IObserver<T> Interface
Definition
Important
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Provides a mechanism for receiving push-based notifications.
generic <typename T>
public interface class IObserverpublic interface IObserver<in T>type IObserver<'T> = interfacePublic Interface IObserver(Of In T)Type Parameters
- 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.- Derived
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.
Methods
| OnCompleted() |
Notifies the observer that the provider has finished sending push-based notifications. |
| OnError(Exception) |
Notifies the observer that the provider has experienced an error condition. |
| OnNext(T) |
Provides the observer with new data. |
