Collision events

Learn OnCollisionEnter, OnCollisionExit and OnCollisionStay.

Collision Events

When two solid Colliders strike one another, Unity automatically fires collision events. For these events to trigger, at least one of the participating objects must carry an active Rigidbody.

// Method called at initial moment of contact
void OnCollisionEnter(Collision collision) {
    Debug.Log("Collision hit: " + collision.gameObject.name);
}

The Collision parameter provides rich data regarding contact points, impact velocity, and the colliding entity.

OnCollisionStay and OnCollisionExit

A physical collision follows a three-phase lifecycle: OnCollisionEnter at initial contact, OnCollisionStay while resting together, and OnCollisionExit upon separation.

void OnCollisionExit(Collision collision) {
    Debug.Log("Objects separated from each other.");
}

For instance, to verify if a character is grounded, grounded state is turned on with Enter and cleared with Exit.

Exercise Print collision event

Assign "OnCollisionEnter" to collisionState and "Wall" to target. Print Event: OnCollisionEnter | Target: Wall to the console.

Show example solution

using System;

class Program
{
    static void Main()
    {
        string collisionState = "OnCollisionEnter";
        string target = "Wall";
        Console.WriteLine("Event: " + collisionState + " | Target: " + target);
    }
}

Test yourself

Which function is called every physics frame while two solid objects remain in contact?
- OnCollisionStart
- OnTriggerOnce
- OnCollisionStay
- OnCollisionStop
Show answer

OnCollisionStay — OnCollisionStay runs regularly on each FixedUpdate step for as long as contact persists.

Define collision start method

void ____(Collision collision)

Show answer

void OnCollisionEnter(Collision collision)

Collision event lifecycle

OnCollisionEnter

Runs once on first contact
OnCollisionStay

Runs every frame while in contact
OnCollisionExit

Triggered when bodies separate
Collision

Holds collision & object data

Next lesson: Trigger events

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