OnCollisionEnter2D and OnTriggerEnter2D in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output when a collision occurs?

Consider this Unity C# script attached to a GameObject with a Rigidbody2D and Collider2D (not set as trigger). What will be printed when this object collides with another object?

Unity

using UnityEngine;

public class CollisionTest : MonoBehaviour
{
    void OnCollisionEnter2D(Collision2D collision)
    {
        Debug.Log($"Collided with {collision.gameObject.name}");
    }

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

A Collided with Enemy

B Triggered by Enemy

C No output

D Both 'Collided with Enemy' and 'Triggered by Enemy'

💡 Hint

Think about the difference between a collider set as trigger and a normal collider.

OnCollisionEnter2D is called when two colliders collide and at least one has a Rigidbody2D and neither collider is set as trigger. OnTriggerEnter2D is called only if one collider is set as trigger. Since the collider is not a trigger, only OnCollisionEnter2D runs.

❓ Predict Output

intermediate

What happens when a trigger collider is entered?

Given this script on a GameObject with a Collider2D set as trigger and a Rigidbody2D, what will be printed when another GameObject enters its trigger area?

Unity

using UnityEngine;

public class TriggerTest : MonoBehaviour
{
    void OnCollisionEnter2D(Collision2D collision)
    {
        Debug.Log("Collision detected");
    }

    void OnTriggerEnter2D(Collider2D other)
    {
        Debug.Log($"Entered trigger by {other.gameObject.name}");
    }
}

A Entered trigger by Player

B No output

C Collision detected

D Both 'Collision detected' and 'Entered trigger by Player'

💡 Hint

Remember which method is called for trigger colliders.

OnTriggerEnter2D is called when a collider marked as trigger is entered by another collider. OnCollisionEnter2D is not called for triggers.

🔧 Debug

advanced

Why does OnTriggerEnter2D not get called?

A developer wrote this script but OnTriggerEnter2D never runs when expected. What is the most likely reason?

Unity

using UnityEngine;

public class TriggerDebug : MonoBehaviour
{
    void OnTriggerEnter2D(Collider2D other)
    {
        Debug.Log("Trigger entered");
    }
}

A The script is missing Start() method

B The collider on this GameObject is not set as trigger

C The Rigidbody2D is missing on this GameObject

D The method name is incorrect, it should be OnTriggerEnter

💡 Hint

Check the collider settings on the GameObject.

OnTriggerEnter2D only runs if the collider is set as trigger. If the collider is not a trigger, this method won't be called.

🧠 Conceptual

advanced

Which statement about OnCollisionEnter2D and OnTriggerEnter2D is true?

Choose the correct statement about how Unity calls OnCollisionEnter2D and OnTriggerEnter2D methods.

A OnTriggerEnter2D requires both colliders to have Rigidbody2D components

B OnCollisionEnter2D is called only if both colliders are triggers

C OnCollisionEnter2D is called when a collider marked as trigger is entered

D OnTriggerEnter2D is called when a collider marked as trigger is entered by another collider with Rigidbody2D

💡 Hint

Think about the role of triggers and Rigidbody2D in collision and trigger events.

OnTriggerEnter2D is called when a collider marked as trigger is entered by another collider that has a Rigidbody2D. OnCollisionEnter2D is called when two colliders collide and neither is a trigger.

❓ Predict Output

expert

What is the output of this script when two objects collide and one has a trigger collider?

Analyze the following script attached to a GameObject with a Rigidbody2D and a Collider2D set as trigger. Another GameObject with Rigidbody2D and a normal collider collides with it. What will be printed?

Unity

using UnityEngine;

public class MixedCollision : MonoBehaviour
{
    void OnCollisionEnter2D(Collision2D collision)
    {
        Debug.Log("Collision detected");
    }

    void OnTriggerEnter2D(Collider2D other)
    {
        Debug.Log("Trigger detected");
    }
}

A Both 'Collision detected' and 'Trigger detected'

B Collision detected

C Trigger detected

D No output

💡 Hint

Remember how Unity treats collisions between trigger and non-trigger colliders.

When one collider is a trigger and the other is not, OnTriggerEnter2D is called on the trigger collider's script. OnCollisionEnter2D is not called.

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