Add ability to change player item in hand. Adds BUKKIT-3318

By: Patrick Seidel <psycowithespn1@aim.com>

paper-api/src/main/java/org/bukkit/event/player/PlayerItemHeldEvent.java

@@ -1,13 +1,15 @@
 package org.bukkit.event.player;
 
 import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
 import org.bukkit.event.HandlerList;
 
 /**
  * Fired when a player changes their currently held item
  */
-public class PlayerItemHeldEvent extends PlayerEvent {
+public class PlayerItemHeldEvent extends PlayerEvent implements Cancellable {
     private static final HandlerList handlers = new HandlerList();
+    private boolean cancel = false;
     private final int previous;
     private final int current;
 
@@ -35,6 +37,14 @@ public class PlayerItemHeldEvent extends PlayerEvent {
         return current;
     }
 
+    public boolean isCancelled() {
+        return cancel;
+    }
+
+    public void setCancelled(boolean cancel) {
+        this.cancel = cancel;
+    }
+
     @Override
     public HandlerList getHandlers() {
         return handlers;

paper-api/src/main/java/org/bukkit/inventory/PlayerInventory.java

@@ -102,6 +102,16 @@ public interface PlayerInventory extends Inventory {
      */
     public int getHeldItemSlot();
 
+    /**
+     * Set the slot number of the currently held item.
+     * <p>
+     * This validates whether the slot is between 0 and 8 inclusive.
+     *
+     * @param slot The new slot number
+     * @throws IllegalArgumentException Thrown if slot is not between 0 and 8 inclusive
+     */
+    public void setHeldItemSlot(int slot);
+
     /**
      * Clears all matching items from the inventory. Setting either value to -1 will skip it's check,
      *  while setting both to -1 will clear all items in your inventory unconditionally.