IObserver(Of T) Interface
Provides a mechanism for receiving push-based notifications.
Assembly: mscorlib (in mscorlib.dll)
Type Parameters
- In T
The object that provides notification information.
This type parameter is contravariant. That is, you can use either the type you specified or any type that is less derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.
The IObserver(Of T) type exposes the following members.
| Name | Description | |
|---|---|---|
![]() | OnCompleted | Notifies the observer that the provider has finished sending push-based notifications. |
![]() | OnError | Notifies the observer that the provider has experienced an error condition. |
![]() | OnNext | Provides the observer with new data. |
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.
An IObserver(Of T) implementation arranges to receive notifications from a provider (an IObservable(Of T) implementation) by passing an instance of itself to the provider's IObservable(Of T).Subscribe method. This method returns an IDisposable object that can be used to unsubscribe the observer before the provider finishes sending notifications.
The IObserver(Of T) interface defines the following three methods that the observer must implement:
The OnNext method, which is typically called by the provider to supply the observer with new data or state information.
The OnError method, which is typically called by the provider to indicate that data is unavailable, inaccessible, or corrupted, or that the provider has experienced some other error condition.
The OnCompleted method, which is typically called by the provider to indicate that it has finished sending notifications to observers.
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) Me.lat = latitude Me.lon = longitude End Sub Public ReadOnly Property Latitude As Double Get Return Me.lat End Get End Property Public ReadOnly Property Longitude As Double Get Return Me.lon End Get End Property End Structure
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 allows 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 enables 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 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 Get 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) Me.Unsubscribe() 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() unsubscriber.Dispose() End Sub End Class
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 observers.Add(observer) 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 _observers.Remove(_observer) 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()) Else observer.OnNext(loc.Value) End If Next End Sub Public Sub EndTransmission() For Each observer In observers.ToArray() If observers.Contains(observer) Then observer.OnCompleted() Next observers.Clear() 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.
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.
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") reporter1.Subscribe(provider) Dim reporter2 As New LocationReporter("MobileGPS") reporter2.Subscribe(provider) provider.TrackLocation(New Location(47.6456, -122.1312)) reporter1.Unsubscribe() provider.TrackLocation(New Location(47.6677, -122.1199)) provider.TrackLocation(Nothing) provider.EndTransmission() 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.
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.
