IObservable<T> Interface

Definition

generic <typename T>
public interface class IObservable

public interface IObservable<out T>

type IObservable<'T> = interface

Public Interface IObservable(Of Out T)

Type Parameters

Derived

System.Diagnostics.DiagnosticListener

System.Web.Hosting.AspNetMemoryMonitor

Examples

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

public struct Location
{
 double lat, lon;

 public Location(double latitude, double longitude)
 {
 this.lat = latitude;
 this.lon = longitude;
 }

 public double Latitude
 { get { return this.lat; } }

 public double Longitude
 { get { return this.lon; } }
}

[<Struct>]
type Location =
 { Latitude: double
 Longitude: double }

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<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 : IObservable<Location>
{
 public LocationTracker()
 {
 observers = new List<IObserver<Location>>();
 }

 private List<IObserver<Location>> observers;

 public IDisposable Subscribe(IObserver<Location> observer)
 {
 if (!observers.Contains(observer))
 observers.Add(observer);
 return new Unsubscriber(observers, observer);
 }

 private class Unsubscriber : IDisposable
 {
 private List<IObserver<Location>>_observers;
 private IObserver<Location> _observer;

 public Unsubscriber(List<IObserver<Location>> observers, IObserver<Location> observer)
 {
 this._observers = observers;
 this._observer = observer;
 }

 public void Dispose()
 {
 if (_observer != null && _observers.Contains(_observer))
 _observers.Remove(_observer);
 }
 }

 public void TrackLocation(Nullable<Location> loc)
 {
 foreach (var observer in observers) {
 if (!loc.HasValue)
 observer.OnError(new LocationUnknownException());
 else
 observer.OnNext(loc.Value);
 }
 }

 public void EndTransmission()
 {
 foreach (var observer in observers.ToArray())
 if (observers.Contains(observer))
 observer.OnCompleted();

 observers.Clear();
 }
}

type Unsubscriber(observers: ResizeArray<IObserver<Location>>, observer: IObserver<Location>) =
 interface IDisposable with
 member _.Dispose() =
 if observer <> null && observers.Contains observer then
 observers.Remove observer |> ignore

type LocationTracker() =
 let observers = ResizeArray<IObserver<Location>>()

 interface IObservable<Location> with
 member _.Subscribe(observer) =
 if observers.Contains observer |> not then
 observers.Add observer
 new Unsubscriber(observers, observer)

 member _.TrackLocation(loc: Nullable<Location>) =
 for observer in observers do
 if not loc.HasValue then
 observer.OnError LocationUnknownException
 else
 observer.OnNext loc.Value

 member _.EndTransmission() =
 for observer in observers.ToArray() do
 if observers.Contains observer then
 observer.OnCompleted()
 observers.Clear()

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.

public class LocationUnknownException : Exception
{
 internal LocationUnknownException()
 { }
}

exception LocationUnknownException

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

Observers register to receive notifications from a LocationTracker object by calling its IObservable<T>.Subscribe method, which assigns a reference to the observer object to a private generic List<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<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<T>.Subscribe method. The following code defines the LocationReporter class.

using System;

public class LocationReporter : IObserver<Location>
{
 private IDisposable unsubscriber;
 private string instName;

 public LocationReporter(string name)
 {
 this.instName = name;
 }

 public string Name
 { get{ return this.instName; } }

 public virtual void Subscribe(IObservable<Location> provider)
 {
 if (provider != null)
 unsubscriber = provider.Subscribe(this);
 }

 public virtual void OnCompleted()
 {
 Console.WriteLine("The Location Tracker has completed transmitting data to {0}.", this.Name);
 this.Unsubscribe();
 }

 public virtual void OnError(Exception e)
 {
 Console.WriteLine("{0}: The location cannot be determined.", this.Name);
 }

 public virtual void OnNext(Location value)
 {
 Console.WriteLine("{2}: The current location is {0}, {1}", value.Latitude, value.Longitude, this.Name);
 }

 public virtual void Unsubscribe()
 {
 unsubscriber.Dispose();
 }
}

open System

type LocationReporter(name) =
 let mutable unsubscriber = Unchecked.defaultof<IDisposable>

 member _.Name = name

 member this.Subscribe(provider: IObservable<Location>) =
 if provider <> null then
 unsubscriber <- provider.Subscribe this

 member _.Unsubscribe() =
 unsubscriber.Dispose()

 interface IObserver<Location> with
 member this.OnCompleted() =
 printfn $"The Location Tracker has completed transmitting data to {name}."
 this.Unsubscribe()

 member _.OnError(_) =
 printfn $"{name}: The location cannot be determined."

 member _.OnNext(value) =
 printfn $"{name}: The current location is {value.Latitude}, {value.Longitude}"

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.

using System;

class Program
{
 static void Main(string[] args)
 {
 // Define a provider and two observers.
 LocationTracker provider = new LocationTracker();
 LocationReporter reporter1 = new LocationReporter("FixedGPS");
 reporter1.Subscribe(provider);
 LocationReporter reporter2 = 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(null);
 provider.EndTransmission();
 }
}
// 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.

open System

// Define a provider and two observers.
let provider = LocationTracker()
let reporter1 = LocationReporter "FixedGPS"
reporter1.Subscribe provider
let reporter2 = LocationReporter "MobileGPS"
reporter2.Subscribe provider

provider.TrackLocation { Latitude = 47.6456; Longitude = -122.1312 }
reporter1.Unsubscribe()
provider.TrackLocation { Latitude = 47.6677; Longitude = -122.1199 }
provider.TrackLocation(Nullable())
provider.EndTransmission()
// 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.

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.

Remarks

The IObserver<T> and IObservable<T> interfaces provide a generalized mechanism for push-based notification, also known as the observer design pattern. The IObservable<T> interface represents the class that sends notifications (the provider); the IObserver<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<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<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<T> methods:

• The current data. The provider can call the IObserver<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<T>.OnError method to notify the observer that some error condition has occurred.

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

See also

• IObserver<T>
• The Observer Design Pattern