OnCollisionEnter Vs. OnTriggerEnter — When to use them?

After a gameObject’s creation and the coding is in place to give them some movement through the space, they will not collide with each other.

They move through other gameObjects without change. They ignore other objects.

A Collider adds a net of coordinates around the gameObject. A cube object has a cube shape net, a capsule object has a surrounding radius and length Collider net.

A ridgidbody is next added to the gameObject.

gameObject Collider with unchecked “Is Trigger”

Shooting a capsule (a Lazer) at a cube (an Enemy) should create the visual of the capsule entering the cube and disappearing. Or if you prefer, the cube disappears when the capsule is touched, or both disappear after touching each other.

Because there is no bounce off of each other, this is an OnTriggerEnter instead of an OnCollisionEnter.

A trigger collision might also be considered a pass-through collision without the followup bounce.

“is Trigger” option now checked

Next determine which objects receive RidgidBody status. Extra unneeded RidgidBody choices make unneeded drag on performance.

“Either of the two objects must have a Ridgid component” — onTriggerEnter.

Help dropdown to search Unity Scripting

Scripting API -> Unity Engine -> Classes -> MonoBehavior -> Messages

Here are some trigger messages:

Example search for OnTrigger messages

Here are some hard collision messages:

Example search for OnCollision messages

Enter: when collision occurs

Stay: during collision

Exit: end of collision

In this example, a Player object shoots (creates) a moving Lazer object that travels to collide with an Enemy object.

Adding a RigidBody component to the Laser object

<Click> on Add Component and search for Rigid

Component search on “Rigid”

When the Rigidbody component is added, “Use Gravity” is checked.

Rigidbody Component in Inspector view

<uncheck> “Use Gravity” to prevent the Laser movement with physics on a collision.

Use Gravity box is unchecked

In the example, Enemy object drop and move downward in the scene, so collisions are possible with the Player object, so logic will need to be added, requiring another Rigidbody — the Enemy object. So add the Rigidbody component to the Enemy object as was done with the Lazer Object

Adding a Rigidbody component to the Enemy object

Now two of the three objects have Rigidbody components without “Use Gravity”. The game logic can be coded.

    private void OnTriggerEnter(Collider other)
    {
        Debug.Log("Hit: " + other.transform.name);
    }

Using OnTriggerEnter (here inside the Enemy class) to first note what the colliding object is. “other” gives us the object. “transform” gives us the root instance of the object. “name” gives us the name component of the object as a string, useful in a Log message.

log messages after a game run with collisions

    private void OnTriggerEnter(Collider other)
    {
        Debug.Log("Hit: " + other.transform.name);

        // if other.name is Player
        // damage player
        // destroy this Enemy

        // if other.name is Laser
        // destroy Laser
        // destroy this enemy

    }

In the code above, notice that the “destroy this …” follows the “destroy that …”

What is a Tag?

A tag links an other object with an object in the game hierarchy.

Tagging a Player object with a Player tag

Player appears in the drop down menu, okay for a Player tag to a Player object.

Adding a Tag to an Enemy object in the Inspector

A Tag to an Enemy object will need to added to the Tag list using the ‘+’ icon.

A new Tag name can be added to the list of Tags

Entering new Tag name. “Enemy” in this example

And then the new name “Enemy” appears in the list as Player appeared.

Updated list with Enemy option

Player has a Player tag. <click> and Enemy has an Enemy tag. Now apply the Overrides to Enemy

Apply All for prefab Enemy Overrides

Lazer is not in the Hierarchy, and the message appears in the Inspector where the Overrides was located for Enemy.

No Overrides for Lazer. Root in Prefab Asset view

Ready to code with a tag? This example is taken from:

Unity — Scripting API: GameObject.tag (unity3d.com)

using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        //Set the tag of this GameObject to Player
        gameObject.tag = "Player";
    }

    private void OnTriggerEnter(Collider other)
    {
        //Check to see if the tag on the collider is equal to Enemy
        if (other.tag == "Enemy")
        {
            Debug.Log("Triggered by Enemy");
        }
    }
}

In our example, keep in mind that the Collision code is inside the Enemy class and inside the Enemy.cs script file. So “this.gameObject” is the Enemy. “other.gameObject” is either the Laser or the Player.

    private void OnTriggerEnter(Collider other)
    {
        Debug.Log("Hit: " + other.transform.name);

        // if other.name is Player
        // damage player
        // destroy this Enemy

        if (other.tag == "Player")
        {
            Debug.Log("Hit Player! Hit Player!");
            Destroy(this.gameObject);

        } else if (other.tag == "Laser")
        // if other.name is Laser
        // destroy Laser
        // destroy this enemy
        {
            Debug.Log("Lazer Hit! Lazer Hit");
            Destroy(other.gameObject);
            Destroy(this.gameObject);
        }
    }