Add WorldDifficultyChangeEvent (#12471)

paper-api/src/main/java/io/papermc/paper/event/world/WorldDifficultyChangeEvent.java

@@ -0,0 +1,59 @@
+package io.papermc.paper.event.world;
+
+import io.papermc.paper.command.brigadier.CommandSourceStack;
+import org.bukkit.Difficulty;
+import org.bukkit.World;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.world.WorldEvent;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
+
+/**
+ * Called when a world's difficulty is changed, either by command or by api.
+ * <p>
+ * If the world is in {@link World#isHardcore() hardcore}
+ * the resulting difficulty will always be {@link Difficulty#HARD}
+ */
+@NullMarked
+public class WorldDifficultyChangeEvent extends WorldEvent {
+
+    private static final HandlerList HANDLER_LIST = new HandlerList();
+
+    private final @Nullable CommandSourceStack commandSource;
+    private final Difficulty difficulty;
+
+    @ApiStatus.Internal
+    public WorldDifficultyChangeEvent(final World world, final @Nullable CommandSourceStack commandSource, final Difficulty difficulty) {
+        super(world);
+        this.commandSource = commandSource;
+        this.difficulty = difficulty;
+    }
+
+    /**
+     * Gets the command source associated with this event.
+     *
+     * @return {@code null} if the difficulty was changed via api, otherwise the {@link CommandSourceStack}.
+     */
+    public @Nullable CommandSourceStack getCommandSource() {
+        return commandSource;
+    }
+
+    /**
+     * Gets the new difficulty of the world.
+     *
+     * @return the new difficulty.
+     */
+    public Difficulty getDifficulty() {
+        return difficulty;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return HANDLER_LIST;
+    }
+
+    public static HandlerList getHandlerList() {
+        return HANDLER_LIST;
+    }
+}