Display image a couple seconds after an enemy spawns?

Question

So right now I have enemies that spawn behind objects with a delay.

What I need is to display an image on the screen 5 seconds after each enemy spawns (it's to show the player it has been "attacked") and that image has to disappear after like 2 seconds so.

I know I have to use WaitForSeconds but I don't know how because everything I've tried isn't working.

Here's my code:

 public class TimedSpawn : MonoBehaviour {

     public GameObject spawnee;
     public bool stopSpawning;
     public float spawnTime;
     public float spawnDelay;

     // Use this for initialization
     void Start () {
         InvokeRepeating ("SpawnObject", spawnTime, spawnDelay);
     }

     public void SpawnObject (){
         Instantiate (spawnee, transform.position, transform.rotation);

         if (stopSpawning) {
             CancelInvoke ("SpawnObject");
         }
     }
 }

Answer 1

Well, all those co-routines and waiting for some seconds are nice but, why not just use Invoke() for this purpose as well?

//...
public void SpawnObject (){
    var warningDelay = 5.0;
    Instantiate (spawnee, transform.position, transform.rotation);
    Invoke("CreateWarningImage", warningDelay);
    if (stopSpawning) {
        CancelInvoke ("SpawnObject");
    }

}
//someTransform is where you want the image to be, assuming you've already defined it
public void CreateWarningImage(){
    Instantiate (warning, someTransform.position, someTransform.rotation);
}

And then, you just put a destroy on the a script's Awake() function which is on the image like so:

var lifetime = 2.0;
function Awake()
{
    Destroy(gameObject, lifetime);
}

Note that I'm assuming you're not spawning hundred thousand enemies per second, so the performance difference between an invoke call (looks for a string) and a co-routine shouldn't matter at all. For the simplest stuff (like doing one thing after some seconds), you shouldn't really need anything other than Invoke().

Answer 2

Use a co routine to accomplish this. I don't know how you want to display the image, but for this example I will use the Unity UI Image. Example below:

public class Example : MonoBehaviour {

public GameObject spawnee;
public bool stopSpawning;
public Image image;

public float spawnTime;
public float spawnDelay;

// Use this for initialization
void Start()
{
    image.gameObject.SetActive(false);
    InvokeRepeating("SpawnObject", spawnTime, spawnDelay);
}

public void SpawnObject()
{
    Instantiate(spawnee, transform.position, transform.rotation);
    StartCoroutine(DisplayMessage(5, 2, image));

    if (stopSpawning)
    {
        CancelInvoke("SpawnObject");
    }
}

public IEnumerator DisplayMessage(float displayOffset, float displayDuration, Image im)
{
    float time = 0f;
    do
    {
        time += Time.deltaTime;
        yield return null;
    }
    while (time < displayOffset);

    im.gameObject.SetActive(true);

    float time2 = 0f;
    do
    {
        time2 += Time.deltaTime;
        yield return null;
    }
    while (time2 < displayDuration);
    im.gameObject.SetActive(false);
}
}

Hope this helps.