Allow players to edit signs through a command.

SignEditorPlugin.java

package com.comphenix.example;

import java.lang.reflect.InvocationTargetException;
import java.util.concurrent.ConcurrentMap;

import org.bukkit.Location;
import org.bukkit.block.Block;
import org.bukkit.block.Sign;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.event.Listener;
import org.bukkit.plugin.java.JavaPlugin;
import com.comphenix.protocol.PacketType;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;
import com.comphenix.protocol.injector.BukkitUnwrapper;
import com.comphenix.protocol.reflect.StructureModifier;
import com.comphenix.protocol.reflect.accessors.Accessors;
import com.comphenix.protocol.reflect.accessors.FieldAccessor;
import com.comphenix.protocol.utility.MinecraftReflection;
import com.google.common.base.Objects;
import com.google.common.collect.MapMaker;

public class SignEditorPlugin extends JavaPlugin implements Listener {
	private ConcurrentMap<Player, Location> editing = new MapMaker().weakKeys().makeMap();
	
	// For accessing the underlying tile entity
	private FieldAccessor tileEntityAccessor;
	private FieldAccessor booleanAccessor;
	private FieldAccessor humanAccessor;
	
	@Override
	public void onEnable() {
		ProtocolLibrary.getProtocolManager().getAsynchronousManager().registerAsyncHandler(
		  new PacketAdapter(this, PacketType.Play.Client.UPDATE_SIGN) {
			@Override
			public void onPacketReceiving(PacketEvent event) {
				Player player = event.getPlayer();
				
				StructureModifier<Integer> ints = event.getPacket().getIntegers();
				Location loc = new Location(player.getWorld(), ints.read(0), ints.read(1), ints.read(2));
				
				// Allow
				if (Objects.equal(editing.get(player), loc)) {
					setEditingPlayer((Sign) loc.getBlock().getState(), player);
				}
			}
		}).syncStart();
	}
	
	private void setEditingPlayer(Sign sign, Player player) {
		// Extract a TileEntitySign from a CraftSign
		if (tileEntityAccessor == null) {
			Class<?> tileEntity = MinecraftReflection.getMinecraftClass("TileEntitySign");
			Class<?> humanClass = MinecraftReflection.getMinecraftClass("EntityHuman");
			
			tileEntityAccessor = Accessors.getFieldAccessor(sign.getClass(), tileEntity, true);
			booleanAccessor = Accessors.getFieldAccessor(tileEntity, boolean.class, true);
			humanAccessor = Accessors.getFieldAccessor(tileEntity, humanClass, true);
		}
		Object tileEntity = tileEntityAccessor.get(sign);
		booleanAccessor.set(tileEntity, true);
		humanAccessor.set(tileEntity, BukkitUnwrapper.getInstance().unwrapItem(player));
	}
	
	@Override
	public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
		if (sender instanceof Player)  {
			Player player = (Player) sender;
			@SuppressWarnings("deprecation")
			Block block = player.getTargetBlock(null, 50);
			
			if (block != null) {
				displaySignEditor(player, block);
			}
		}
		return true;
	}
	
	/**
	 * Display a sign editor for the given player.
	 * <p>
	 * The block could also be client-side.
	 * @
	 * @param sign - the sign to edit.
	 */
	private void displaySignEditor(Player player, Block sign) {
		if (!player.getWorld().equals(sign.getWorld()))
			throw new IllegalArgumentException("Player and sign must be in the same world.");
		PacketContainer editSignPacket = new PacketContainer(PacketType.Play.Server.OPEN_SIGN_ENTITY);
		
		// Permit this
		editing.put(player, sign.getLocation());
		
		editSignPacket.getIntegers().
			write(0, sign.getX()).
			write(1, sign.getY()).
			write(2, sign.getZ());
		
		try {
			ProtocolLibrary.getProtocolManager().sendServerPacket(player, editSignPacket);
		} catch (InvocationTargetException e) {
			throw new RuntimeException("Cannot send packet.", e);
		}
	}
}