1

(Apologies for the useless tags, but this site seems not to accept “spring”, “trampoline” or “collision” as valid tags, even though results for them came up when I entered those words in the tag suggestion box!)

Hi all,

I’ve got a 2D Game with a player and a trampoline, and I want the player to bounce up when he lands on the trampoline. However, when I tried this:

using UnityEngine;
using System.Collections;

public class Springy : MonoBehaviour {
	public int springForce;
	public Collision collision;
	public GameObject subject;
	// Use this for initialization
	void Start () {
	}
	
	// Update is called once per frame
	void Update () {
		OnCollisionEnter ();
	}
	void OnCollisionEnter()
	{
		collision.gameObject.rigidbody2D.AddForce(new Vector2(0f, springForce));
		Debug.Log("We bounced...or did we?");
	}
}

it compiles correctly, but running the game immediately causes the following error:

NullReferenceException: Object reference not set to an instance of an object
Springy.OnCollisionEnter () (at Assets/Springy.cs:18)
Springy.Update () (at Assets/Springy.cs:14)

I am a little bamboozled by the Collision object, and am not sure what I’m doing wrong. Can someone else shed some light on this?

Thanks.

3

I have been working on a Trampoline class based on a few answers in this forum. If you want a consistent bounce every time, I found it works better if you set the velocity of the colliding object to 0 and then add the force right after. I know this has already been answered, but I figured this might help those who are finding this question on Google.

using UnityEngine;
using System.Collections;

public class Trampoline : MonoBehaviour {
	public float springForce = 1000;
	private Collision2D collision;
	private bool bouncing = false;

	void OnCollisionEnter2D(Collision2D coll) {
		if (!bouncing) {
			bouncing = true;
			collision = coll;
		}
	}

	void FixedUpdate () {
		if (bouncing) {
			var rb = collision.gameObject.GetComponent<Rigidbody2D> ();
			rb.velocity = new Vector3 (0, 0, 0);
			rb.AddForce (new Vector2 (0f, springForce));
			bouncing = false;
		}
	}
}
2
  1. You should not call OnCollisionEnter from inside Update()!!! OnCollisionEnter gets automatically called if there’s a collision on a MonoBehaviour (see Note below)
  2. OnCollisionEnter has one parameter of type “Collision”. So, when this method gets automatically called, the system provides the Collision object parameter, which contains several useful info about the collision:

So, you should declare in your code OnCollisionEnter together with its parameter. Otherwise, it’s not the same as the OnCollisionEnter that gets called automatically, i.e. OnCollisionEnter(Collision collision){}

Accordingly, this is how your code should look like:

using UnityEngine;
using System.Collections;

public class Springy : MonoBehaviour {

public int springForce;
public Collision collision;
public GameObject subject;

// Use this for initialization
void Start () {
}

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

}

void OnCollisionEnter(Collision collision)
{
   collision.gameObject.rigidbody2D.AddForce(new Vector2(0f, springForce));
   Debug.Log("We bounced...or did we?");
}

}

Like this, collision.* (just above the Debug.Log statement) refers to the parameter “collision”, of type “Collision” that is passed by the system, when it automatically calls OnCollisionEnter.

I hope this makes it clear now.

Note: collision events are only sent if one of the colliders also has a non-kinematic rigidbody attached: