OnCollisionEnter2D and OnTriggerEnter2D in Unity

Introduction

These functions help your game know when two objects touch or overlap. This lets you make things happen, like scoring points or playing sounds.

When you want to detect if a player hits a wall or enemy.

When you want to know if a player picks up an item by touching it.

When you want to trigger a door to open when the player enters a zone.

When you want to stop a character from passing through solid objects.

When you want to detect if a projectile hits a target.

Syntax

Unity

using UnityEngine;

public class CollisionExample : MonoBehaviour
{
    // Called when this object collides with another with a Rigidbody2D and Collider2D
    private void OnCollisionEnter2D(Collision2D collision)
    {
        // Your code here
    }

    // Called when this object's trigger collider overlaps another collider
    private void OnTriggerEnter2D(Collider2D other)
    {
        // Your code here
    }
}

OnCollisionEnter2D is called when two solid objects with Rigidbody2D and Collider2D touch and collide.

OnTriggerEnter2D is called when an object with a Collider2D set as a trigger overlaps another collider.

Examples

This runs when your object bumps into another solid object.

Unity

private void OnCollisionEnter2D(Collision2D collision)
{
    Debug.Log("Collided with " + collision.gameObject.name);
}

This runs when your object enters a trigger zone.

Unity

private void OnTriggerEnter2D(Collider2D other)
{
    Debug.Log("Entered trigger of " + other.gameObject.name);
}

Make sure your objects have Rigidbody2D components for these methods to work.

Unity

// Edge case: What if the object has no Rigidbody2D?
// Neither method will be called because physics needs Rigidbody2D to detect collisions or triggers.

Set the collider's 'Is Trigger' checkbox to switch between collision and trigger behavior.

Unity

// Edge case: What if the collider is not set as trigger?
// OnTriggerEnter2D won't run; instead OnCollisionEnter2D will run if colliders are solid.

Sample Program

This script prints messages when the object collides with another solid object or enters a trigger zone. Attach it to a GameObject with Rigidbody2D and Collider2D components. Make sure some colliders are triggers and some are not to see both messages.

Unity

using UnityEngine;

public class CollisionTriggerDemo : MonoBehaviour
{
    private void OnCollisionEnter2D(Collision2D collision)
    {
        Debug.Log($"Collision detected with {collision.gameObject.name}");
    }

    private void OnTriggerEnter2D(Collider2D other)
    {
        Debug.Log($"Trigger entered by {other.gameObject.name}");
    }
}

Important Notes

Time complexity: These methods run only when collisions or triggers happen, so they do not slow down your game continuously.

Space complexity: Minimal, only storing collision info temporarily.

Common mistake: Forgetting to add Rigidbody2D to objects, so these methods never get called.

Use OnCollisionEnter2D for solid object hits and OnTriggerEnter2D for zones or pickups that don't block movement.

Summary

OnCollisionEnter2D detects solid collisions between objects with Rigidbody2D and Collider2D.

OnTriggerEnter2D detects when an object enters a trigger collider (a collider set as a trigger).

Both let you run code when objects touch or overlap, helping your game respond to player actions and events.

Practice

1. Which Unity method is called when two solid objects with Rigidbody2D and Collider2D components collide physically?

easy

A. OnTriggerEnter2D

B. OnCollisionEnter2D

C. OnCollisionExit2D

D. OnTriggerExit2D

Solution

1. Step 1: Understand collision detection methods

   OnCollisionEnter2D is called when two solid objects collide physically, both having Rigidbody2D and Collider2D components without trigger enabled.

2. Step 2: Differentiate triggers from collisions

   OnTriggerEnter2D is called when an object enters a trigger collider, which is a collider set as a trigger, not a solid collision.

3. Final Answer:

   OnCollisionEnter2D -> Option B

4. Quick Check:

   Physical collision = OnCollisionEnter2D [OK]

Hint: Solid collisions use OnCollisionEnter2D, triggers use OnTriggerEnter2D [OK]

Common Mistakes:

• Confusing trigger events with collision events
• Using OnTriggerEnter2D for solid collisions
• Assuming OnCollisionEnter2D works with trigger colliders

2. Which of the following is the correct method signature for detecting trigger entry in a 2D Unity script?

easy

A. void OnTriggerEnter(Collider other)

B. void OnCollisionEnter2D(Collider2D other)

C. void OnTriggerEnter2D(Collider other)

D. void OnTriggerEnter2D(Collider2D other)

Solution

1. Step 1: Identify correct method name and parameter type

   The method for trigger detection in 2D physics is OnTriggerEnter2D and it takes a Collider2D parameter.

2. Step 2: Check parameter types and method names

   void OnTriggerEnter2D(Collider2D other) matches the exact signature: void OnTriggerEnter2D(Collider2D other). Other options have wrong parameter types or method names.

3. Final Answer:

   void OnTriggerEnter2D(Collider2D other) -> Option D

4. Quick Check:

   Trigger 2D method = OnTriggerEnter2D(Collider2D) [OK]

Hint: Use OnTriggerEnter2D with Collider2D parameter for 2D triggers [OK]

Common Mistakes:

• Using 3D Collider instead of Collider2D
• Mixing OnTriggerEnter with OnTriggerEnter2D
• Wrong parameter type in method signature

3. What will happen when the following Unity script is attached to a GameObject with a Collider2D set as a trigger, and another Rigidbody2D object enters it?

void OnTriggerEnter2D(Collider2D other) {
    Debug.Log("Trigger entered by " + other.name);
}

void OnCollisionEnter2D(Collision2D collision) {
    Debug.Log("Collision with " + collision.gameObject.name);
}

medium

A. Logs "Trigger entered by [object name]" when the other object enters the trigger collider.

B. Logs "Collision with [object name]" when the other object enters the trigger collider.

C. Logs both messages when the other object enters the trigger collider.

D. No logs appear because the methods are incorrect.

Solution

1. Step 1: Understand trigger vs collision behavior

   The OnTriggerEnter2D method is called when an object enters a trigger collider. Since the collider is set as a trigger, this method will run.

2. Step 2: Check collision method call conditions

   OnCollisionEnter2D only runs on solid collisions, not triggers. So it will not be called here.

3. Final Answer:

   Logs "Trigger entered by [object name]" when the other object enters the trigger collider. -> Option A

4. Quick Check:

   Trigger collider entered = OnTriggerEnter2D logs [OK]

Hint: Trigger colliders call OnTriggerEnter2D, not OnCollisionEnter2D [OK]

Common Mistakes:

• Expecting OnCollisionEnter2D to run on triggers
• Assuming both methods run simultaneously
• Confusing Collider and Collision parameter types

4. A developer wrote this code but the OnTriggerEnter2D method never runs when expected. What is the likely problem?

void OnTriggerEnter(Collider2D other) {
    Debug.Log("Entered trigger");
}

medium

A. The method must return a boolean value.

B. The parameter type should be Collider, not Collider2D.

C. The method name should be OnTriggerEnter2D, not OnTriggerEnter.

D. The method should be private, not public.

Solution

1. Step 1: Check method name correctness

   The method for 2D trigger detection must be named exactly OnTriggerEnter2D. The code uses OnTriggerEnter, which is for 3D physics.

2. Step 2: Confirm parameter type and method signature

   The parameter Collider2D is correct for 2D triggers, but the method name mismatch prevents Unity from calling it.

3. Final Answer:

   The method name should be OnTriggerEnter2D, not OnTriggerEnter. -> Option C

4. Quick Check:

   Correct method name = OnTriggerEnter2D [OK]

Hint: Use exact method names for 2D events: OnTriggerEnter2D [OK]

Common Mistakes:

• Using 3D method names for 2D physics
• Wrong parameter types
• Expecting return values from event methods

5. You want to detect when a player touches a collectible item in a 2D game without blocking movement. Which setup and method should you use to detect the event correctly?

hard

A. Set the collectible's Collider2D as a trigger and use OnTriggerEnter2D in the player's script.

B. Set the collectible's Collider2D as non-trigger and use OnCollisionEnter2D in the player's script.

C. Set both player and collectible colliders as triggers and use OnCollisionEnter2D.

D. Use OnTriggerEnter with 3D colliders on both objects.

Solution

1. Step 1: Choose collider settings for collectibles

   To allow the player to pass through collectibles without blocking, the collectible's Collider2D must be set as a trigger.

2. Step 2: Select correct detection method

   Use OnTriggerEnter2D in the player's script to detect when the player enters the collectible's trigger collider.

3. Final Answer:

   Set the collectible's Collider2D as a trigger and use OnTriggerEnter2D in the player's script. -> Option A

4. Quick Check:

   Trigger collider + OnTriggerEnter2D = collectible detection [OK]

Hint: Use triggers for collectibles to avoid blocking player movement [OK]

Common Mistakes:

• Using solid colliders causing player to stop
• Using OnCollisionEnter2D with triggers
• Mixing 3D and 2D physics components