using System;

using System.Collections.Generic;

​

PrinterService ps = new();

​

var pss = new PaperSupplierService(ps);

var tss = new TonerSupplierService(ps);

var cs = new CleanerService(ps);

​

ps.Print("abc");

ps.Print("def");

ps.Print("ghi");

​

interface IObserver<TNotification>

{

  void Notify(TNotification notification);

}

​

class Observable<T, TNotification>

  where T : IObserver<TNotification> 

{

  List<T> _observers = new ();

​

  public void Add(T observer)

    => _observers.Add(observer);

  public void Remove(T observer)

    => _observers.Remove(observer);

  public void NotifyAll(TNotification notification)

  {

    foreach (var observer in _observers)

    {

      observer.Notify(notification);

    } 

  }

}

​

class PrinterServiceNotification

{

  public string DocumentId {get; private set;}

  public PrinterServiceNotification(string documentId)

     => DocumentId = documentId;

} 

​

class PrinterService

{

  public readonly Observable<IPrinterServiceObserver, PrinterServiceNotification> 

    BeforePrinting = new();

  public readonly Observable<IPrinterServiceObserver, PrinterServiceNotification> 

    AfterPrinting = new();

  public void Print(string documentId)

  {

    BeforePrinting.NotifyAll(new PrinterServiceNotification(documentId));

    Console.WriteLine($"Printing {documentId}...");   

    AfterPrinting.NotifyAll(new PrinterServiceNotification(documentId));

  }

}

​

interface IPrinterServiceObserver

  : IObserver<PrinterServiceNotification>

{}

​

class PaperSupplierService

  : IPrinterServiceObserver

{

  public PaperSupplierService(PrinterService ps)

    => ps.BeforePrinting.Add(this);

  public void Notify(PrinterServiceNotification notification)

  {

    Console.WriteLine($"PSS: {notification.DocumentId}");

  }

}

​

class TonerSupplierService

  : IPrinterServiceObserver

{

  public TonerSupplierService(PrinterService ps)

    => ps.BeforePrinting.Add(this);

  public void Notify(PrinterServiceNotification notification)

    => Console.WriteLine($"TSS: {notification.DocumentId}");

}

​

class CleanerService

  : IPrinterServiceObserver

{

  public CleanerService(PrinterService ps)

    => ps.AfterPrinting.Add(this);

  public void Notify(PrinterServiceNotification notification)

    =>  Console.WriteLine($"CS: {notification.DocumentId}");

}