IObserver<T> Interface

Definition

generic <typename T> public interface class IObserver

public interface IObserver<in T>

type IObserver<'T> = interface

Public Interface IObserver(Of In T)

Type Parameters

Derived

Microsoft.Extensions.DiagnosticAdapter.DiagnosticSourceAdapter

System.Web.Hosting.LowPhysicalMemoryObserver

System.Web.Hosting.RecycleLimitObserver

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 LocationReporter class provides the IObserver<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<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 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 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 : Exception { internal LocationUnknownException() { } } 

exception LocationUnknownException 

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<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.

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.

An IObserver<T> implementation arranges to receive notifications from a provider (an IObservable<T> implementation) by passing an instance of itself to the provider's IObservable<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<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.

See also

• IObservable<T>