Defines a provider for push-based notification.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- value
- Type: T
The current notification information.
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.
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; } }
}
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();
}
}
public class LocationUnknownException : Exception
{
internal LocationUnknownException()
{ }
}
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();
}
}
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.
출처 : http://msdn.microsoft.com/en-us/library/dd990377.aspx
