Spawning Enemies Using Coroutines

Spawning Enemies Using Coroutines

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Justin Horner
·Apr 7, 2022·

We've made a lot of progress on the prototype; congrats on making it this far! Now, let's add a spawn manager to will spawn enemies over time.

The Spawn Manager

First, we need a host GameObject for the spawn manager script. You know how to do that by now, so go ahead and create it and also create a new script named SpawnManager in the scripts folder, and add it as a component.

In the spawn manager script, I'll add a field named enemyPrefab and make it configurable via the inspector with the SerializeField attribute. To keep our hierarchy clean, I'll also create an enemyContainer field to be the parent for our enemy prefab instances.

I don't need the Update method, so I'll remove it and in it's place I'll add a Coroutine method. I will have an upcoming article to talk about Coroutines in more detail. For now, just know that this will allow us to continually spawn enemy instances.

I want to spawn a new enemy every 5 seconds as a default, but also allow for this to be adjusted via the inspector. The coroutine itself is a simple loop that yields for a provided amount of seconds, and then instantiates a new enemy instance at a random horizontal location.

public class SpawnManager : MonoBehaviour
    [SerializeField] float spawnInterval = 5.0f;
    [SerializeField] GameObject enemyPrefab;
    [SerializeField] Transform enemyContainer;

    bool isStopped;
    Enemy enemy = null;

    // Start is called before the first frame update
    void Start()
        enemy = enemyPrefab.GetComponent<Enemy>();


I've assigned the enemyPrefab and the enemyContainer references from the editor. We've already created variables in the enemy script that define the min and max X and Y values that we will want to use when spawning a new enemy, so in Start I am using the GetComponent method to get the Enemy component.

The min and max X and Y variables on the Enemy class are private, so we need to make read-only public accessors so we can use the values, but not be able to change them. I'll add the properties just beneath the variables in Enemy.cs.

public float BoundaryYMax { get { return boundarYMax; } }
public float BoundaryXMin { get { return boundaryXMin; } }
public float BoundaryXMax { get { return boundaryXMax; } }

And here's the spawn coroutine:

IEnumerator SpawnRoutine()
    while (!isStopped)
        float randomXPosition = Random.Range(enemy.BoundaryXMin, enemy.BoundaryXMax);
        Vector3 spawnPosition = new Vector3(randomXPosition, enemy.BoundaryYMax);

        Instantiate(enemyPrefab, spawnPosition, Quaternion.identity, enemyContainer);
        yield return new WaitForSeconds(spawnInterval);

Here you can see we are creating a new random position for newly spawned enemies and waiting for the time set in our spawnInterval. After playing the game for a bit, you'll end up with lots of enemies on screen!

Spawning Enemies


We now have a spawn manager that we can build upon as our prototype evolves.

Take care.
Stay awesome.

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