Multiple enemies get hit by arrow instead of one

Question

I am working on my first Tower Defense game and I have a problem where multiple enemies get hit by a projectile instead of one.

Enemy with its collider:

The problem is that when the enemies get close to each other, their colliders overlap and one arrow deals damage to multiple units.

Arrow and Enemy both have trigger colliders 2D.

Code for Arrow.cs component:

    private void OnTriggerEnter2D(Collider2D collision) {
    Enemy enemy = collision.GetComponent<Enemy>();

         if (enemy != null) {
             IDamagable damagable = enemy.GetComponent<IDamagable>();
             if (damagable != null) {
                 damagable.ReceiveDamage(DamageType.PHYSICAL, attackType, damage);
             } else {
                 Debug.Log("Unit is missing the IDamagable script");
             }
             Destroy(gameObject);
        }
    }

Seems like collision detection and OnTriggerEnter2D is called in the same step and this function runs multiple times even if the game object is destroyed.

Answer 1

As I have target set during initialization

private GameObject targetEnemy;
private int damage;
private AttackType attackType;

public void Initialize(GameObject targetEnemy, int damage, AttackType attackType) {
    this.targetEnemy = targetEnemy;
    this.damage = damage;
    this.attackType = attackType;
}

I just had to add additional check if collision happened for taget enemy, not some enemy that was close to he target.

    private void OnTriggerEnter2D(Collider2D collision) {
    Enemy enemy = collision.GetComponent<Enemy>();

    if (enemy != null && targetEnemy == collision.gameObject) {
        ...receive damage
    }
}

Answer 2

I disagree with the suggestion to keep track of the target. What you can do instead is have some bookkeeping in the projectile about whether it already hit a target or not (a boolean field should do the job). And if it already has, then you can just short-circuit in the collision code and not apply damage.

So something like this:

private bool hasHitAnEnemy = false;

private void OnTriggerEnter2D(Collider2D collision) {
    if(hasHitAnEnemy) { 
        return;
    }

    Enemy enemy = collision.GetComponent<Enemy>();

    if (enemy != null) {
         IDamagable damagable = enemy.GetComponent<IDamagable>();
         if (damagable != null) {
             damagable.ReceiveDamage(DamageType.PHYSICAL, attackType, damage);
             hasHitAnEnemy = true;
         } else {
             Debug.Log("Unit is missing the IDamagable script");
         }
         Destroy(gameObject);
    }
}