Tower Defender - Part 2

Spawn enemies at random locations

The purpose of the blog is to document my journey in building a simple Tower Defense game. In this second part I show how I made the enemy objects randomly spawn from different areas of the platform.

The following C# features will be covered:

• Coroutines for time between spawns.
• Random.Range for randomize spawning.
• Simple and reusable C# code

Spawn Manager

For the SpawnManager I used an empty game object and rested it’s location to 0. Then I added two empty game objects as children which act as the actual spawning points. For better visibility I added an Icon to them (green diamond).

Explain logic to spawn objects randomly

With the SpawnManager and SpawnPoints in place, the next step is to create a C# script and add it to the SpawnManager game object.

Variables defined at the start:

• System.Collections has been added since I am using Coroutines to spread the enemy spawn between frames.
• Add several private variables via a SerializedField to be able to change values directly in the game editor.

The Start method:

• I like to Spawn enemy objects every 1-2 seconds. I could use the Update method and an If statement to count down to do this but the task would run every frame. In order to reduce the CPU load and be more effective, a coroutine seems to be better to do this kind of tasks.
• StartCoroutine is a method and part of the coroutine. It is needed to set the custom method I wrote “SpawnObjects”.

Custom function - The IEnumerator SpawnObjects:

• Spawns new enemy prefabs as long the amountToSpawn value is greater zero.
• At every iteration creates a random number that is used to determine at which spawnPoint the enemy prefab is initiated.
• Checks first that we have still objects left to spawn before running the initiate method (Null check).
• Wait the amount specified in timeBetweenSpawn before running again.

The complete C# code for the Spawn Manager script

using System.Collections;
using UnityEngine;

public class EnemySpawner : MonoBehaviour
{
    [SerializeField] private GameObject enemyToSpawn;
    [SerializeField] private int amountToSpawn = 6;
    [SerializeField] private float timeBetweenSpawn = 1f;
    [SerializeField] private GameObject[] spawnPoints;

    private void Start()
    {
        StartCoroutine(SpawnObjects());
    }

    private IEnumerator SpawnObjects()
    {
        while (amountToSpawn > 0)
        {
            // Spawn an object at a random spawn point
            int randomIndex = Random.Range(0, spawnPoints.Length);
            GameObject spawnPoint = spawnPoints[randomIndex];
            if (enemyToSpawn != null)
            {
                Instantiate(enemyToSpawn, spawnPoint.transform.position, spawnPoint.transform.rotation);
            }

            // Wait for the specified time before spawning another object
            yield return new WaitForSeconds(timeBetweenSpawn);

            // Decrement the spawn count
            amountToSpawn--;
        }
    }
}

Game preview