OnCollisionEnter2D and OnTriggerEnter2D in Unity - Step-by-Step Execution

Concept Flow - OnCollisionEnter2D and OnTriggerEnter2D

Object A moves

↓

Collision or Trigger detected?

Yes No↓

Is Collider Trigger?

↓

Call OnTriggerEnter2D

↓

Execute collision/trigger response

When two objects touch, Unity checks if the collider is a trigger or not, then calls the matching method to handle the event.

Execution Sample

Unity

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

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

This code logs a message when the object collides or triggers with another object.

Execution Table

Step	Event Type	Collider Trigger?	Method Called	Logged Output
1	Object A touches Object B	No	OnCollisionEnter2D	Collision with ObjectB
2	Object A enters Object C's trigger area	Yes	OnTriggerEnter2D	Trigger with ObjectC
3	Object A moves without touching	N/A	None	No output

💡 Execution stops when no collision or trigger event occurs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
collision.gameObject.name	null	ObjectB	ObjectB	ObjectB
other.gameObject.name	null	null	ObjectC	ObjectC
Logged Output	empty	Collision with ObjectB	Trigger with ObjectC	No output

Key Moments - 3 Insights

Why does OnTriggerEnter2D get called instead of OnCollisionEnter2D?

Because the collider involved has its 'Is Trigger' property checked, Unity calls OnTriggerEnter2D (see execution_table row 2).

What happens if two colliders touch but neither is a trigger?

OnCollisionEnter2D is called to handle the physical collision (see execution_table row 1).

Why is there no output when objects don't touch or enter triggers?

Because neither collision nor trigger events occur, so no methods are called (see execution_table row 3).

Visual Quiz - 3 Questions

Look at the execution_table, what method is called when Object A touches Object B and the collider is not a trigger?

A Update

B OnCollisionEnter2D

C OnTriggerEnter2D

D Start

💡 Hint

Check execution_table row 1 under 'Method Called'

At which step does OnTriggerEnter2D get called?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

Look at execution_table row 2 for 'Method Called'

If Object C's collider was not set as a trigger, what method would be called when Object A touches it?

A OnTriggerEnter2D

B No method called

C OnCollisionEnter2D

D OnCollisionExit2D

💡 Hint

Recall that OnCollisionEnter2D is called when colliders are not triggers (see key_moments answer 2)

Concept Snapshot

OnCollisionEnter2D and OnTriggerEnter2D are Unity event methods.
OnCollisionEnter2D runs when two colliders physically collide (Is Trigger unchecked).
OnTriggerEnter2D runs when an object enters a trigger collider (Is Trigger checked).
Use these to detect and respond to collisions or trigger events in 2D games.

Full Transcript

In Unity 2D, when two objects touch, the engine checks if the collider is a trigger or not. If it is a trigger, OnTriggerEnter2D is called. If not, OnCollisionEnter2D is called. For example, if Object A touches Object B and neither collider is a trigger, OnCollisionEnter2D runs and logs a collision message. If Object A enters Object C's trigger area, OnTriggerEnter2D runs and logs a trigger message. If no collision or trigger happens, no method runs and no output is logged. This helps you respond differently to physical collisions and trigger zones in your game.

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