Tower Defender - Part 3

Detect and shoot at nearest enemy

The purpose of the blog is to document my journey in building a simple Tower Defense game. In this third part I show how to rotate the Tower towards and shoot at the nearest enemy.

The following C# features will be covered:

• Transform
• Vector3
• Quaternion
• GetComponent
• foreach

Tower Setup

For the Tower I created a cylinder as base and on top a capsule as the fire head that is within an empty game objet Rotate. I then saved the Tower as a Prefab, so that it can be used and spawned multiple times going forward.

Explain logic for the detect and shoot script

With the tower being in place, the next step is to create two C# scripts.

• One for detecting and shooting at nearby enemies
• One that controls the bullet movement.

Variables defined at the start:

In order to select in the Unity UI what game objects are needed for this script, I created a few SerializeField’s. I also defined some variables like the Tower turn speed, fire rate and range.

The Start method:

Checking 60 times or more per second if an enemy is in range in the Update method would be very CPU consuming. Instead I use a coroutine defined in the Start method to call the UpdateEnemyInRange every 2 time per second.

The Update method:

• Prevent that no resources are wasted if there is no target. The If statement is taking care about it and returns every frame if there is no target.

• Define the position of the target (closed enemy) and tell the Tower to rotate towards the closest enemy in range.

• Call the Shoot() method if a set of conditions is fulfilled. The enemy need to be in range and the tower need to rotate and look at the enemy. Reset the time between shooting.

Custom function - Shoot:

• Get the pullet component.
• Calls the Seek method to make the bullet follow the target.
• Instantiate the bullet at the position of the Tower fire point.

Custom function - IEnumerator UpdateEnemyInRange:

• Check two time a second all enemies in range of the tower and define a target that is the closest enemy in range.

The OnDrawGizmosSelected method does the following:

• Draws a a white wire frame sphere around the Tower to visually see the range.

The complete C# code for the the detect and shoot script

using System.Collections;
using UnityEngine;

public class TowerMoveAndShoot : MonoBehaviour
{
    [SerializeField] private GameObject projectilePrefab;
    [SerializeField] private Transform partToRotate;
    [SerializeField] private Transform firePoint;
    [SerializeField] private float range = 3f;
    [SerializeField] private float turnSpeed = 8f;
    [SerializeField] private float fireRate = 2f;
    [SerializeField] private string enemyTag = "Enemy";

    private Transform target;
    private float fireCountdown;

    private void Start()
    {
        StartCoroutine(UpdateEnemyInRange());
        fireCountdown = 1f / fireRate;
    }

    private void Update()
    {
        if (target == null)
        {
            return;
        }

        Vector3 dir = target.position - transform.position;
        Quaternion lookRotation = Quaternion.LookRotation(dir);
        Vector3 rotation = Quaternion.Lerp(partToRotate.rotation, lookRotation, Time.deltaTime * turnSpeed).eulerAngles;
        partToRotate.rotation = Quaternion.Euler(0f, rotation.y, 0f);

        if (target != null && fireCountdown <= 0f && Vector3.Dot(partToRotate.forward, (target.position - transform.position).normalized) >= 0.8f)
        {
            Shoot();
            fireCountdown = 1f / fireRate;
        }
        else
        {
            fireCountdown -= Time.deltaTime;
        }
    }

    private void Shoot()
    {
        Projectile projectile = cannonGO.GetComponent<Projectile>();
        projectile?.Seek(target);
        GameObject cannonGO = Instantiate(projectilePrefab, firePoint.position, firePoint.rotation);
    }

    private IEnumerator UpdateEnemyInRange()
    {
        while (true)
        {
            yield return new WaitForSeconds(0.5f);

            GameObject[] enemies = GameObject.FindGameObjectsWithTag(enemyTag);
            float shortestDistance = Mathf.Infinity;
            GameObject nearestEnemy = null;

            foreach (GameObject enemy in enemies)
            {
                float distanceToEnemy = Vector3.Distance(transform.position, enemy.transform.position);
                if (distanceToEnemy < shortestDistance)
                {
                    shortestDistance = distanceToEnemy;
                    nearestEnemy = enemy;
                }
            }

            target = (nearestEnemy != null && shortestDistance <= range) ? nearestEnemy.transform : null;
        }
    }

    private void OnDrawGizmosSelected()
    {
        Gizmos.color = Color.white;
        Gizmos.DrawWireSphere(transform.position, range);
    }
}

Game preview with Towers rotating towards the closet enemy and initiate bullet prefab

Explain logic for the Bullet script:

In order to make separate the the bullet functionality from the tower, I used a separate script.

Above the Update method:

• A private variable to store the tower target data.

• A variable to set the bullet move speed.

Custom function - Seek:

• This method is taking the target information from the Tower script and stores it into the local target variable.

The Update method:

• First check if we have a target. If not then Destroy the bullet and exit out of the Update method.

• If there is a target the direction of the target will be determined and stored into the dir variable.

• The IF block checks if the the bullet has reached the target and if so calls the custom HitTarget function which destroys the bullet game object.

• If the bullet has not reached the target yet, then the bullet position will be updated every frame to make the bullet move in a constant speed (normalized) towards the target.

Custom function - HitTarget:

• Destroys the bullet game object when called.

The complete C# code for the the detect and shoot script

using UnityEngine;

public class Bullet : MonoBehaviour
{
    private Transform target;
    [SerializeField] private float moveSpeed = 5f;


    public void Seek(Transform _target)
    {
        target = _target;
    }

    // Update is called once per frame
    void Update()
    {
        if (target == null)
        {
            Destroy(gameObject);
            return;
        }

        Vector3 dir = target.position - transform.position;
        float distanceThisFrame = moveSpeed * Time.deltaTime;

        if (dir.magnitude <= distanceThisFrame)
        {
            HitTarget();
            return;
        }

        transform.position += dir.normalized * distanceThisFrame;

    }

    void HitTarget()
    {
        Destroy(gameObject);
    }
}

Game preview