Cancel inventory clicks with TinyProtocol

CancelInventoryClick.java

package com.comphenix.example;

import org.bukkit.entity.Player;
import org.bukkit.event.Listener;
import org.bukkit.inventory.ItemStack;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.example.Reflection.FieldAccessor;
import com.comphenix.example.Reflection.MethodInvoker;
import net.minecraft.util.io.netty.channel.Channel;

public class CancelInventoryClick extends JavaPlugin {
    // net.minecraft.server.ItemStack
    private Class<Object> nmsStackClass = Reflection.getUntypedClass("{nms}.ItemStack");
    
    // Explosion packet
    private Class<?> inventoryClickPacket = Reflection.getClass("{nms}.PacketPlayInWindowClick");
    
    // Convert an NMS item stack to a CraftBukkit item stack
    private MethodInvoker getItemStack = Reflection.getMethod("{obc}.inventory.CraftItemStack", "asCraftMirror", nmsStackClass);
    
    // 0 is the player inventory
    private FieldAccessor<Integer> windowId = Reflection.getField(inventoryClickPacket, int.class, 0); 
    private FieldAccessor<Integer> slotIndex = Reflection.getField(inventoryClickPacket, int.class, 1);
    private FieldAccessor<Integer> button = Reflection.getField(inventoryClickPacket, int.class, 2);
    private FieldAccessor<Short> actionNumber = Reflection.getField(inventoryClickPacket, short.class, 0);
    private FieldAccessor<Object> itemStack = Reflection.getField(inventoryClickPacket, nmsStackClass, 0);
    private FieldAccessor<Integer> clickMode = Reflection.getField(inventoryClickPacket, int.class, 3);
    
    private TinyProtocol protocol;
    
    @Override
    public void onEnable() {
        protocol = new TinyProtocol(this) {
            @Override
            public Object onPacketInAsync(final Player sender, Channel channel, Object packet) {
                if (windowId.hasField(packet)) {
                    ItemStack stack = (ItemStack) getItemStack.invoke(null, itemStack.get(packet));
                    int index = slotIndex.get(packet);
                    
                    System.out.println("Clicked: " + stack + " at " + index);
                    
                    // To cancel
                    if (index == 0) {
                        // THE CURRENT METHOD IS ASYNC, SO WE CANNOT CALL BUKKIT API DIRECTLY.
                        // Instead, we have to use the scheduler
                        getServer().getScheduler().scheduleSyncDelayedTask(CancelInventoryClick.this, new Runnable() {
                            @SuppressWarnings("deprecation")
                            @Override
                            public void run() {
                                // This is needed to properly restore the inventory state on the client side
                                sender.updateInventory();
                            }
                        });
                        return null;
                    }
                }
                return packet;
            }
                
            @Override
            public Object onPacketOutAsync(Player reciever, Channel channel, Object packet) {
                return packet;
            }
        };
    }
}