Change from Cancelable to using a Result. This means you can force despawn mobs you

don't want around anymore. Also, deferred check to once every 20 ticks. May tune it
down further or make it a config if this event is a lag issue.

common/net/minecraftforge/event/ForgeEventFactory.java

@@ -60,9 +60,11 @@ public class ForgeEventFactory
         return MinecraftForge.EVENT_BUS.post(new LivingSpawnEvent.SpecialSpawn(entity, world, x, y, z));
     }
 
-    public static boolean canEntityDespawn(EntityLiving entity)
+    public static Result canEntityDespawn(EntityLiving entity)
     {
-        return !MinecraftForge.EVENT_BUS.post(new AllowDespawn(entity));
+        AllowDespawn event = new AllowDespawn(entity);
+        MinecraftForge.EVENT_BUS.post(event);
+        return event.getResult();
     }
     
     public static List getPotentialSpawns(WorldServer world, EnumCreatureType type, int x, int y, int z, List oldList)

common/net/minecraftforge/event/entity/living/LivingSpawnEvent.java

@@ -49,19 +49,21 @@ public class LivingSpawnEvent extends LivingEvent
     }
     
     /**
-     * Fired if the mob may be allowed to despawn. Cancellation will prevent possible despawning. This is fired every tick for 
-     * every despawnable entity. Be efficient in your handlers.
+     * Fired each tick for despawnable mobs to allow control over despawning.
+     * {@link Result#DEFAULT} will pass the mob on to vanilla despawn mechanics.
+     * {@link Result#ALLOW} will force the mob to despawn.
+     * {@link Result#DENY} will force the mob to remain.
+     * This is fired every tick for every despawnable entity. Be efficient in your handlers.
      * 
      * Note: this is not fired <em>if</em> the mob is definitely going to otherwise despawn. It is fired to check if
-     * the mob can be allowed to despawn according to standard mob spawning rules. See {@link EntityLiving#despawnEntity}
+     * the mob can be allowed to despawn. See {@link EntityLiving#despawnEntity}
      * 
      * @author cpw
      *
      */
-    @Cancelable
+    @HasResult
     public static class AllowDespawn extends LivingSpawnEvent
     {
-
         public AllowDespawn(EntityLiving entity)
         {
             super(entity, entity.worldObj, (float)entity.posX, (float)entity.posY, (float)entity.posZ);

patches/minecraft/net/minecraft/entity/EntityLiving.java.patch

@@ -1,17 +1,18 @@
 --- ../src_base/minecraft/net/minecraft/entity/EntityLiving.java
 +++ ../src_work/minecraft/net/minecraft/entity/EntityLiving.java
-@@ -31,6 +31,10 @@
+@@ -31,6 +31,11 @@
  import net.minecraft.util.MathHelper;
  import net.minecraft.world.World;
  import net.minecraft.world.WorldServer;
 +import net.minecraftforge.common.ForgeHooks;
 +import net.minecraftforge.common.MinecraftForge;
++import net.minecraftforge.event.Event.Result;
 +import net.minecraftforge.event.ForgeEventFactory;
 +import net.minecraftforge.event.entity.living.LivingSpawnEvent.AllowDespawn;
  
  public abstract class EntityLiving extends EntityLivingBase
  {
-@@ -141,6 +145,7 @@
+@@ -141,6 +146,7 @@
      public void setAttackTarget(EntityLivingBase par1EntityLivingBase)
      {
          this.attackTarget = par1EntityLivingBase;
@@ -19,18 +20,29 @@
      }
  
      /**
-@@ -548,6 +553,10 @@
+@@ -547,9 +553,21 @@
+      */
      protected void despawnEntity()
      {
++        Result result = null;
          if (this.persistenceRequired)
-+        {
-+            this.entityAge = 0;
-+        }
-+        else if (this.canDespawn() && !ForgeEventFactory.canEntityDespawn(this))
          {
              this.entityAge = 0;
++        }
++        else if (this.entityAge % 20 == 1 && (result = ForgeEventFactory.canEntityDespawn(this)) != Result.DEFAULT)
++        {
++            if (result == Result.DENY)
++            {
++                this.entityAge = 0;
++            }
++            else
++            {
++                this.setDead();
++            }
          }
-@@ -726,8 +735,6 @@
+         else
+         {
+@@ -726,8 +744,6 @@
          return this.worldObj.checkNoEntityCollision(this.boundingBox) && this.worldObj.getCollidingBoundingBoxes(this, this.boundingBox).isEmpty() && !this.worldObj.isAnyLiquid(this.boundingBox);
      }