Merge pull request 'Add Event Cancellation' (#3) from f/cancel-event into develop

Reviewed-on: https://git.kske.dev/zdm/event-bus/pulls/3

README.md

@@ -51,6 +51,15 @@ public class SimpleEventListener implements EventListener {
 In this case, an event bus is created and used locally.
 In a more sophisticated example the class would acquire an external event bus that is used by multiple classes.
 
+Note that creating static event handlers like this
+
+```java
+    @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
 
 On certain occasions its practical for an event handler to accept both events of the specified type, as well as subclasses of that event.
@@ -60,6 +69,23 @@ To include subtypes for an event handler, use the `includeSubtypes` parameter as
 @Event(includeSubtypes = true)
 ```
 
+## 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`.
+
+Priority can be set on the `@Event` annotation like that:
+```java
+@Event(priority=100)
+```
+
+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.
+
+If no priority is set or multiple handlers have the same priority, the order of execution is undefined.
+
 ## Parameter-less event handlers
 
 In some cases an event handler is not interested in the dispatched event instance.
@@ -74,6 +100,30 @@ 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
+
+There are cases when it would be useful to stop event propagation after a certain condition has been fulfilled.
+Event Bus provides a mechanism to consume events:
+
+```java
+@Event(eventType = SimpleEvent.class, priority=1000)
+private void onSimpleEvent() {
+	EventBus.getInstance().cancel();
+}
+
+@Event(eventType = SimpleEvent.class, priority=900)
+private void onSimpleEvent2() {
+	System.out.println("Will not be printed!");
+}
+```
+
+In this example, the second method will not be executed as the event will no longer be forwarded.
+Any event handler with a lower priority than the one canceling it will not get executed.
+
+**Important:**
+Please avoid cancelling events when (multiple) event handlers have the same priority as the one cancelling it:
+It is undefined whether those will be executed or not.
+
 ## Installation
 
 Event Bus is currently hosted at [kske.dev](https://kske.dev).
@@ -91,7 +141,7 @@ To include it inside your project, just add the Maven repository and the depende
     <dependency>
         <groupId>dev.kske</groupId>
         <artifactId>event-bus</artifactId>
-        <version>0.0.4</version>
+        <version>0.1.0</version>
     </dependency>
 </dependencies>
 ```

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>dev.kske</groupId>
 	<artifactId>event-bus</artifactId>
-	<version>0.0.4</version>
+	<version>0.1.0</version>
 
 	<name>Event Bus</name>
 	<description>An event handling framework for Java utilizing annotations.</description>
@@ -63,6 +63,7 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-source-plugin</artifactId>
+				<version>3.2.1</version>
 				<executions>
 					<execution>
 						<id>attach-sources</id>
@@ -75,6 +76,7 @@
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-javadoc-plugin</artifactId>
+				<version>3.2.0</version>
 				<executions>
 					<execution>
 						<id>attach-javadocs</id>

src/main/java/dev/kske/eventbus/Event.java

@@ -10,11 +10,12 @@ import java.lang.annotation.*;
  * comply with the following specifications:
  * <ul>
  * <li>Declared inside a class that implements {@link EventListener}</li>
- * <li>Specifying an event type by either</li>
+ * <li>Specifying an event type by either
  * <ul>
  * <li>Declaring one parameter of a type that implements {@link IEvent}</li>
  * <li>Defining the class of the event using the {@link Event#eventType()} value</li>
  * </ul>
+ * </li>
  * <li>Return type of {@code void}</li>
  * </ul>
  *
@@ -32,6 +33,7 @@ public @interface Event {
 	 * <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;
@@ -39,6 +41,7 @@ public @interface Event {
 	/**
 	 * 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;
@@ -49,6 +52,7 @@ public @interface Event {
 	 * <p>
 	 * 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
 	 */
 	Class<? extends IEvent> eventType() default USE_PARAMETER.class;

src/main/java/dev/kske/eventbus/EventBus.java

@@ -1,5 +1,7 @@
 package dev.kske.eventbus;
 
+import java.lang.System.Logger;
+import java.lang.System.Logger.Level;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 
@@ -17,7 +19,19 @@ import java.util.concurrent.ConcurrentHashMap;
  */
 public final class EventBus {
 
-	private static EventBus singletonInstance;
+	/**
+	 * Holds the state of the dispatching process on one thread.
+	 *
+	 * @since 0.1.0
+	 */
+	private static final class DispatchState {
+
+		boolean isDispatching, isCancelled;
+	}
+
+	private static volatile EventBus singletonInstance;
+
+	private static final Logger logger = System.getLogger(EventBus.class.getName());
 
