Paper Mob Goal API Example

/*
This is a basic example of Paper's Mob Goal API. It simply adds a new goal
to every villager when they are added to the world. The goal is a simple task
of following the closest player holding an emerald (item, block, or ore).
*/

import com.destroystokyo.paper.entity.ai.Goal;
import com.destroystokyo.paper.entity.ai.GoalKey;
import com.destroystokyo.paper.entity.ai.GoalType;
import com.destroystokyo.paper.event.entity.EntityAddToWorldEvent;
import org.bukkit.Bukkit;
import org.bukkit.GameMode;
import org.bukkit.NamespacedKey;
import org.bukkit.entity.EntityType;
import org.bukkit.entity.Mob;
import org.bukkit.entity.Player;
import org.bukkit.entity.Villager;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.inventory.ItemStack;
import org.bukkit.inventory.PlayerInventory;
import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.Collection;
import java.util.EnumSet;

public final class MyPlugin extends JavaPlugin {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(new Listener() {
            @EventHandler
            public void on(EntityAddToWorldEvent event) {
                if (event.getEntityType() == EntityType.VILLAGER) {
                    Villager villager = (Villager) event.getEntity();
                    FollowEmeraldsGoal goal = new FollowEmeraldsGoal(MyPlugin.this, villager);
                    if (!Bukkit.getMobGoals().hasGoal(villager, goal.getKey())) {
                        Bukkit.getMobGoals().addGoal(villager, 3, goal);
                    }
                }
            }
        }, this);
    }

    public static class FollowEmeraldsGoal implements Goal<Villager> {
        private final GoalKey<Villager> key;
        private final Mob mob;
        private Player closestPlayer;
        private int cooldown;

        public FollowEmeraldsGoal(Plugin plugin, Mob mob) {
            this.key = GoalKey.of(Villager.class, new NamespacedKey(plugin, "follow_emeralds"));
            this.mob = mob;
        }

        @Override
        public boolean shouldActivate() {
            if (cooldown > 0) {
                --cooldown;
                return false;
            }
            closestPlayer = getClosestPlayer();
            if (closestPlayer == null) {
                return false;
            }
            return isHoldingEmerald(closestPlayer);
        }

        @Override
        public boolean shouldStayActive() {
            return shouldActivate();
        }

        @Override
        public void start() {
        }

        @Override
        public void stop() {
            mob.getPathfinder().stopPathfinding();
            mob.setTarget(null);
            cooldown = 100;
        }

        @Override
        public void tick() {
            mob.setTarget(closestPlayer);
            if (mob.getLocation().distanceSquared(closestPlayer.getLocation()) < 6.25) {
                mob.getPathfinder().stopPathfinding();
            } else {
                mob.getPathfinder().moveTo(closestPlayer, 1.0D);
            }
        }

        @Override
        public GoalKey<Villager> getKey() {
            return key;
        }

        @Override
        public EnumSet<GoalType> getTypes() {
            return EnumSet.of(GoalType.MOVE, GoalType.LOOK);
        }

        private Player getClosestPlayer() {
            Collection<Player> nearbyPlayers = mob.getWorld().getNearbyPlayers(mob.getLocation(), 10.0, player ->
                    !player.isDead() && player.getGameMode() != GameMode.SPECTATOR && player.isValid());
            double closestDistance = -1.0;
            Player closestPlayer = null;
            for (Player player : nearbyPlayers) {
                double distance = player.getLocation().distanceSquared(mob.getLocation());
                if (closestDistance != -1.0 && !(distance < closestDistance)) {
                    continue;
                }
                closestDistance = distance;
                closestPlayer = player;
            }
            return closestPlayer;
        }

        private boolean isHoldingEmerald(Player target) {
            PlayerInventory inv = target.getInventory();
            return isEmerald(inv.getItemInMainHand()) || isEmerald(inv.getItemInOffHand());
        }

        private boolean isEmerald(ItemStack stack) {
            switch (stack.getType()) {
                case EMERALD:
                case EMERALD_BLOCK:
                case EMERALD_ORE:
                    return true;
                default:
                    return false;
            }
        }
    }
}