2021-02-09 09:52:26 +01:00
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package dev.kske.eventbus.core;
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2020-09-02 16:15:31 +02:00
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2020-09-20 15:27:51 +02:00
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import java.lang.System.Logger;
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import java.lang.System.Logger.Level;
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2021-02-21 10:36:06 +01:00
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import java.lang.reflect.InvocationTargetException;
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2020-09-02 16:15:31 +02:00
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import java.util.*;
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import java.util.concurrent.ConcurrentHashMap;
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/**
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* Event listeners can be registered at an event bus to be notified when an event is dispatched.
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* <p>
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2020-09-06 15:32:54 +02:00
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* A singleton instance of this class can be lazily created and acquired using the
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* {@link EventBus#getInstance()} method.
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* <p>
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2020-09-02 16:15:31 +02:00
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* This is a thread-safe implementation.
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*
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* @author Kai S. K. Engelbart
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* @since 0.0.1
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* @see Event
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*/
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public final class EventBus {
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2020-10-02 17:50:11 +02:00
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/**
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* Holds the state of the dispatching process on one thread.
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*
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* @since 0.1.0
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*/
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private static final class DispatchState {
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boolean isDispatching, isCancelled;
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}
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2021-03-14 11:18:11 +01:00
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/**
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* The priority assigned to every event handler without an explicitly defined priority.
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*
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* @since 1.1.0
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* @see Priority
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*/
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public static final int DEFAULT_PRIORITY = 100;
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2020-09-24 17:41:08 +02:00
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private static volatile EventBus singletonInstance;
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2020-09-06 15:32:54 +02:00
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2020-09-20 15:27:51 +02:00
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private static final Logger logger = System.getLogger(EventBus.class.getName());
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2020-09-06 15:32:54 +02:00
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/**
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* Produces a singleton instance of the event bus. It is lazily initialized on the first call.
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*
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* @return a singleton instance of the event bus.
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* @since 0.0.2
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*/
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public static EventBus getInstance() {
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2020-09-24 17:41:08 +02:00
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EventBus instance = singletonInstance;
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if (instance == null)
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synchronized (EventBus.class) {
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if ((instance = singletonInstance) == null) {
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logger.log(Level.DEBUG, "Initializing singleton event bus instance");
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instance = singletonInstance = new EventBus();
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}
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}
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return instance;
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2020-09-06 15:32:54 +02:00
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}
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2021-02-15 14:43:34 +01:00
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private final Map<Class<?>, TreeSet<EventHandler>> bindings =
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new ConcurrentHashMap<>();
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private final Set<Object> registeredListeners =
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ConcurrentHashMap.newKeySet();
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private final ThreadLocal<DispatchState> dispatchState =
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ThreadLocal.withInitial(DispatchState::new);
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/**
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* Dispatches an event to all event handlers registered for it in descending order of their
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* priority.
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*
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* @param event the event to dispatch
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2021-02-21 10:36:06 +01:00
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* @throws EventBusException if an event handler isn't accessible or has an invalid signature
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2020-09-02 16:15:31 +02:00
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* @since 0.0.1
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*/
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public void dispatch(Object event) {
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Objects.requireNonNull(event);
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logger.log(Level.INFO, "Dispatching event {0}", event);
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2020-10-02 17:50:11 +02:00
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// Set dispatch state
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var state = dispatchState.get();
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state.isDispatching = true;
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2021-02-19 16:04:49 +01:00
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Iterator<EventHandler> handlers = getHandlersFor(event.getClass());
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if (handlers.hasNext()) {
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while (handlers.hasNext())
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if (state.isCancelled) {
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logger.log(Level.INFO, "Cancelled dispatching event {0}", event);
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state.isCancelled = false;
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break;
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} else {
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try {
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handlers.next().execute(event);
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} catch (InvocationTargetException e) {
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if (event instanceof DeadEvent || event instanceof ExceptionEvent)
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// Warn about system event not being handled
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logger.log(Level.WARNING, event + " not handled due to exception", e);
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else
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// Dispatch exception event
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dispatch(new ExceptionEvent(this, event, e.getCause()));
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}
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}
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} else if (event instanceof DeadEvent || event instanceof ExceptionEvent) {
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2021-02-20 21:40:48 +01:00
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// Warn about the dead event not being handled
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logger.log(Level.WARNING, "{0} not handled", event);
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} else {
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// Dispatch dead event
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dispatch(new DeadEvent(this, event));
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}
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// Reset dispatch state
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state.isDispatching = false;
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logger.log(Level.DEBUG, "Finished dispatching event {0}", event);
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2020-09-02 16:15:31 +02:00
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}
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/**
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* Searches for the event handlers bound to an event class. This includes polymorphic handlers
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* that are bound to a supertype of the event class.
