Unity - How to have two of the same GameObjects collide, destroy each other, and spawn a new GameObject at the target position?

Question

So I want to have two balls collide, destroy themselves, and then have another ball spawn at their place (preferably with a specific velocity). When I try to attach the script to the ball, however, both instances of the ball are destroyed on contact, then immediately spawns two prefabs of the ball, since they both have the code. This causes both balls to spawn and destroy each other over and over. I have this script attached to the balls:

private Vector3 ballPosition;

void OnTriggerEnter2D (Collider2D other) {

    if (other.gameObject.tag == "Ball") {

        ballPosition = new Vector3 ((transform.position.x + other.transform.position.x) / 2, (transform.position.y + other.transform.position.y) / 2, 0.0f);
        StartCoroutine ("RespawnBall");
    }
}

IEnumerator RespawnBall () {

    Instantiate (gameObject, ballPosition, Quaternion.identity);
    Destroy (gameObject);
    yield return null;
}

How do I make this code destroy both balls, then spawn only one instance of the prefab?

Answer 1

You can also use a boolean on your script that is set so only the first ball having its OnTriggerEnter2D() method call will spawn a new ball :

private Vector3 ballPosition;
public bool spawnNewBall;

void Start() {
    spawnNewBall = true;
}

void OnTriggerEnter2D (Collider2D other) {

    if (other.gameObject.tag == "Ball") {

        if (spawnNewBall) {
            other.GetComponent</*YourScriptName*/>().spawnNewBall = false;
        }

        ballPosition = new Vector3 ((transform.position.x + other.transform.position.x) / 2, (transform.position.y + other.transform.position.y) / 2, 0.0f);
        StartCoroutine ("RespawnBall");
    }
}

IEnumerator RespawnBall () {
    if (spawnNewBall) {
        Instantiate (gameObject, ballPosition, Quaternion.identity);
    }
    Destroy (gameObject);
    yield return null;
}

---

More simply, just do this:

public class TwoToOne : MonoBehaviour {

    public bool doNothing;

    void OnCollisionEnter (Collision col)
        {
        if (doNothing) return;

        col.gameObject.GetComponent<TwoToOne>().doNothing = true;
        Destroy(col.gameObject);

        GameObject newCube = Instantiate(gameObject);

        Destroy(gameObject);
        }

    }

That's a totally common script or pattern in Unity.

So, conceptually you just "destroy the other one"; since one of the scripts has to run first, it works out fine. In different systems from Unity you can do one of these things:

• A, "destroy" the other game object in some way. That would be DestroyImmediate(col.gameObject) in Unity.

• B, "destroy" or disable the other script in some way. that would be col.gameObject.GetComponent<TwoToOne>().enabled = false in Unity

• C, "flag" (set a boolean) on the other game object in some way - as shown here.

As it happens, in Unity specifically you can't do A or B, so you just do C. B is the most elegant solution, but it doesn't work in Unity5, so for now do C!

Answer 2

You can solve this with the use of delegates OR as @JoeBlow suggested, the use of UnityEvents from an Event Manager.

In the following example I have opt'd to use delegates.

There are 2 scripts. One which sits on the spheres (which you already have) and the second, a static class which can be referenced from any other script in the application without instantiation.

---

EventManager.cs

using UnityEngine;
using System.Collections;

public static class EventManager{

    // Create our delegate with expected params.
    // NOTE params must match SphereScript.PostCollision declaration
    public delegate void CollideEvent(string message);

    // Create the delegate instance. This is the one we will invoke.
    public static event CollideEvent PostCollision;

    // Called whenever an object has collided with another
    public static void Collision(GameObject obj1, GameObject obj2, Vector3 collisionPoint){
        if (obj1.GetComponent<sphereScript>().isAlive && obj2.GetComponent<sphereScript>().isAlive) {

            //Kill the 2 objects which haev collided.
            obj1.GetComponent<sphereScript> ().Kill ();
            obj2.GetComponent<sphereScript> ().Kill ();

            //Create a cube.
            GameObject cube = GameObject.CreatePrimitive (PrimitiveType.Cube);
            cube.transform.position = collisionPoint;

            // Invoke delegate invocation list
            PostCollision("Something is dead");
        }
    }
}

SphereScript .cs

using UnityEngine;
using System.Collections;

public class sphereScript : MonoBehaviour {
    // Am I alive?
    public bool isAlive;

    // Use this for initialization
    void Start () {
        // Add a function we want to be called when the EventManager invokes PostCollision
        EventManager.PostCollision += PostCollision;
        isAlive = true;
    }

    // Update is called once per frame
    void Update () {

    }

    //Invoked from EventManager.PostCollision delegate
    void PostCollision(string message){
        if(isAlive)
            Debug.Log (this.name + " message received: " + message);
    }

    // Called when it is time to destroy this gameobject
    public void Kill(){
        isAlive = false;
        Destroy (this.gameObject);
    }

    //On collision with another object
    void OnCollisionEnter2D(Collision2D collision){
        if (collision.gameObject.GetComponent<sphereScript>()) {
            EventManager.Collision (this.gameObject, collision.gameObject, collision.contacts [0].point);
        }
    }

    // Called after this object has been destroyed
    void OnDestroy(){
        // cleanup events for performance.
        EventManager.PostCollision -= PostCollision;
    }
}

---

So why use this method?

This method is ideal for controlling the release of multiple 'happenings' from a single location. In this case, we trigger a function on each ball each of the colliding balls. The EventManager then creates a single cube in their wake and following this, notifies any remaining cubes that a collision has occurred elsewhere in the scene.

Answer 3

what you can do is create another script which only destroys the ball but not spawn and attach it to the 2nd ball. the first ball will have both destroy and respawn feature. so when both balls destroy each other, one will respawn and the other will not because you did not attach the spawn feature to the 2nd ball