 	/**
 	 * Produces a singleton instance of the event bus. It is lazily initialized on the first call.
@@ -26,14 +40,22 @@ public final class EventBus {
 	 * @since 0.0.2
 	 */
 	public static EventBus getInstance() {
-		if (singletonInstance == null)
-			singletonInstance = new EventBus();
-		return singletonInstance;
+		EventBus instance = singletonInstance;
+		if (instance == null)
+			synchronized (EventBus.class) {
+				if ((instance = singletonInstance) == null) {
+					logger.log(Level.DEBUG, "Initializing singleton event bus instance");
+					instance = singletonInstance = new EventBus();
+				}
+			}
+		return instance;
 	}
 
 	private final Map<Class<? extends IEvent>, TreeSet<EventHandler>> bindings
 		= new ConcurrentHashMap<>();
 	private final Set<EventListener> registeredListeners = ConcurrentHashMap.newKeySet();
+	private final ThreadLocal<DispatchState> dispatchState
+		= ThreadLocal.withInitial(DispatchState::new);
 
 	/**
 	 * Dispatches an event to all event handlers registered for it in descending order of their
@@ -44,7 +66,25 @@ public final class EventBus {
 	 */
 	public void dispatch(IEvent event) {
 		Objects.requireNonNull(event);
-		getHandlersFor(event.getClass()).forEach(handler -> handler.execute(event));
+		logger.log(Level.INFO, "Dispatching event {0}", event);
+
+		// Set dispatch state
+		var state = dispatchState.get();
+		state.isDispatching = true;
+
+		for (var handler : getHandlersFor(event.getClass()))
+			if (state.isCancelled) {
+				logger.log(Level.INFO, "Cancelled dispatching event {0}", event);
+				state.isCancelled = false;
+				break;
+			} else {
+				handler.execute(event);
+			}
+
+		// Reset dispatch state
+		state.isDispatching = false;
+
+		logger.log(Level.DEBUG, "Finished dispatching event {0}", event);
 	}
 
 	/**
@@ -71,6 +111,20 @@ public final class EventBus {
 		return new ArrayList<>(handlers);
 	}
 
+	/**
+	 * Cancels an event that is currently dispatched from inside an event handler.
+	 *
+	 * @throws EventBusException if the calling thread is not an active dispatching thread
+	 * @since 0.1.0
+	 */
+	public void cancel() {
+		var state = dispatchState.get();
+		if (state.isDispatching && !state.isCancelled)
+			state.isCancelled = true;
+		else
+			throw new EventBusException("Calling thread not an active dispatching thread!");
+	}
+
 	/**
 	 * Registers an event listener at this event bus.
 	 *
@@ -84,6 +138,8 @@ public final class EventBus {
 		Objects.requireNonNull(listener);
 		if (registeredListeners.contains(listener))
 			throw new EventBusException(listener + " already registered!");
+		logger.log(Level.INFO, "Registering event listener {0}", listener.getClass().getName());
+		boolean handlerBound = false;
 
 		registeredListeners.add(listener);
 		for (var method : listener.getClass().getDeclaredMethods()) {
@@ -97,9 +153,18 @@ public final class EventBus {
 			var handler = new EventHandler(listener, method, annotation);
 			if (!bindings.containsKey(handler.getEventType()))
 				bindings.put(handler.getEventType(), new TreeSet<>());
+			logger.log(Level.DEBUG, "Binding event handler {0}", handler);
 			bindings.get(handler.getEventType())
 				.add(handler);
+			handlerBound = true;
 		}
+
+		if(!handlerBound)
+			logger.log(
+				Level.WARNING,
+				"No event handlers bound for event listener {0}",
+				listener.getClass().getName()
+			);
 	}
 
 	/**
@@ -110,11 +175,17 @@ public final class EventBus {
 	 */
 	public void removeListener(EventListener listener) {
 		Objects.requireNonNull(listener);
+		logger.log(Level.INFO, "Removing event listener {0}", listener.getClass().getName());
+
 		for (var binding : bindings.values()) {
 			var it = binding.iterator();
-			while (it.hasNext())
-				if (it.next().getListener() == listener)
+			while (it.hasNext()) {
+				var handler = it.next();
+				if (handler.getListener() == listener) {
+					logger.log(Level.DEBUG, "Unbinding event handler {0}", handler);
 					it.remove();
+				}
+			}
 		}
 		registeredListeners.remove(listener);
 	}
@@ -125,6 +196,7 @@ public final class EventBus {
 	 * @since 0.0.1
 	 */
 	public void clearListeners() {
+		logger.log(Level.INFO, "Clearing event listeners");
 		bindings.clear();
 		registeredListeners.clear();
 	}

src/main/java/dev/kske/eventbus/EventBusException.java

@@ -1,8 +1,9 @@
 package dev.kske.eventbus;
 
 /**
- * This runtime exception is thrown when an event bus error occurs. This can either occur while
- * registering event listeners with invalid handlers, or when an event handler throws an exception.
+ * This runtime exception is thrown when an event bus error occurs. This can
+ * either occur while registering event listeners with invalid handlers, or when
+ * an event handler throws an exception.
  *
  * @author Kai S. K. Engelbart
  * @since 0.0.1
@@ -11,10 +12,21 @@ public class EventBusException extends RuntimeException {
 
 	private static final long serialVersionUID = 1L;
 
