How do I queue up targets in a TD?

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Hi all,

Here's a problem that's been bugging me for a few days now; I couldn't figure it out on my own, so I thought I'd ask here. I've just started making a Tower Defense game, and have quickly set up a couple temporary scripts, one attached to a cube that automatically moves in a straight line past my tower, and one on my tower. A child of the tower is a cylinder collider made into a trigger.

At first I tried making the trigger itself note when the cube entered its range, but since I couldn't figure that out, I made the cube send a message to the tower when the cube hit the trigger, and from there made the tower look at the cube until it left the trigger area. That worked fine, until I added in multiple cubes. The way I had set it up meant it screwed up when there were multiple cubes in the trigger area, but I can't for the life of me figure out how to make it so that:

The first cube is targeted by the tower,
When that cube leaves the trigger area or is destroyed, the tower targets the next one, and
Continues to cycle through all of the possible targets when the current one is destroyed or goes out of range

Here's the scripts I already have, if it helps in any way. This one's on the turret:

var targetedEnemy : GameObject; var inRange : boolean = false;

function Update() { if (targetedEnemy && inRange == true) { transform.LookAt(targetedEnemy.transform); } }

function EnteringRange(recievedGameObject : GameObject) { targetedEnemy = recievedGameObject; inRange = true; }

function LeavingRange() { inRange = false; }

And this one's on the cube:

var thisObject : GameObject; var otherObject : GameObject;

function Start() { thisObject = gameObject; }

function Update () { transform.Translate(0.025, 0, 0); }

function OnTriggerEnter (hit : Collider) { otherObject = hit.gameObject; otherObject.SendMessageUpwards("EnteringRange", thisObject); }

function OnTriggerExit (hit : Collider) { otherObject.SendMessageUpwards("LeavingRange"); }

Thanks for your help!

Update:

I tried changing the inRange variable to an int, and every time an enemy came in range it'd add 1 to the variable, and every time one left it'd subtract 1, which made it not screw up, however the turret would then target the newest enemy to enter the range, whereas I wanted it to select the first one to enter, i.e the one closest to leaving.

Answer 1

You talk about the common targetting modes in TD games. There are a lot TDs where you can choose the mode. I'll choose the Onslaught TD game as reference.
It have modes like:

Newest
Oldest
Nearest
Furthest
Strongest
Weakest

Beside those criteria that choose what you want to target it's important whether the tower should Lock on target or not. I guess lock on target is what you want.
Every of those modes I've listed above needs to check all enemies that are in range to decide what's the desired target. If you use "lock on target", you don't need to look for another one until the current leaves the range or dies.

Unity's triggersystem is really great, but only for triggering events. For selecting I would recommend to use a function inside Physics. For such range detecting tasks Physics.OverlapSphere comes in handy. It returns all colliders (also those marked "asTrigger") that are inside a certain radius.

Your tower only have some states that you should seperate:

No target: keep on searching for targets
Have a target
- Lock on target: keep that target and check if we loose it
- Without lock: Just like no target

Here's a small example of the idea i was talking about:
edit: This will pick the strongest target.

var currentTarget : Transform; var range : float = 5.0;

function SearchTarget() { var newTarget : Enemy = null; var collidersInRange = Physics.OverlapSphere(transform.position,range); for (var currentCollider : Collider in collidersInRange) { var currentEnemy : Enemy = currentCollider.GetComponent.<Enemy>(); if (newTarget == null) { newTarget = currentEnemy; } else if (newTarget.health < currentEnemy.health) { newTarget = currentEnemy; } } if (newTarget != null) currentTarget = newTarget.transform; }

function Update () { if (currentTarget == null) { SearchTarget(); } else { var targetDistance = (currentTarget.position - transform.position).magnitude; if (targetDistance > range) { currentTarget = null; } else { ShootAtTarget(); // or maybe AimAtTarget(); } } }

As long as the tower have a target there's nothing else in "the towers mind". Only if the current target gets destroyed (that would cause the variable to go null) or when it leaves the range it would look for a new target.

How do I queue up targets in a TD?

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