A lightweight Scriptable Object event system for Unity.

BaseGameEvent.cs

using System;
using System.Collections.Generic;
using UnityEngine;
 
public abstract class BaseGameEvent : ScriptableObject
{
    #if UNITY_EDITOR
        [TextArea] public string developerDescription = "";
    #endif

    private readonly List<Action> listeners = new ();
 
    public void Raise()
    {
        for (var i = listeners.Count - 1; i >= 0; i--)
        {
            listeners[i].Invoke();
        }
    }
    public void RegisterListener(Action listener)
    {
        if (!listeners.Contains(listener)) { listeners.Add(listener); }
    }
    public void UnregisterListener(Action listener)
    {
        if (listeners.Contains(listener)) { listeners.Remove(listener); }
    }
}

public abstract class BaseGameEvent<T1> : BaseGameEvent
{
    private readonly List<Action<T1>> listeners = new ();
 
    public void Raise(T1 t1)
    {
        for (var i = listeners.Count - 1; i >= 0; i--)
        {
            listeners[i].Invoke(t1);
        }
    }
    public void RegisterListener(Action<T1> listener)
    {
        if (!listeners.Contains(listener)) { listeners.Add(listener); }
    }
    public void UnregisterListener(Action<T1> listener)
    {
        if (listeners.Contains(listener)) { listeners.Remove(listener); }
    }
}

public abstract class BaseGameEvent<T1,T2> : BaseGameEvent
{
    private readonly List<Action<T1,T2>> listeners = new ();
 
    public void Raise(T1 t1, T2 t2)
    {
        for (var i = listeners.Count - 1; i >= 0; i--)
        {
            listeners[i].Invoke(t1, t2);
        }
    }
    public void RegisterListener(Action<T1,T2> listener)
    {
        if (!listeners.Contains(listener)) { listeners.Add(listener); }
    }
    public void UnregisterListener(Action<T1,T2> listener)
    {
        if (listeners.Contains(listener)) { listeners.Remove(listener); }
    }
}

/* USE:
*************

[CreateAssetMenu()]
public class GameEvent : BaseGameEvent {}
public class IntGameEvent : BaseGameEvent<int> {}
public class IntStringGameEvent : BaseGameEvent<int,string> {}

**************
*/

SampleUsageCoin.cs

/// 
/// examples of the three components required to utilize the system:
///

// the Scriptable Object Event definition
[CreateAssetMenu]
public class CoinGameEvent : BaseGameEvent<int> 
{}

// the MonoBehaviour on each Coin to collect
public class Coin : MonoBehaviour
{
  // the Scriptable Object Event we created via the Create Asset menu
  [SerializeField] private CoinGameEvent coinEvent;
  
  // raise our coin event whenever the user picks up a coin
  void OnTriggerEnter2D(Collider2D other)
  {  
    coinEvent.Raise(1);
    Destroy(gameobject);
  }
}

// this can also be a ScriptableObject since they still have OnEnable() and OnDisable() methods
public class CoinManager : MonoBehaviour
{
  int coins;

  // register to Listen for the Coin Event
  void OnEnable() => coinEvent.RegisterListener(AddCoins);
  void OnDisable() => coinEvent.UnregisterListener(AddCoins);

  // add the number of coins in the Event to our total
  void AddCoins(int value) => coins += value;

}