+	/**
+	 * Creates a new event bus exception.
+	 *
+	 * @param message the message to display
+	 * @param cause   the cause of this exception
+	 */
 	public EventBusException(String message, Throwable cause) {
 		super(message, cause);
 	}
 
+	/**
+	 * Creates a new event bus exception.
+	 *
+	 * @param message the message to display
+	 */
 	public EventBusException(String message) {
 		super(message);
 	}

src/main/java/dev/kske/eventbus/EventHandler.java

@@ -77,6 +77,11 @@ final class EventHandler implements Comparable<EventHandler> {
 		return priority == 0 ? hashCode() - other.hashCode() : priority;
 	}
 
+	@Override
+	public String toString() {
+		return String.format("EventHandler[method=%s, annotation=%s]", method, annotation);
+	}
+
 	/**
 	 * Executes the event handler.
 	 *

src/test/java/dev/kske/eventbus/CancelTest.java

@@ -0,0 +1,52 @@
+package dev.kske.eventbus;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.*;
+
+/**
+ * Tests the event cancellation mechanism of the event bus.
+ *
+ * @author Kai S. K. Engelbart
+ * @author Leon Hofmeister
+ * @since 0.1.0
+ */
+class CancelTest implements EventListener {
+
+	EventBus	bus;
+	int			hits;
+
+	/**
+	 * Constructs an event bus and registers this test instance as an event listener.
+	 *
+	 * @since 0.1.0
+	 */
+	@BeforeEach
+	void registerListener() {
+		bus = new EventBus();
+		bus.registerListener(this);
+	}
+
+	/**
+	 * Tests {@link EventBus#cancel()} with two event handlers, of which the first cancels the
+	 * event.
+	 *
+	 * @since 0.1.0
+	 */
+	@Test
+	void testCancellation() {
+		bus.dispatch(new SimpleEvent());
+		assertEquals(1, hits);
+	}
+
+	@Event(eventType = SimpleEvent.class, priority = 100)
+	void onSimpleFirst() {
+		++hits;
+		bus.cancel();
+	}
+
+	@Event(eventType = SimpleEvent.class, priority = 50)
+	void onSimpleSecond() {
+		++hits;
+	}
+}

src/test/java/dev/kske/eventbus/DispatchTest.java

@@ -0,0 +1,58 @@
+package dev.kske.eventbus;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.*;
+
+/**
+ * Tests the dispatching mechanism of the event bus.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ */
+class DispatchTest implements EventListener {
+
+	EventBus	bus;
+	static int	hits;
+
+	/**
+	 * Constructs an event bus and registers this test instance as an event listener.
+	 *
+	 * @since 0.0.1
+	 */
+	@BeforeEach
+	void registerListener() {
+		bus = new EventBus();
+		bus.registerListener(this);
+	}
+
+	/**
+	 * Tests {@link EventBus#dispatch(IEvent)} with multiple handler priorities, a subtype handler
+	 * and a static handler.
+	 *
+	 * @since 0.0.1
+	 */
+	@Test
+	void testDispatch() {
+		bus.dispatch(new SimpleEventSub());
+		bus.dispatch(new SimpleEvent());
+	}
+
+	@Event(eventType = SimpleEvent.class, includeSubtypes = true, priority = 200)
+	void onSimpleEventFirst() {
+		++hits;
+		assertTrue(hits == 1 || hits == 2);
+	}
+
+	@Event(eventType = SimpleEvent.class, priority = 150)
+	static void onSimpleEventSecond() {
+		++hits;
+		assertEquals(3, hits);
+	}
+
+	@Event(priority = 100)
+	void onSimpleEventThird(SimpleEvent event) {
+		++hits;
+		assertEquals(4, hits);
+	}
+}

src/test/java/dev/kske/eventbus/EventBusTest.java

@@ -1,49 +0,0 @@
-package dev.kske.eventbus;
-
-import static org.junit.jupiter.api.Assertions.*;
-
-import org.junit.jupiter.api.*;
-
-/**
- * Tests the of the event bus library.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-class EventBusTest implements EventListener {
-
-	int hits;
-
-	@BeforeEach
-	public void registerListener() {
-		EventBus.getInstance().registerListener(this);
-	}
-
-	@Test
-	void testDispatch() {
-		EventBus.getInstance().dispatch(new SimpleEventSub());
-		EventBus.getInstance().dispatch(new SimpleEvent());
-	}
-
-	@Event(
-		eventType = SimpleEvent.class,
-		includeSubtypes = true,
-		priority = 200
-	)
-	private void onSimpleEventFirst() {
-		++hits;
-		assertTrue(hits == 1 || hits == 2);
-	}
-
-	@Event(eventType = SimpleEvent.class, priority = 150)
-	private void onSimpleEventSecond() {
-		++hits;
-		assertEquals(3, hits);
-	}
-
-	@Event(priority = 50)
-	private void onSimpleEventThird(SimpleEvent event) {
-		++hits;
-		assertEquals(4, hits);
-	}
-}