = other.priority - this.priority;
 		if (priority == 0)
 			priority = listener.hashCode() - other.listener.hashCode();
 		return priority == 0 ? hashCode() - other.hashCode() : priority;
@@ -79,7 +85,9 @@ final class EventHandler implements Comparable<EventHandler> {
 
 	@Override
 	public String toString() {
-		return String.format("EventHandler[method=%s, annotation=%s]", method, annotation);
+		return String.format(
+			"EventHandler[method=%s, eventType=%s, useParameter=%b, polymorphic=%b, priority=%d]",
+			method, eventType, useParameter, polymorphic, priority);
 	}
 
 	/**
@@ -89,17 +97,13 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * @throws EventBusException if the handler throws an exception
 	 * @since 0.0.1
 	 */
-	void execute(IEvent event) throws EventBusException {
+	void execute(Object event) throws EventBusException {
 		try {
-			if (annotation.eventType().equals(USE_PARAMETER.class))
+			if (useParameter)
 				method.invoke(listener, event);
 			else
 				method.invoke(listener);
-		} catch (
+		} catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
-			IllegalAccessException
-			| IllegalArgumentException
-			| InvocationTargetException e
-		) {
 			throw new EventBusException("Failed to invoke event handler!", e);
 		}
 	}
@@ -108,29 +112,25 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * @return the listener containing this handler
 	 * @since 0.0.1
 	 */
-	EventListener getListener() { return listener; }
+	Object getListener() { return listener; }
 
 	/**
-	 * @return the event annotation
+	 * @return the event type this handler listens for
-	 * @since 0.0.1
-	 */
-	Event getAnnotation() { return annotation; }
-
-	/**
-	 * @return the priority of the event annotation
-	 * @since 0.0.1
-	 */
-	int getPriority() { return annotation.priority(); }
-
-	/**
-	 * @return whether this handler includes subtypes
-	 * @since 0.0.4
-	 */
-	boolean includeSubtypes() { return annotation.includeSubtypes(); }
-
-	/**
-	 * @return the event type this handler listens to
 	 * @since 0.0.3
 	 */
-	Class<? extends IEvent> getEventType() { return eventType; }
+	Class<?> getEventType() { return eventType; }
+
+	/**
+	 * @return the priority of this handler
+	 * @since 0.0.1
+	 * @see Priority
+	 */
+	int getPriority() { return priority; }
+
+	/**
+	 * @return whether this handler is polymorphic
+	 * @since 1.0.0
+	 * @see Polymorphic
+	 */
+	boolean isPolymorphic() { return polymorphic; }
 }

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

@@ -1,12 +0,0 @@
-package dev.kske.eventbus.core;
-
-/**
- * Marker interface for event listeners. Event listeners can contain event handling methods to which
- * events can be dispatched.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see Event
- * @see EventBus
- */
-public interface EventListener {}

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

@@ -1,12 +0,0 @@
-package dev.kske.eventbus.core;
-
-/**
- * Marker interface for event objects. Event objects can be used as event handler parameters and
- * thus can be dispatched to the event bus.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see Event
- * @see EventBus
- */
-public interface IEvent {}

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/main/java/dev/kske/eventbus/core/Priority.java

@@ -0,0 +1,30 @@
+package dev.kske.eventbus.core;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.*;
+
+/**
+ * Defines the priority of an event handler. Handlers are executed in descending order of their
+ * priority.
+ * <p>
+ * Handlers without this annotation have the default priority of 100.
+ * <p>
+ * The execution order of handlers with the same priority is undefined.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ * @see Event
+ */
+@Documented
+@Retention(RUNTIME)
+@Target(METHOD)
+public @interface Priority {
+
+	/**
+	 * @return the priority of the event handler
+	 * @since 1.0.0
+	 */
+	int value();
+}

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

@@ -11,7 +11,7 @@ import org.junit.jupiter.api.*;
  * @author Leon Hofmeister
  * @since 0.1.0
  */
-class CancelTest implements EventListener {
+class CancelTest {
 
 	EventBus	bus;
 	int			hits;
@@ -39,13 +39,15 @@ class CancelTest implements EventListener {
 		assertEquals(1, hits);
 	}
 
-	@Event(eventType = SimpleEvent.class, priority = 100)
+	@Event(SimpleEvent.class)
+	@Priority(100)
 	void onSimpleFirst() {
 		++hits;
 		bus.cancel();
 	}
 
-	@Event(eventType = SimpleEvent.class, priority = 50)
+	@Event(SimpleEvent.class)
+	@Priority(50)
 	void onSimpleSecond() {
 		++hits;
 	}

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

@@ -10,7 +10,7 @@ import org.junit.jupiter.api.*;
  * @author Kai S. K. Engelbart
  * @since 0.0.1
  */
-class DispatchTest implements EventListener {
+class DispatchTest {
 
 	EventBus	bus;
 	static int	hits;
@@ -27,7 +27,7 @@ class DispatchTest implements EventListener {
 	}
 
 	/**
-	 * Tests {@link EventBus#dispatch(IEvent)} with multiple handler priorities, a subtype handler
+	 * Tests {@link EventBus#dispatch(Object)} with multiple handler priorities, a subtype handler
 	 * and a static handler.
 	 *
 	 * @since 0.0.1
@@ -38,19 +38,22 @@ class DispatchTest implements EventListener {
 		bus.dispatch(new SimpleEvent());
 	}
 
-	@Event(eventType = SimpleEvent.class, includeSubtypes = true, priority = 200)
+	@Event(SimpleEvent.class)
+	@Priority(200)
+	@Polymorphic
 	void onSimpleEventFirst() {
 		++hits;
 		assertTrue(hits == 1 || hits == 2);
 	}
 
-	@Event(eventType = SimpleEvent.class, priority = 150)
+	@Event(SimpleEvent.class)
+	@Priority(150)
 	static void onSimpleEventSecond() {
 		++hits;
 		assertEquals(3, hits);
 	}
 
-	@Event(priority = 100)
+	@Event
 	void onSimpleEventThird(SimpleEvent event) {
 		++hits;
 		assertEquals(4, hits);

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

@@ -6,4 +6,4 @@ package dev.kske.eventbus.core;
  * @author Kai S. K. Engelbart
  * @since 0.0.1
  */
-public class SimpleEvent implements IEvent {}
+public class SimpleEvent {}