OnCollisionEnter2D and OnTriggerEnter2D in Unity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to detect a collision with another object.

Unity

void [1](Collision2D collision) {
    Debug.Log("Collision detected with " + collision.gameObject.name);
}

Drag options to blanks, or click blank then click option'

AUpdate

BOnTriggerEnter2D

CStart

DOnCollisionEnter2D

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnTriggerEnter2D instead of OnCollisionEnter2D for collision detection.

Using Start or Update methods which are unrelated.

The method OnCollisionEnter2D is called when this object collides with another collider and both have Rigidbody2D components.

2fill in blank

medium

Complete the code to detect when another object enters a trigger collider.

Unity

void [1](Collider2D other) {
    Debug.Log("Trigger entered by " + other.gameObject.name);
}

Drag options to blanks, or click blank then click option'

AOnCollisionExit2D

BOnTriggerExit2D

COnTriggerEnter2D

DOnCollisionEnter2D

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnCollisionEnter2D instead of OnTriggerEnter2D for triggers.

Confusing enter and exit methods.

The method OnTriggerEnter2D is called when another collider enters a trigger collider attached to this object.

3fill in blank

hard

Fix the error in the method signature to detect collision correctly.

Unity

private void [1](Collision2D collision) {
    Debug.Log("Hit " + collision.gameObject.name);
}

Drag options to blanks, or click blank then click option'

AOnCollisionEnter2D

BOnTriggerEnter2D

COnTriggerEnter

DOnCollisionEnter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3D collision methods with 2D physics.

Mismatching method name and parameter type.

For 2D physics, the method must be OnCollisionEnter2D and use Collision2D as parameter.

4fill in blank

hard

Fill both blanks to create a method that logs when an object exits a trigger.

Unity

void [1](Collider2D [2]) {
    Debug.Log("Trigger exited by " + [2].gameObject.name);
}

Drag options to blanks, or click blank then click option'

AOnTriggerExit2D

BOnCollisionExit2D

Cother

Dcollision

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnCollisionExit2D instead of OnTriggerExit2D.

Using 'collision' as parameter name for trigger methods.

The method OnTriggerExit2D detects when a collider leaves a trigger. The parameter is usually named other to represent the other collider.

5fill in blank

hard

Fill all three blanks to create a method that detects collision and logs the tag of the other object if it is 'Enemy'.

Unity

void [1](Collision2D [2]) {
    if ([3].gameObject.tag == "Enemy") {
        Debug.Log("Collided with Enemy");
    }
}

Drag options to blanks, or click blank then click option'

AOnCollisionEnter2D

Bcollision

Cother

DOnTriggerEnter2D

Attempts:

3 left

💡 Hint

Common Mistakes

Using OnTriggerEnter2D instead of OnCollisionEnter2D for collisions.

Using wrong parameter names or inconsistent names.

The method OnCollisionEnter2D detects collisions. The parameter is often named collision. We check the tag of the colliding object using this parameter.

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