using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var product = new Product

        {

            Id = 1453,

            Price = 1000M,

            Stock = 0

        };

​

        product.Attach(new ProductPriceObserver());

        product.Attach(new ProductStockObserver());

​

        product.UpdatePrice(999M);

        product.UpdateStock(10);

    }

}

​

public record ProductObserverArgs

{

    public int Id { get; set; }

    public int? PreviousStock { get; init; }

    public int? NewStock { get; init; }

    public decimal? PreviousPrice { get; init; }

    public decimal? NewPrice { get; init; }

}

​

public interface IProductObserver

{

    void Update(ProductObserverArgs args);

}

​

public class ProductStockObserver : IProductObserver

{

    public void Update(ProductObserverArgs args)

    {

        if (args.PreviousPrice.GetValueOrDefault() > 0 || args.NewStock == 0) return;

​

        //TODO Inject a service and get users who subscribe to the product.

        Console.WriteLine($"The product is in stock. Stock : {args.NewStock}");

    }

}

​

public class ProductPriceObserver : IProductObserver

{

    public void Update(ProductObserverArgs args)

    {

        if (args.NewPrice.GetValueOrDefault() >= args.PreviousPrice.GetValueOrDefault()) return;

​

        //TODO Inject a service and get users who subscribe to the product.

        Console.WriteLine($"The product price dropped. {args.PreviousPrice} -> {args.NewPrice}");

    }

}

​

public class ObservableProduct

{

    private List<IProductObserver> Observers { get; set; } = new();

​

    public void Attach(IProductObserver observer)

    {

        Observers.Add(observer);

    }

​

    public void Detach(IProductObserver observer)

    {

        Observers.Remove(observer);

    }

​

    protected void Notify(ProductObserverArgs args)

    {

        foreach (var observer in Observers)

        {

            observer.Update(args);

        }

    }

}

​

public class Product : ObservableProduct

{

    public int Id { get; set; }

    public decimal Price { get; set; }

    public int Stock { get; set; }

​

    public void UpdateStock(int stock)

    {

        var previousStock = Stock;

        Stock = stock;

​

        Notify(new ProductObserverArgs

        {

            Id = Id,

            PreviousStock = previousStock,

            NewStock = stock

        });

    }

​

    public void UpdatePrice(decimal price)

    {

        var previousPrice = Price;

        Price = price;

​

        Notify(new ProductObserverArgs

        {

            Id = Id,

            PreviousPrice = previousPrice,

            NewPrice = price

        });

    }

}

​

​