Design patterns Observer using IObservable and IObserver (C#)


Example

IObserver<T> and IObservable<T> interfaces can be used to implement observer pattern in .NET

  • IObservable<T> interface represents the class that sends notifications
  • IObserver<T> interface represents the class that receives them
public class Stock {
  private string Symbol { get; set; }
  private decimal Price { get; set; }
}

public class Investor : IObserver<Stock> {
  public IDisposable unsubscriber;
  public virtual void Subscribe(IObservable<Stock> provider) {
    if(provider != null) {
      unsubscriber = provider.Subscribe(this);
    }
  }
  public virtual void OnCompleted() {
    unsubscriber.Dispose();
  }
  public virtual void OnError(Exception e) {
  }
  public virtual void OnNext(Stock stock) {
  }
}

public class StockTrader : IObservable<Stock> {
  public StockTrader() {
    observers = new List<IObserver<Stock>>();
  }
  private IList<IObserver<Stock>> observers;
  public IDisposable Subscribe(IObserver<Stock> observer) {
    if(!observers.Contains(observer)) {
      observers.Add(observer);
    }
    return new Unsubscriber(observers, observer);
  }
  public class Unsubscriber : IDisposable {
    private IList<IObserver<Stock>> _observers;
    private IObserver<Stock> _observer;

    public Unsubscriber(IList<IObserver<Stock>> observers, IObserver<Stock> observer) {
      _observers = observers;
      _observer = observer;
    }

    public void Dispose() {
      Dispose(true);
    }
    private bool _disposed = false;
    protected virtual void Dispose(bool disposing) {
      if(_disposed) {
        return;
      }
      if(disposing) {
        if(_observer != null && _observers.Contains(_observer)) {
          _observers.Remove(_observer);
        }
      }
      _disposed = true;
    }
  }
  public void Trade(Stock stock) {
    foreach(var observer in observers) {
      if(stock== null) {
        observer.OnError(new ArgumentNullException());
      }
      observer.OnNext(stock);
    }
  }
  public void End() {
    foreach(var observer in observers.ToArray()) {
      observer.OnCompleted();
    }
    observers.Clear();
  }
}

Usage

...
var provider = new StockTrader();
var i1 = new Investor();
i1.Subscribe(provider);
var i2 = new Investor();
i2.Subscribe(provider);

provider.Trade(new Stock());
provider.Trade(new Stock());
provider.Trade(null);
provider.End();
...

REF: Design patterns and practices in .NET: the Observer pattern