inventory class example

Inventory.cs

using System.Collections.Generic;
using UnityEngine;

namespace Shaninja.Scripts {
    public interface IInventory
    {
        void AddCoin();
        void SetTotalCoins(int total);
        void Reset();

        void RegisterCoinListener(CoinListener coinListener);

        interface CoinListener
        {
            void OnCoinsUpdated(int possessed, int total);
        }
    }

    public class Inventory: IInventory
    {
        int _coins;
        int _totalCoins;

        static Inventory _instance;
        readonly List<IInventory.CoinListener> _coinListeners = new();

        public static Inventory Instance
        {
            get
            {
                return _instance ??= new Inventory();
            }
        }

        public void AddCoin()
        {
            SetCoins(++_coins);
        }

        public void SetCoins(int coins)
        {
            if (coins < 0) return;
            _coins = coins;
            EmitCoinUpdate();
        }

        public void SetTotalCoins(int total)
        {
            _totalCoins = total;
            EmitCoinUpdate();
        }

        public void Reset()
        {
            _coins = 0;
            _totalCoins = 0;
        }

        public void RegisterCoinListener(IInventory.CoinListener coinListener)
        {
            _coinListeners.Add(coinListener);
            EmitCoinUpdate();
        }

        void EmitCoinUpdate()
        {
            Debug.Log($"Listeners: {_coinListeners.Count}; {_coins}/{_totalCoins}");
            _coinListeners.ForEach(listener => listener.OnCoinsUpdated(_coins, _totalCoins));
        }
    }
}