BaseBehaviour.cs

using UnityEngine;

namespace LD32
{
    public class BaseBehaviour : MonoBehaviour
    {
        BehaviourMessageBus _messageBus;
        public BehaviourMessageBus MessageBus
        {
            get
            {
                return _messageBus;
            }
        }

        public virtual void Awake()
        {
            //resolve MessageBus before we do anything else:
            _messageBus = GetComponent<BehaviourMessageBus>();

            if (_messageBus == null)
            {
                //we couldn't find a message bus -- we'll have to
                //create one
                _messageBus = gameObject.AddComponent<BehaviourMessageBus>();
            }
        }

        void OnDestroy()
        {
            MessageBus.OnDestroy.Invoke(this);
        }

    }

}

BehaviourMessageBus.cs

using UnityEngine;
using System.Collections;

namespace LD32
{

    public class BehaviourMessageBus : MonoBehaviour
    {
        IntEvent _damage = new IntEvent();
        public IntEvent Damage { get { return _damage; } }
        
        BehaviourEvent _onDestroy = new BehaviourEvent();
        public BehaviourEvent OnDestroy { get { return _onDestroy; } }


        //other events can go here, using the above as a template
        //their arguments are simply determined by their type
    }

}

BulletController.cs

using UnityEngine;
using System.Collections;

namespace LD32
{
    public class BulletController : BaseBehaviour
    {
        int damageAmount;
    
        void OnCollisionEnter2D(Collision2D collision)
        {
            var targetBus = collision.gameObject.GetComponent<BehaviourMessageBus>();
            
            if (targetBus != null)
            {
                targetBus.Damage.Invoke(this.damageAmount);
            }
        }
    }
}

BulletControllerSimplified.cs

using UnityEngine;
using System.Collections;

namespace LD32
{
    public class BulletController : BaseBehaviour
    {
        int damageAmount;
    
        void OnCollisionEnter2D(Collision2D collision)
        {
            collision.gameObject.GetMessageBus().Damage.Invoke(this.damageAmount);
        }
    }
}

Events.cs

using UnityEngine;
using UnityEngine.Events;

namespace LD32
{
    public class FloatEvent : UnityEvent<float> { }
    public class IntEvent : UnityEvent<int> { }
    public class GameObjectEvent : UnityEvent<GameObject> { }
    public class NoArgEvent : UnityEvent { }
    public class VectorEvent : UnityEvent<Vector2> { }
}

ExtensionMethods.cs

using UnityEngine;

namespace LD32
{

    public static class ExtensionMethods
    {
        public static BehaviourMessageBus GetMessageBus(this GameObject go)
        {
            var messageBus = go.GetComponent<BehaviourMessageBus>();

            if (messageBus == null)
            {
                //no message bus yet -- make one!
                messageBus = go.AddComponent<BehaviourMessageBus>();
            }

            return messageBus;
        }

        public static BehaviourMessageBus GetMessageBus(this Component comp)
        {
            return comp.gameObject.GetMessageBus();
        }
    }

}

Health.cs

using UnityEngine;
using System.Collections;

namespace LD32
{
    public class Health : BaseBehaviour
    {
        int _health;

        public int maxHealth;
        public int health
        {
            get
            {
                return _health;
            }
        }

        public override void Awake()
        {
            base.Awake();
            MessageBus.Damage.AddListener(Damage);
        }

        public override void Start()
        {
            _health = maxHealth;
        }

        public void Damage(int damageAmount)
        {
            _health -= damageAmount;

            if (_health <= 0)
            {
                Destroy(this.gameObject);
            }
        }
    }

}