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*
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* @param eventClass the event class to use for the search
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* @return an iterator over the applicable handlers in descending order of priority
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* @since 0.0.1
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*/
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private Iterator<EventHandler> getHandlersFor(Class<?> eventClass) {
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2020-09-20 12:20:29 +02:00
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// Get handlers defined for the event class
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TreeSet<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
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// Get polymorphic handlers
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for (var binding : bindings.entrySet())
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if (binding.getKey().isAssignableFrom(eventClass))
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for (var handler : binding.getValue())
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2021-02-15 10:55:30 +01:00
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if (handler.isPolymorphic())
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handlers.add(handler);
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return handlers.iterator();
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}
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2020-10-02 17:50:11 +02:00
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/**
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* Cancels an event that is currently dispatched from inside an event handler.
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*
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* @throws EventBusException if the calling thread is not an active dispatching thread
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* @since 0.1.0
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*/
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public void cancel() {
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var state = dispatchState.get();
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if (state.isDispatching && !state.isCancelled)
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state.isCancelled = true;
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else
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throw new EventBusException("Calling thread not an active dispatching thread!");
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}
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2020-09-02 16:15:31 +02:00
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/**
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* Registers an event listener at this event bus.
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*
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* @param listener the listener to register
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* @throws EventBusException if the listener is already registered or a declared event handler
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2020-09-08 19:47:21 +02:00
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* does not comply with the specification
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* @since 0.0.1
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* @see Event
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*/
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2021-02-15 14:43:34 +01:00
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public void registerListener(Object listener) throws EventBusException {
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Objects.requireNonNull(listener);
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if (registeredListeners.contains(listener))
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throw new EventBusException(listener + " already registered!");
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logger.log(Level.INFO, "Registering event listener {0}", listener.getClass().getName());
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boolean handlerBound = false;
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2021-03-14 11:18:11 +01:00
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// Predefined handler polymorphism
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boolean polymorphic = false;
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if (listener.getClass().isAnnotationPresent(Polymorphic.class))
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polymorphic = listener.getClass().getAnnotation(Polymorphic.class).value();
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// Predefined handler priority
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int priority = 100;
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if (listener.getClass().isAnnotationPresent(Priority.class))
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priority = listener.getClass().getAnnotation(Priority.class).value();
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2020-09-02 16:15:31 +02:00
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registeredListeners.add(listener);
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for (var method : listener.getClass().getDeclaredMethods()) {
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Event annotation = method.getAnnotation(Event.class);
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// Skip methods without annotations
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if (annotation == null)
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continue;
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2020-09-08 19:47:21 +02:00
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// Initialize and bind the handler
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var handler = new EventHandler(listener, method, annotation, polymorphic, priority);
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2021-02-15 09:25:16 +01:00
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bindings.putIfAbsent(handler.getEventType(), new TreeSet<>());
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2020-09-20 15:27:51 +02:00
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logger.log(Level.DEBUG, "Binding event handler {0}", handler);
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2020-09-08 19:47:21 +02:00
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bindings.get(handler.getEventType())
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.add(handler);
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2020-09-20 15:27:51 +02:00
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handlerBound = true;
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}
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2021-02-15 09:25:16 +01:00
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if (!handlerBound)
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logger.log(
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Level.WARNING,
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"No event handlers bound for event listener {0}",
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listener.getClass().getName());
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2020-09-02 16:15:31 +02:00
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}
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/**
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* Removes a specific listener from this event bus.
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*
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* @param listener the listener to remove
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* @since 0.0.1
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*/
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2021-02-15 14:43:34 +01:00
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public void removeListener(Object listener) {
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Objects.requireNonNull(listener);
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2020-09-20 15:27:51 +02:00
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logger.log(Level.INFO, "Removing event listener {0}", listener.getClass().getName());
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2020-09-02 16:15:31 +02:00
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for (var binding : bindings.values()) {
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var it = binding.iterator();
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2020-09-20 15:27:51 +02:00
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while (it.hasNext()) {
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var handler = it.next();
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if (handler.getListener() == listener) {
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logger.log(Level.DEBUG, "Unbinding event handler {0}", handler);
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it.remove();
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}
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}
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2020-09-02 16:15:31 +02:00
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}
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registeredListeners.remove(listener);
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}
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/**
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* Removes all event listeners from this event bus.
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*
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* @since 0.0.1
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*/
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public void clearListeners() {
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logger.log(Level.INFO, "Clearing event listeners");
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2020-09-02 16:15:31 +02:00
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bindings.clear();
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registeredListeners.clear();
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}
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/**
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* Provides an unmodifiable view of the event listeners registered at this event bus.
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*
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* @return all registered event listeners
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* @since 0.0.1
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*/
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2021-02-15 14:43:34 +01:00
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public Set<Object> getRegisteredListeners() {
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return Collections.unmodifiableSet(registeredListeners);
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}
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}
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