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.