Pseudo code for Unity3d EventManager. Basically this is implementation of Observer pattern but with some twists for Unity3d

Unity3DEventManager.cs

using System;
using System.Collections.Generic;

namespace EventManager
{
	public class EventManager
	{
		private static EventManager instance = null;
		public static EventManager Instance
		{
			get { return instance; }
			set { }
		}

		private Dictionary<EVENT_TYPE, List<IListener>> Listeners = new Dictionary<EVENT_TYPE, List<IListener>>();

		void Awake()
		{
			if (instance == null)
			{
				instance = this;
				// DontDestroyOnLoad(gameObject);
			}
			else
			{
				//DestroyImmediate(this);
			}
		}

		public void AddListener(EVENT_TYPE eventType, IListener listener)
		{
			// List of listeners for this event
			List<IListener> ListenList = null;

			// Check existing event type key. If exists, add to
			if (Listeners.TryGetValue(eventType, out ListenList))
			{
				// List exists, so add new item
				ListenList.Add(listener);
				return;
			}

			//Otherwise create new list as dictionary key
			ListenList = new List<IListener>();
			ListenList.Add(listener);
			Listeners.Add(eventType, ListenList);
		}

		public void PostNotification(EVENT_TYPE eventType, object sender, object param = null)
		{
			// Notify all listeners of an event

			// List of listeners for this event only
			List<IListener> ListenList = null;

			// If no event exists, then exit
			if (!Listeners.TryGetValue(eventType, out ListenList))
			{
				return;
			}

			// Entry exists. Now notify appropriate listeners
			for (int i = 0; i < ListenList.Count; i++)
			{
				if (!ListenList[i].Equals(null))
				{
					ListenList[i].OnEvent(eventType, sender, param);
				}
			}
		}

		public void RemoveEvent(EVENT_TYPE eventType)
		{
			Listeners.Remove(eventType);
		}

		// Remove all redundant entries from the Dictionary
		public void RemoveRedundancies()
		{
			// Create new dictionary
			Dictionary<EVENT_TYPE, List<IListener>> TempListeners = new Dictionary<EVENT_TYPE, List<IListener>>();

			// Cycle through all dictionary entries
			foreach (KeyValuePair<EVENT_TYPE, List<IListener>> item in Listeners)
			{
				// Cycle all listeners, remove null objects
				for (int i = item.Value.Count - 1; i >= 0; i--)
				{
					// if null, then remove item
					if (item.Value[i].Equals(null))
					{
						item.Value.RemoveAt(i);
					}
				}

				// If items remain in list, then add to tmp dictionary
				if (item.Value.Count > 0)
				{
					TempListeners.Add(item.Key, item.Value);
				}
			}

			// Replace listeners object with new dictionary
			Listeners = TempListeners;
		}

		// Called on scene change. Clean up dictionary
		void OnLevelWasLoaded()
		{
			RemoveRedundancies();
		}
	}

	// EVENT MANAGER END!!!!!!!

	public enum EVENT_TYPE
	{
		INIT,
		END,
		AMMO_EMPTY,
		HEALTH_CHANGE,
		DEAD
	}
	public interface IListener
	{
		void OnEvent(EVENT_TYPE eventType, Object Sender, Object Param = null);
	}

	public class MyCustomListener : IListener
	{
		public void OnEvent(EVENT_TYPE eventType, Object Sender, object Param = null)
		{
			throw new NotImplementedException();
		}
	}
}