Citizens2/main/src/main/java/net/citizensnpcs/trait/FollowTrait.java

package net.citizensnpcs.trait;

import java.util.UUID;

import org.bukkit.Bukkit;
import org.bukkit.entity.Entity;
import org.bukkit.entity.Player;
import org.bukkit.entity.Projectile;
import org.bukkit.event.EventHandler;
import org.bukkit.event.entity.EntityDamageByEntityEvent;
import org.bukkit.event.player.PlayerTeleportEvent.TeleportCause;

import net.citizensnpcs.Settings.Setting;
import net.citizensnpcs.api.ai.flocking.Flocker;
import net.citizensnpcs.api.ai.flocking.RadiusNPCFlock;
import net.citizensnpcs.api.ai.flocking.SeparationBehavior;
import net.citizensnpcs.api.persistence.Persist;
import net.citizensnpcs.api.trait.Trait;
import net.citizensnpcs.api.trait.TraitName;
import net.citizensnpcs.util.Util;

/**
 * Persists a {@link Player} to follow while spawned. Optionally allows protecting of the player as well.
 */
@TraitName("followtrait")
public class FollowTrait extends Trait {
    private Entity entity;
    private Flocker flock;
    @Persist
    private UUID followingUUID;
    @Persist
    private double margin = -1;
    @Persist
    private boolean protect;

    public FollowTrait() {
        super("followtrait");
    }

    private void cancelNavigationIfActive() {
        if (npc.getNavigator().isNavigating() && entity != null && npc.getNavigator().getEntityTarget() != null
                && entity == npc.getNavigator().getEntityTarget().getTarget()) {
            npc.getNavigator().cancelNavigation();
        }
    }

    /**
     * Sets the {@link Entity} to follow
     */
    public void follow(Entity follow) {
        cancelNavigationIfActive();
        followingUUID = follow == null ? null : follow.getUniqueId();
        entity = null;
    }

    public Entity getFollowing() {
        return entity;
    }

    public double getFollowingMargin() {
        return margin;
    }

    /**
     * Returns whether the trait is actively following a {@link Entity}.
     */
    public boolean isActive() {
        return npc.isSpawned() && entity != null;
    }

    public boolean isEnabled() {
        return followingUUID != null;
    }

    @Override
    public void onDespawn() {
        flock = null;
    }

    @EventHandler
    private void onEntityDamage(EntityDamageByEntityEvent event) {
        if (isActive() && protect && event.getEntity().equals(entity)) {
            Entity damager = event.getDamager();
            if (event.getEntity() instanceof Projectile) {
                Projectile projectile = (Projectile) event.getEntity();
                if (projectile.getShooter() instanceof Entity) {
                    damager = (Entity) projectile.getShooter();
                }
            }
            npc.getNavigator().setTarget(damager, true);
        }
    }

    @Override
    public void onSpawn() {
        flock = new Flocker(npc, new RadiusNPCFlock(4, 4), new SeparationBehavior(1));
    }

    @Override
    public void run() {
        if (entity == null || !entity.isValid()) {
            if (followingUUID == null)
                return;
            entity = Bukkit.getPlayer(followingUUID);
            if (entity == null) {
                entity = Util.getEntity(followingUUID);
            }
            if (entity == null)
                return;
        }
        if (!isActive())
            return;

        if (!npc.getEntity().getWorld().equals(entity.getWorld())) {
            if (Setting.FOLLOW_ACROSS_WORLDS.asBoolean()) {
                npc.teleport(entity.getLocation(), TeleportCause.PLUGIN);
            }
            return;
        }
        if (!npc.getNavigator().isNavigating()) {
            npc.getNavigator().setTarget(entity, false);
            if (margin > 0) {
                npc.getNavigator().getLocalParameters().distanceMargin(margin);
            }
        } else {
            flock.run();
        }
    }

    public void setFollowingMargin(double margin) {
        this.margin = margin;
        cancelNavigationIfActive();
    }

    /**
     * Sets whether to protect the followed Entity (similar to wolves in Minecraft, attack whoever attacks the entity).
     */
    public void setProtect(boolean protect) {
        this.protect = protect;
        cancelNavigationIfActive();
    }
}