How to detect if a GameObject is depawned?

Hi,

I have an  Enemy object with the  Killable script attached.  I have a Turret running a third party script that is aiming, shooting and sending damage to the killable (via raycasting).  However when the Enemy is destroyed (de-spawned)  the turret does not pick any other enemies because it's 3rd party script doesn't know how to detect that the target has been despawned.

So my question is what's the best method to detect this?


Additional Info / Stuff I've tried.
during the initial (3rd party script) DetectTarget method i have coded access to the killable script via:


if(target == null)
{
 target = otherGO.transform;
 
 /* store the EnemyKillable script */
 EnemyKillable = target.root.gameObject.GetComponent<Killable>();
}
Which i know works as later i call:

EnemyKillable.TakeDamage(myKillable.atckPoints.Value, myKillable);

but I need to detect when the Enemy is "killed"...?

the 3rd party script uses:


void OnTriggerExit(Collider other)
{
 if(other.transform.root == target)
 {
   target = null;
 }
}
But this does not seem to work if the Enemy is de-spawned rather than 'Destroyed'. and I have tried doing 

if(!target.root.gameObject.activeInHierarchy) { target = null; }
but activeInHierarchy always = true

any pointers woud be gratefully received.  I note there is a OnDespawned event mentioned in the docs, but I am not sure what I need to do in my script to subscribe to this event?


I have tried to simply put:

void OnDespawned () {
 Debug.Log("I GOT DESPAWNED!!  AWW");
 target = null;
 EnemyKillable = null;
}
however I suspect I need to do something else to get the method to fire?? - help  :-D