Simple event base class for Unity.

BaseGameEvent.cs

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

namespace WizBangify
{

  public delegate void Subscription(bool add);

  public abstract class BaseGameEvent<T> where T : BaseGameEvent<T>
  {

    private bool hasFired;
    public delegate void EventListener(T info);
    public static event EventListener listeners;


    public static void AddListener(EventListener listener)
    {
      listeners -= listener; // Prevents duplicates...
      listeners += listener;
    }

    public static void RemoveListener(EventListener listener)
    {
      listeners -= listener;
    }

    public static Subscription Subscribe(EventListener listener, bool addByDefault = true)
    {
      if (addByDefault)
      {
        AddListener(listener);
      }

      return (bool addListener) =>
      {
        if (addListener)
        {
          AddListener(listener);
        }
        else
        {
          RemoveListener(listener);
        }
      };
    }
    public static Subscription SubscriptionFor(EventListener listener)
    {
      return Subscribe(listener, false);
    }

    public string Name
    {
      get
      {
        return this.GetType().Name;
      }
    }

    protected T FireEvent()
    {
      if (hasFired)
      {
        throw new Exception("This event has already fired, to prevent infinite loops you can't refire an event");
      }

      Debug.Log("[Firing event: " + this.Name + "]");

      hasFired = true;

      if (listeners != null)
      {
        listeners(this as T);
      }

      return this as T;
    }
  }

  public class SubscriptionManager
  {
    private List<Subscription> subscriptions = new List<Subscription>();

    public int Count
    {
      get
      {
        return this.subscriptions.Count;
      }
    }

    public void Add(Subscription subscription)
    {
      subscriptions.Add(subscription);
    }

    public void AddAll(params Subscription[] bulkSubscriptions)
    {
      foreach (Subscription subscription in bulkSubscriptions)
      {
        subscriptions.Add(subscription);
      }
    }

    public bool Remove(Subscription subscription)
    {
      return subscriptions.Remove(subscription);
    }

    public void UnsubscribeAll()
    {
      foreach (Subscription subscription in subscriptions)
      {
        subscription(false);
      }
    }

    public void SubscribeAll()
    {
      foreach (Subscription subscription in subscriptions)
      {
        subscription(true);
      }
    }

    public void Clear()
    {
      subscriptions.Clear();
    }
  }
}

ExampleEvent.cs

public class GameStartEvent : WizBangify.BaseGameEvent<GameStartEvent>
{

  // You can add parameters too, if you like...
  static public GameStartEvent Fire() { 
    return new GameStartEvent().FireEvent(); 
  }
}

ExampleUsage.cs

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

namespace WizBangify
{

  public class ExampleUsage : MonoBehaviour
  {
    private SubscriptionManager subscriptions = new SubscriptionManager();

    void Awake()
    {
      // Subscribe to event
      subscriptions.Add(GameStartEvent.SubScribe(OnGameStart, false))
    }
    
    void Start()
    {
      // At some point, someone Fires the event (like this)...
      GameStartEvent.Fire();
    }
    
    void OnGameStart() 
    {
      // Handle event...
    }

    void OnEnable()
    {
      subscriptions.SubscribeAll();
    }
    void OnDisable()
    {
      subscriptions.UnsubscribeAll();
    }

  }

}