A trigger is a collider that detects when other physics objects enter or exit it, without blocking their movement. They're useful for things like pickup zones, death planes, checkpoints, and area detection.

Setting Up a Trigger

Any collider can become a trigger. In the inspector, tick the Is Trigger checkbox on a BoxCollider, SphereCollider, CapsuleCollider, or any other collider component.

To do it in code:

var box = AddComponent<BoxCollider>();
box.Scale = new Vector3( 200, 200, 100 );
box.IsTrigger = true;

Reacting to Trigger Events

Add Component.ITriggerListener to any component on the same GameObject as the trigger collider (or a child). Implement OnTriggerEnter and OnTriggerExit:

public sealed class PickupZone : Component, Component.ITriggerListener
{
	public void OnTriggerEnter( Collider other )
	{
		Log.Info( $"{other.GameObject.Name} entered the pickup zone" );
	}

	public void OnTriggerExit( Collider other )
	{
		Log.Info( $"{other.GameObject.Name} left the pickup zone" );
	}
}

You can also use the overloads that give you both colliders if you need to know which trigger was hit (useful when one GameObject has multiple triggers):

public void OnTriggerEnter( Collider self, Collider other )
{
	Log.Info( $"{other.GameObject.Name} entered {self.GameObject.Name}" );
}

And there are GameObject variants if you only care about the entering object, not its specific collider:

public void OnTriggerEnter( GameObject other ) { }
public void OnTriggerExit( GameObject other ) { }

The default player controller setup uses two child colliders for the player: a capsule for the body and a box for the feet. When entering a trigger, you might get two events (one for each collider), and neither of them will be the actual player GameObject. Use FindMode.InAncestors to check for the player component in the parent GameObject.

Checking What's Inside (Polling)

If you'd rather poll than use events, Collider.Touching gives you all colliders currently overlapping this trigger:

[Property] public BoxCollider Box { get; set; }

protected override void OnUpdate()
{
    foreach ( var collider in Box.Touching )
    {
        Log.Info( $"{collider.GameObject.Name} is inside the box" );
    }
}

The trigger fires OnTriggerExit automatically when a collider inside the trigger is disabled or destroyed, so you don't need to manually clean up the list for disabled objects.