How to destroy an object, and then instantiate it again later?

Question

I'm creating an enemy spawner in Unity (2D). At long last, I have something that is quite close to working, but the problem is that I want it to destroy all instances of the enemy when the player can no longer see the spawn area.

That is, the player happens upon the spawn area to find several enemies, but if they move so that the camera can no longer see the spawn area, I want those enemies to be deleted and then a different set of the same enemies to be instantiated when the player (camera) happens upon the spawn area again. But when I attempt this initial delete, the script component no longer has an object to instantiate when the player returns.

Here's a snippet of what I have:

    if (!notOnScreen && spawned == false)
    {
        int numberOfEnemies = Random.Range(1, enemyMax);

        for (int i = 1; i <= numberOfEnemies; i++)
        {

            spawnX = Random.Range(minX + 1, maxX - 1);
            spawnY = Random.Range(minY + 1, maxY - 1);

            Vector3 spawnpoint = new Vector3 (spawnX, spawnY, 0);

            if (enemies.Length == 1) {
                typeOfEnemy = 0;
            }
            else if (enemies.Length > 1) {
                typeOfEnemy = Random.Range(0, enemies.Length);
            }
            else {
                typeOfEnemy = -1;
            }

            Instantiate(enemies[typeOfEnemy], spawnpoint, Quaternion.identity);
        }

        spawned = true;
    }

    if (notOnScreen) {
        spawned = false;

        currentEnemies = GameObject.FindGameObjectsWithTag("Enemy");
        for (int i = 0; i <= currentEnemies.Length; i++) {
            Destroy(currentEnemies[i]);
        }
    }


}

}

I'm sure there's a way to preserve an instance of the enemy so that they can be instantiated again later, but I'm not sure how (sorry--very new to this). I thought about cheating and having one enemy remain at a different z coordinate behind the map (as this is a 2D game), but that doesn't seem like an appropriate solution. Any thoughts or advice?

Answer 1

I agree with Pascal Lohscheidt that for a case like this, where you're repeatedly spawning & despawning many enemies, recycling them may be a better option.

But, for the general problem of saving an out-of-scene object that you want to be able to spawn on demand, you can use Prefabs.

Take one of the object you want to spawn multiple times, and drag it into your Project folder. This creates a Prefab asset, that exists independent of any scene. Now your spawning scripts can reference the prefab as the source to copy newly-instantiated objects from, instead of relying on an object that's already in the scene.

One often-overlooked trick for Prefabs is that they can be referred to by the type of any component they contain. So if you ultimately want to access an Enemy component on your newly-spawned critter, you can declare its prefab variable as public Enemy enemyPrefab; and instantiate it as Enemy myEnemy = Instantiate<Enemy>(enemyPrefab, ...) to get a reference to the newly created enemy's Enemy component without GetComponent or as. This still spawns a copy of the whole Prefab, not just a free-floating Enemy component. ;)

Answer 2

If I understand you right, you want to reposition the enemies at a random position anytime you look at them again. A really Simple way would be to have an array (maybe a list) of enemy objects and method which replaces them and every time the player camera leaves the spawn area you simply disable the objects and replace them and any time your camera enters the area you simply enable them. That would be solution if you really want the same set. Code example:

GameObject[] enemies = FindGameObjectsWithTag(...);

if(camLeaves)
{
   foreach(GameObject enemy in enemies) //place them into another 
                                           position
   {
      enemy.transform.position = new Vector(RandomX, 0, RandomY);
      enemy.SetActive(false);
   }
}

if(camEnters)
{
   foreach(GameObject enemy in enemies)
   {
       enemy.SetActive(true);
   }
}

Anyway this way you won´t lose your references. But that are always the same enemies. And also if an enemy dies you won´t have any reference to him anymore. Maybe it´s worth using a list and removing the instance once it dies. Or you ask anytime you change the positions or enable & disable the objects for new reference with the method FindObjectsWithTag(). If you have questions or when I understood something wrong. Feel free to comment below & ask :)