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IObservable(Of T) Interface

Defines a provider for push-based notification.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

Public Interface IObservable(Of Out T)

Type Parameters

Out T

The object that provides notification information.

This type parameter is covariant. That is, you can use either the type you specified or any type that is more derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IObservable(Of T) type exposes the following members.

Public methodSubscribeNotifies the provider that an observer is to receive notifications.

The IObserver(Of T) and IObservable(Of T) interfaces provide a generalized mechanism for push-based notification, also known as the observer design pattern. The IObservable(Of T) interface represents the class that sends notifications (the provider); the IObserver(Of T) interface represents the class that receives them (the observer). T represents the class that provides the notification information. In some push-based notifications, the IObserver(Of T) implementation and T can represent the same type.

The provider must implement a single method, Subscribe, that indicates that an observer wants to receive push-based notifications. Callers to the method pass an instance of the observer. The method returns an IDisposable implementation that enables observers to cancel notifications at any time before the provider has stopped sending them.

At any given time, a given provider may have zero, one, or multiple observers. The provider is responsible for storing references to observers and ensuring that they are valid before it sends notifications. The IObservable(Of T) interface does not make any assumptions about the number of observers or the order in which notifications are sent.

The provider sends the following three kinds of notifications to the observer by calling IObserver(Of T) methods:

  • The current data. The provider can call the IObserver(Of T).OnNext method to pass the observer a T object that has current data, changed data, or fresh data.

  • An error condition. The provider can call the IObserver(Of T).OnError method to notify the observer that some error condition has occurred.

  • No further data. The provider can call the IObserver(Of T).OnCompleted method to notify the observer that it has finished sending notifications.

The following example illustrates the observer design pattern. It defines a Location class that contains latitude and longitude information.

Public Structure Location
   Dim lat, lon As Double

   Public Sub New(ByVal latitude As Double, ByVal longitude As Double) = latitude
      Me.lon = longitude
   End Sub

   Public ReadOnly Property Latitude As Double
      End Get
   End Property

   Public ReadOnly Property Longitude As Double
         Return Me.lon
      End Get
   End Property
End Structure

The LocationTracker class provides the IObservable(Of T) implementation. Its TrackLocation method is passed a nullable Location object that contains the latitude and longitude data. If the Location value is not Nothing, the TrackLocation method calls the OnNext method of each observer.

Public Class LocationTracker : Implements IObservable(Of Location)

   Public Sub New()
      observers = New List(Of IObserver(Of Location))
   End Sub

   Private observers As List(Of IObserver(Of Location))

   Public Function Subscribe(ByVal observer As System.IObserver(Of Location)) As System.IDisposable _
                            Implements System.IObservable(Of Location).Subscribe
      If Not observers.Contains(observer) Then
      End If
      Return New Unsubscriber(observers, observer)
   End Function

   Private Class Unsubscriber : Implements IDisposable
      Private _observers As List(Of IObserver(Of Location))
      Private _observer As IObserver(Of Location)

      Public Sub New(ByVal observers As List(Of IObserver(Of Location)), ByVal observer As IObserver(Of Location))
         Me._observers = observers
         Me._observer = observer
      End Sub

      Public Sub Dispose() Implements IDisposable.Dispose
         If _observer IsNot Nothing AndAlso _observers.Contains(_observer) Then
         End If
      End Sub
   End Class

   Public Sub TrackLocation(ByVal loc As Nullable(Of Location))
      For Each observer In observers
         If Not loc.HasValue Then
            observer.OnError(New LocationUnknownException())
         End If
   End Sub

   Public Sub EndTransmission()
      For Each observer In observers.ToArray()
         If observers.Contains(observer) Then observer.OnCompleted()
   End Sub
End Class

If the Location value is Nothing, the TrackLocation method instantiates a LocationNotFoundException object, which is shown in the following example. It then calls each observer's OnError method and passes it the LocationNotFoundException object. Note that LocationNotFoundException derives from Exception, but does not add any new members.

Public Class LocationUnknownException : Inherits Exception
   Friend Sub New()
   End Sub
End Class

Observers register to receive notifications from a TrackLocation object by calling its IObservable(Of T).Subscribe method, which assigns a reference to the observer object to a private generic List(Of T) object. The method returns an Unsubscriber object, which is an IDisposable implementation that enables observers to stop receiving notifications. The LocationTracker class also includes an EndTransmission method. When no further location data is available, the method calls each observer's OnCompleted method and then clears the internal list of observers.

In this example, the LocationReporter class provides the IObserver(Of T) implementation. It displays information about the current location to the console. Its constructor includes a name parameter, which enables the LocationReporter instance to identify itself in its string output. It also includes a Subscribe method, which wraps a call to the provider's Subscribe method. This allows the method to assign the returned IDisposable reference to a private variable. The LocationReporter class also includes an Unsubscribe method, which calls the IDisposable.Dispose method of the object that is returned by the IObservable(Of T).Subscribe method. The following code defines the LocationReporter class.

Public Class LocationReporter : Implements IObserver(Of Location)
   Dim unsubscriber As IDisposable
   Dim instName As String

   Public Sub New(ByVal name As String)
      Me.instName = name
   End Sub

   Public ReadOnly Property Name As String
         Return instName
      End Get
   End Property

   Public Overridable Sub Subscribe(ByVal provider As IObservable(Of Location))
      If provider Is Nothing Then Exit Sub
      unsubscriber = provider.Subscribe(Me)
   End Sub

   Public Overridable Sub OnCompleted() Implements System.IObserver(Of Location).OnCompleted
      Console.WriteLine("The Location Tracker has completed transmitting data to {0}.", Me.Name)
   End Sub

   Public Overridable Sub OnError(ByVal e As System.Exception) Implements System.IObserver(Of Location).OnError
      Console.WriteLine("{0}: The location cannot be determined.", Me.Name)
   End Sub

   Public Overridable Sub OnNext(ByVal value As Location) Implements System.IObserver(Of Location).OnNext
      Console.WriteLine("{2}: The current location is {0}, {1}", value.Latitude, value.Longitude, Me.Name)
   End Sub

   Public Overridable Sub Unsubscribe()
   End Sub
End Class

The following code then instantiates the provider and the observer.

Module Module1
   Dim provider As LocationTracker

   Sub Main()
      ' Define a provider and two observers.
      provider = New LocationTracker()
      Dim reporter1 As New LocationReporter("FixedGPS")
      Dim reporter2 As New LocationReporter("MobileGPS")

      provider.TrackLocation(New Location(47.6456, -122.1312))
      provider.TrackLocation(New Location(47.6677, -122.1199))
   End Sub
End Module
' The example displays output similar to the following:
'       FixedGPS: The current location is 47.6456, -122.1312
'       MobileGPS: The current location is 47.6456, -122.1312
'       MobileGPS: The current location is 47.6677, -122.1199
'       MobileGPS: The location cannot be determined.
'       The Location Tracker has completed transmitting data to MobileGPS.

.NET Framework

Supported in: 4

.NET Framework Client Profile

Supported in: 4

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.