feat: add leash holding support to the entity api (#2026)

Co-authored-by: DeidaraMC <DeidaraMC>

src/main/java/net/minestom/server/entity/Entity.java

@@ -163,6 +163,10 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
     protected boolean removed;
 
     private final Set<Entity> passengers = new CopyOnWriteArraySet<>();
+
+    private final Set<Entity> leashedEntities = new CopyOnWriteArraySet<>();
+    private Entity leashHolder;
+
     protected EntityType entityType; // UNSAFE to change, modify at your own risk
 
     // Network synchronization, send the absolute position of the entity each X milliseconds
@@ -474,6 +478,15 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
             }
             player.sendPacket(getPassengersPacket());
         }
+        // Leashes
+        if (leashHolder != null && (player.equals(leashHolder) || leashHolder.isViewer(player))) {
+            player.sendPacket(getAttachEntityPacket());
+        }
+        for (Entity entity : leashedEntities) {
+            if (entity.isViewer(player)) {
+                player.sendPacket(entity.getAttachEntityPacket());
+            }
+        };
         // Head position
         player.sendPacket(new EntityHeadLookPacket(getEntityId(), position.yaw()));
     }
@@ -492,6 +505,7 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
                 if (passenger != player) passenger.updateOldViewer(player);
             }
         }
+        leashedEntities.forEach(entity -> player.sendPacket(new AttachEntityPacket(entity, null)));
         player.sendPacket(destroyPacketCache);
     }
 
@@ -1100,6 +1114,40 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
         return new SetPassengersPacket(getEntityId(), passengers.stream().map(Entity::getEntityId).toList());
     }
 
+    /**
+     * Gets the entities that this entity is leashing.
+     *
+     * @return an unmodifiable list containing all the leashed entities
+     */
+    public @NotNull Set<Entity> getLeashedEntities() {
+        return Collections.unmodifiableSet(leashedEntities);
+    }
+
+    /**
+     * Gets the current leash holder.
+     *
+     * @return the entity leashing this entity, null if no leash holder
+     */
+    public @Nullable Entity getLeashHolder() {
+        return leashHolder;
+    }
+
+    /**
+     * Sets the leash holder to this entity.
+     *
+     * @param entity the new leash holder
+     */
+    public void setLeashHolder(@Nullable Entity entity) {
+        if (leashHolder != null) leashHolder.leashedEntities.remove(this);
+        if (entity != null) entity.leashedEntities.add(this);
+        this.leashHolder = entity;
+        sendPacketToViewersAndSelf(getAttachEntityPacket());
+    }
+
+    protected @NotNull AttachEntityPacket getAttachEntityPacket() {
+        return new AttachEntityPacket(this, leashHolder);
+    }
+
     /**
      * Entity statuses can be found <a href="https://wiki.vg/Entity_statuses">here</a>.
      *
@@ -1545,6 +1593,9 @@ public class Entity implements Viewable, Tickable, Schedulable, Snapshotable, Ev
         final Entity vehicle = this.vehicle;
         if (vehicle != null) vehicle.removePassenger(this);
 
+        Set<Entity> leashedEntities = getLeashedEntities();
+        leashedEntities.forEach(entity -> entity.setLeashHolder(null));
+
         MinecraftServer.process().dispatcher().removeElement(this);
         this.removed = true;
         if (permanent) {