For Unity3D Triggers, lets you attach UnityEvents to Trigger events

TriggerEvent.cs

using UnityEngine;
using UnityEngine.Events;
using System.Collections;

public class TriggerEvent : MonoBehaviour {
	public bool EnterOnce;
	bool enterFire;
	public UnityEvent EnterEvent;
	public bool StayOnce;
	bool stayFire;
	public UnityEvent StayEvent;
	public bool ExitOnce;
	bool exitFire;
	public UnityEvent ExitEvent;
	void OnTriggerEnter(Collider other){
		if(!EnterOnce||(EnterOnce&&!enterFire)){
			EnterEvent.Invoke();
		}
	}
	void OnTriggerStay(Collider other){
		if(!StayOnce||(StayOnce&&!stayFire)){
			StayEvent.Invoke();
		}
	}
	void OnTriggerExit(Collider other){
		if(!ExitOnce||(ExitOnce&&!exitFire)){
			ExitEvent.Invoke();
		}
	}
}