IObservable(Of T) Interface
Defines a provider for push-based notification.
Assembly: mscorlib (in mscorlib.dll)
| Name | Description | |
|---|---|---|
![]() | Subscribe(IObserver(Of T)) | Notifies 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) 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 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 null, 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 null, the TrackLocation method instantiates a LocationUnknownException object, which is shown in the following example. It then calls each observer's OnError method and passes it the LocationUnknownException object. Note that LocationUnknownException 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.
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 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 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.
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
