Implement an event handler

src/main/java/emu/grasscutter/plugin/PluginManager.java

 package emu.grasscutter.plugin;
 
 import emu.grasscutter.Grasscutter;
+import emu.grasscutter.server.event.Event;
+import emu.grasscutter.server.event.EventHandler;
+import emu.grasscutter.server.event.Listener;
 import emu.grasscutter.utils.Utils;
+import org.reflections.Reflections;
 
 import java.io.File;
 import java.io.InputStreamReader;
 import java.lang.reflect.Method;
 import java.net.URL;
 import java.net.URLClassLoader;
-import java.util.Arrays;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
+import java.util.*;
 
 /**
  * Manages the server's plugins & the event system.
  */
 public final class PluginManager {
     private final Map<String, Plugin> plugins = new HashMap<>();
+    private final Map<Plugin, List<Listener>> listeners = new HashMap<>();
     
     public PluginManager() {
         this.loadPlugins(); // Load all plugins from the plugins directory.
@@ -113,4 +115,38 @@ public final class PluginManager {
             plugin.onDisable();
         });
     }
+
+    /**
+     * Registers a plugin's event listener.
+     * @param plugin The plugin instance.
+     * @param listener The event listener.
+     */
+    public void registerListener(Plugin plugin, Listener listener) {
+        this.listeners.computeIfAbsent(plugin, k -> new ArrayList<>()).add(listener);
+    }
+    
+    /**
+     * Invoke the provided event on all registered event listeners.
+     * @param event The event to invoke.
+     */
+    public void invokeEvent(Event event) {
+        this.listeners.values().stream()
+                .flatMap(Collection::stream)
+                .forEach(listener -> this.invokeOnListener(listener, event));
+    }
+    
+    /**
+     * Attempts to invoke the event on the provided listener.
+     */
+    private void invokeOnListener(Listener listener, Event event) {
+        try {
+            Class<?> listenerClass = listener.getClass();
+            Method[] methods = listenerClass.getMethods();
+            for (Method method : methods) {
+                if(!method.isAnnotationPresent(EventHandler.class)) return;
+                if(!method.getParameterTypes()[0].isAssignableFrom(event.getClass())) return;
+                method.invoke(listener, event);
+            }
+        } catch (Exception ignored) { }
+    }
 }
\ No newline at end of file

src/main/java/emu/grasscutter/server/event/Cancellable.java

+package emu.grasscutter.server.event;
+
+/**
+ * Implementing this interface marks an event as cancellable.
+ */
+public interface Cancellable {
+    void cancel();
+}

src/main/java/emu/grasscutter/server/event/Event.java

+package emu.grasscutter.server.event;
+
+import emu.grasscutter.Grasscutter;
+
+/**
+ * A generic server event.
+ */
+public abstract class Event {
+    private boolean cancelled = false;
+
+    /**
+     * Return the cancelled state of the event.
+     */
+    public boolean isCanceled() {
+        return this.cancelled;
+    }
+
+    /**
+     * Cancels the event if possible.
+     */
+    public void cancel() throws IllegalAccessException {
+        if(!(this instanceof Cancellable))
+            throw new IllegalAccessException("Event is not cancellable.");
+        this.cancelled = true;
+    }
+
+    /**
+     * Pushes this event to all listeners.
+     */
+    public void call() {
+        Grasscutter.getPluginManager().invokeEvent(this);
+    }
+}

src/main/java/emu/grasscutter/server/event/EventHandler.java

+package emu.grasscutter.server.event;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+/**
+ * Declares a class as an event listener/handler.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+public @interface EventHandler {
+}
\ No newline at end of file

src/main/java/emu/grasscutter/server/event/Listener.java

+package emu.grasscutter.server.event;
+
+/**
+ * Implementing this interface declares a class as an event listener.
+ */
+public interface Listener {
+}

src/main/java/emu/grasscutter/server/event/ServerEvent.java

+package emu.grasscutter.server.event;
+
+/**
+ * An event that is related to the internals of the server.
+ */
+public abstract class ServerEvent extends Event {
+    protected final Type type;
+    
+    public ServerEvent(Type type) {
+        this.type = type;
+    }
+    
+    public enum Type {
+        DISPATCH,
+        GAME
+    }
+}