OnCollision - Accessing properties of object collided with

Unity Engine
1

Hello,

This has completely brought my Ludum Dare weekend to a halt (booo), but I’ve tried for hours to figure out how to do this / what Unity is doing to no avail.

Basically I want to do this:

• Fire rigidbody projectile
• Projectile hits a GameObject tagged with ‘prop’ which also has a Prop.CS on it
• Inside Prop.CS are strings like ‘lookAtDescription’ (blah blah…stuff unique to that prop)
• Print the string on the UI Canvas

No matter what I did today, I couldn’t get Unity to access or show the string. At first, it looked like the issue I had was my UI text simply wouldn’t update, even though I could query the variable and text object with Print() and get the correct data. Then, with more digging around and debugging it looked like for the single frame of the collision things were getting set, but then immeditally going blank / null.

Let me show you how I set it up:

  1. This is what a prop in the level would have on it. Then in the inspector it has a string setup. Simple.
public class Prop : MonoBehaviour {

       public string lookAtResponse;

}
  1. A rigidbody projectile is then instantiated from a fire script:
if (Input.GetMouseButtonDown(0))
        {
            switch (ammoState.activeAmmo)
            {
                case 1:
                    projectileInstance = Instantiate(LookAtProjectile, barrelEnd.position, barrelEnd.rotation) as Rigidbody;
                    projectileInstance.AddForce(barrelEnd.forward * 300);
                    break;

                case 2:
                    projectileInstance = Instantiate(TalkToProjectile, barrelEnd.position, barrelEnd.rotation) as Rigidbody;
                    projectileInstance.AddForce(barrelEnd.forward * 300);
                    break;
            }
        }
  1. The projectile then has a check on it to see if it collided with a prop, and then if so, to create a reference to the prop.cs string ‘lookAtResponse’. It then passes this string through to a method on the canvas.
public class LookAtHit : MonoBehaviour {
 
    Prop propHit;
    public UIObserver uIObserver;
 

    public void OnCollisionEnter(Collision collision)
    {
   
        //If we collide with a prop
        if(collision.gameObject.tag == "Prop")
        {
            //Get the prop stuff
            propHit = collision.gameObject.GetComponent<Prop>();

            //Call method on UI Observer and pass through the prop description
            uIObserver.OnPropHit(propHit.lookAtResponse);

            //Destory the projectile
           Destroy(gameObject);
        }
    }
}
  1. The canvas then had a method (and links) to a text element in the UI
   public void OnPropHit(string lookAtText)
    {
        mainText.text = lookAtText;
 
    }

No matter what I did, the text never changed. No errors. No warnings. No suggestion of any missing references…etc

If anyone has any suggestions or insight they’d be massively appreciated.

Many thanks!

2

That sounds logical.

From part of what you said, if you’re sure the text is correct but then unset a moment later, I would try to ensure that nothing else is setting the text (to an empty string, for instance).
Also, it doesn’t hurt to double check the inspector to see if the text is there correctly, but just not shown properly visually (can happen sometimes).

Other than that, all I can suggest is that you Debug.Log at various stages to see when it doesn’t go according to plan in your code.

3

I still haven’t figured this out, but I suspect it’s something to do with the fact I’m trying to alter the UI and that it’s all stemming from a single frame collision call, not an Update().

4

Did you try the things I suggested?

It shouldn’t matter that you call it from the collision point.