How to implement an observer

The observer design pattern requires a division between an observer, which registers for notifications, and a provider, which monitors data and sends notifications to one or more observers. This article discusses how to create an observer. A related article, How to implement a provider, discusses how to create a provider.

Create an observer

To create an observer, implement the System.IObserver<T> interface. The following steps describe each member you need to define.

1. Define the observer type that implements the System.IObserver<T> interface.

   The following code defines a type named TemperatureReporter that is a constructed System.IObserver<T> implementation with a generic type argument of Temperature.

   namespace TemperatureSample;
   
   public sealed class TemperatureReporter : IObserver<Temperature>
   

   Namespace Global.TemperatureSample
   
       Public NotInheritable Class TemperatureReporter
           Implements IObserver(Of Temperature)
   

2. If the observer needs to unsubscribe before the provider calls IObserver<T>.OnCompleted, define a private variable to hold the IDisposable returned by IObservable<T>.Subscribe, and define a subscription method.

   The private variable unsubscriber stores the IDisposable object. The Subscribe method calls the provider's Subscribe method and assigns the returned object to unsubscriber.

   namespace TemperatureSample;
   
   public sealed class TemperatureReporter : IObserver<Temperature>
   {
       private IDisposable? _unsubscriber;
       private Temperature? _last;
   
       public void Subscribe(IObservable<Temperature> provider)
       {
           ArgumentNullException.ThrowIfNull(provider);
           _unsubscriber = provider.Subscribe(this);
       }
   

   Namespace Global.TemperatureSample
   
       Public NotInheritable Class TemperatureReporter
           Implements IObserver(Of Temperature)
   
           Private _unsubscriber As IDisposable
           Private _last As Temperature?
   
           Public Sub Subscribe(provider As IObservable(Of Temperature))
               ArgumentNullException.ThrowIfNull(provider)
               _unsubscriber = provider.Subscribe(Me)
           End Sub
   

3. Define an Unsubscribe method that lets the observer stop receiving notifications before the provider calls IObserver<T>.OnCompleted.

   public void Unsubscribe() => _unsubscriber?.Dispose();
   

   Public Sub Unsubscribe()
       _unsubscriber?.Dispose()
   End Sub
   

4. Implement the three methods defined by IObserver<T>: IObserver<T>.OnNext, IObserver<T>.OnError, and IObserver<T>.OnCompleted.

   The OnError and OnCompleted methods can be stub implementations. The OnError method shouldn't handle the passed Exception as an exception, and OnCompleted can call the provider's IDisposable.Dispose implementation.

   public void OnCompleted() =>
       Console.WriteLine("Additional temperature data won't be transmitted.");
   
   // OnError is informational; observers shouldn't treat it as an exception to handle.
   public void OnError(Exception error) { }
   
   public void OnNext(Temperature value)
   {
       Console.WriteLine($"The temperature is {value.Degrees}°C at {value.Date:g}");
   
       if (_last is Temperature previous)
       {
           TimeSpan elapsed = value.Date.ToUniversalTime() - previous.Date.ToUniversalTime();
           Console.WriteLine($"   Change: {value.Degrees - previous.Degrees}° in {elapsed:g}");
       }
   
       _last = value;
   }
   

   Public Sub OnCompleted() Implements IObserver(Of Temperature).OnCompleted
       Console.WriteLine("Additional temperature data won't be transmitted.")
   End Sub
   
   ' OnError is informational; observers shouldn't treat it as an exception to handle.
   Public Sub OnError(error_ As Exception) Implements IObserver(Of Temperature).OnError
   End Sub
   
   Public Sub OnNext(value As Temperature) Implements IObserver(Of Temperature).OnNext
       Console.WriteLine($"The temperature is {value.Degrees}°C at {value.Date:g}")
   
       If _last.HasValue Then
           Dim previous = _last.Value
           Dim elapsed = value.Date.ToUniversalTime() - previous.Date.ToUniversalTime()
           Console.WriteLine($"   Change: {value.Degrees - previous.Degrees}° in {elapsed:g}")
       End If
   
       _last = value
   End Sub
   

Complete example

The following example shows the complete source code for the TemperatureReporter class, which provides the IObserver<T> implementation for a temperature monitoring application.

namespace TemperatureSample;

public sealed class TemperatureReporter : IObserver<Temperature>
{
    private IDisposable? _unsubscriber;
    private Temperature? _last;

    public void Subscribe(IObservable<Temperature> provider)
    {
        ArgumentNullException.ThrowIfNull(provider);
        _unsubscriber = provider.Subscribe(this);
    }

    public void Unsubscribe() => _unsubscriber?.Dispose();

    public void OnCompleted() =>
        Console.WriteLine("Additional temperature data won't be transmitted.");

    // OnError is informational; observers shouldn't treat it as an exception to handle.
    public void OnError(Exception error) { }

    public void OnNext(Temperature value)
    {
        Console.WriteLine($"The temperature is {value.Degrees}°C at {value.Date:g}");

        if (_last is Temperature previous)
        {
            TimeSpan elapsed = value.Date.ToUniversalTime() - previous.Date.ToUniversalTime();
            Console.WriteLine($"   Change: {value.Degrees - previous.Degrees}° in {elapsed:g}");
        }

        _last = value;
    }
}

Namespace Global.TemperatureSample

    Public NotInheritable Class TemperatureReporter
        Implements IObserver(Of Temperature)

        Private _unsubscriber As IDisposable
        Private _last As Temperature?

        Public Sub Subscribe(provider As IObservable(Of Temperature))
            ArgumentNullException.ThrowIfNull(provider)
            _unsubscriber = provider.Subscribe(Me)
        End Sub

        Public Sub Unsubscribe()
            _unsubscriber?.Dispose()
        End Sub

        Public Sub OnCompleted() Implements IObserver(Of Temperature).OnCompleted
            Console.WriteLine("Additional temperature data won't be transmitted.")
        End Sub

        ' OnError is informational; observers shouldn't treat it as an exception to handle.
        Public Sub OnError(error_ As Exception) Implements IObserver(Of Temperature).OnError
        End Sub

        Public Sub OnNext(value As Temperature) Implements IObserver(Of Temperature).OnNext
            Console.WriteLine($"The temperature is {value.Degrees}°C at {value.Date:g}")

            If _last.HasValue Then
                Dim previous = _last.Value
                Dim elapsed = value.Date.ToUniversalTime() - previous.Date.ToUniversalTime()
                Console.WriteLine($"   Change: {value.Degrees - previous.Degrees}° in {elapsed:g}")
            End If

            _last = value
        End Sub
    End Class

End Namespace

Related content

• IObserver<T>
• Observer design pattern
• How to implement a provider
• Best practices for the observer design pattern