Trigger and Collision Events in Unity Explained

Introduction

Collisions and triggers are core components of most Unity games. Whether it’s detecting when a player hits an enemy, opens a door, or collects a coin — trigger and collision events let you handle real-time interactions between GameObjects.

In this guide, you’ll learn:

• The difference between collisions and triggers
• How to use OnCollisionEnter, OnTriggerEnter, and related methods
• What components (like Collider and Rigidbody) are required
• Real-world use cases and best practices

Let’s break it down step-by-step

Colliders vs Triggers — What’s the Difference?

✅ Colliders

• Used for physical collisions
• GameObjects bounce or stop when they collide
• Must include a Rigidbody for dynamic physics
• Example: A car crashing into a wall

✅ Triggers

• Used for detection only, without physics reaction
• Pass through each other like “ghosts”
• Useful for pickups, zones, UI triggers, etc.
• Example: Player entering a healing zone

Required Components

Component	Needed for Collision?	Needed for Trigger?
Collider	✅	✅
Rigidbody	✅ (one object must have)	✅ (for 3D trigger detection)
isTrigger = true	❌	✅

⚙️ Collision Detection Methods

These C# methods are built into Unity’s MonoBehaviour class:

OnCollisionEnter

Called when two colliders physically collide.

void OnCollisionEnter(Collision collision)
{
    Debug.Log("Collided with: " + collision.gameObject.name);
}

OnTriggerEnter

Called when a GameObject enters a trigger collider.

void OnTriggerEnter(Collider other)
{
    if (other.CompareTag("Player"))
    {
        Debug.Log("Player entered the zone!");
    }
}

✅ You can also use OnCollisionExit, OnTriggerExit, and their Stay variants.

Real-World Example: Coin Collection

Let’s create a simple trigger zone that destroys a coin when touched by the player:

public class Coin : MonoBehaviour
{
    void OnTriggerEnter(Collider other)
    {
        if (other.CompareTag("Player"))
        {
            Debug.Log("Coin collected!");
            Destroy(gameObject);
        }
    }
}

Make sure the coin has:

• A Collider with “Is Trigger” checked
• No Rigidbody (or use isKinematic = true if it has one)

And the player has:

• A Collider
• A Rigidbody

Unity Physics Flow Tips

• Triggers do not apply force — they’re for detection only.
• For collision physics like bouncing, use non-trigger colliders.
• Only one object in a collision must have a Rigidbody.
• Rigidbody on both objects? You’ll get more realistic physics behavior.

Pro Tips for Clean Event Handling

✅ 1. Use Tags

Assign tags like "Player", "Enemy", or "Pickup" for detection.

if (other.CompareTag("Pickup"))

✅ 2. Don’t Rely on Name Checks

Avoid this (not scalable):

if (other.name == "Player")

Use tags or component checking instead.

✅ 3. Separate Collision Logic

Avoid putting all logic in one script. Keep trigger behavior separate for each object when possible.

✅ 4. Use LayerMask for Filtering

if (((1 << other.gameObject.layer) & layerMask) != 0)

Let’s you restrict trigger events to specific layers.

❌ Common Mistakes to Avoid

• Forgetting to check “Is Trigger” for trigger zones
• Expecting trigger events without Rigidbody present
• Using OnTriggerEnter2D with 3D colliders (or vice versa)
• Handling all collisions in one script without delegation

Suggested Use Cases

• Enemy patrol detection zone → Trigger alert
• Damage player on spike trap → OnCollisionEnter
• Collect coins → OnTriggerEnter
• Portal to new scene → OnTriggerEnter + SceneManager.LoadScene()

Final Thoughts

Mastering Unity’s trigger and collision events unlocks powerful interactions for your game. From pickups and power-ups to damage systems and scene changes — understanding how physics and triggers work is essential.

Always remember:

• Use collisions for physical responses
• Use triggers for detection
• Set up your colliders and Rigidbody components correctly

Are you handling collisions like a pro — or are your objects passing through each other silently?