OnCollisionEnter2D and OnTriggerEnter2D in Unity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is OnCollisionEnter2D used for in Unity?

OnCollisionEnter2D is a method called when two 2D physics objects with colliders and rigidbodies physically collide. It detects solid collisions.

beginner

How does OnTriggerEnter2D differ from OnCollisionEnter2D?

OnTriggerEnter2D is called when a collider marked as a trigger overlaps another collider. It detects overlaps without physical collision response.

intermediate

What must be true about colliders for OnTriggerEnter2D to be called?

At least one collider must have Is Trigger enabled, and at least one object must have a Rigidbody2D.

intermediate

What parameters does OnCollisionEnter2D receive and what info can you get from it?

It receives a Collision2D object, which contains info like the other collider, contact points, and relative velocity.

beginner

Why would you use OnTriggerEnter2D instead of OnCollisionEnter2D?

Use OnTriggerEnter2D to detect when objects overlap without blocking movement, like picking up items or entering zones.

Which method is called when two solid 2D objects collide physically?

A OnCollisionEnter2D

B OnTriggerEnter2D

C OnOverlapEnter2D

D OnContactEnter2D

OnCollisionEnter2D is triggered by physical collisions between 2D colliders.

What must be enabled on a collider to use OnTriggerEnter2D?

A Is Solid

B Is Trigger

C Is Kinematic

D Is Static

The collider must have Is Trigger enabled to detect overlaps without physical collision.

Which component is required on at least one object for collision or trigger events to work?

A Rigidbody2D

B BoxCollider2D

C SpriteRenderer

D Animator

A Rigidbody2D is needed to detect physics collisions or triggers in Unity 2D.

What kind of information does the Collision2D parameter provide in OnCollisionEnter2D?

A The trigger zone size

B Only the other object's name

C Contact points and relative velocity

D The object's health

Collision2D gives details like contact points and relative velocity during collision.

When would you prefer OnTriggerEnter2D over OnCollisionEnter2D?

A To change physics materials

B To detect physical impacts only

C To animate sprites

D To detect overlaps without blocking movement

OnTriggerEnter2D is used for detecting overlaps like pickups or zones without physical collision response.

Explain the difference between OnCollisionEnter2D and OnTriggerEnter2D in Unity.

Think about solid hits versus passing through areas.

Describe a simple use case for OnTriggerEnter2D in a 2D game.

Imagine a player walking into a special area or collecting coins.

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