IObservable(Of T).Subscribe Method
Notifies the provider that an observer is to receive notifications.
Assembly: mscorlib (in mscorlib.dll)
'Declaration Function Subscribe ( _ observer As IObserver(Of T) _ ) As IDisposable
Return ValueType: System.IDisposable
A reference to an interface that allows observers to stop receiving notifications before the provider has finished sending them.
The method must be called to register an observer for push-based notifications. A typical implementation of the method does the following:
It stores a reference to the observer in a collection object, such as a List(Of T) object.
It returns a reference to an IDisposable interface. This enables observers to unsubscribe (that is, to stop receiving notifications) before the provider has finished sending them and called the subscriber's OnCompleted method.
At any given time, a particular instance of an IObservable(Of T) implementation is responsible for handling all subscriptions and notifying all subscribers. Unless the documentation for a particular IObservable(Of T) implementation indicates otherwise, observers should make no assumptions about the IObservable(Of T) implementation, such as the order of notifications that multiple observers will receive.
The following example illustrates the method for an application that reports latitude and longitude information. It defines an IList(Of T) collection object that stores references to all observers. It also returns a private class named Unsubscriber that implements the IDisposable interface and enables subscribers to stop receiving event notifications. See the Example section of the IObservable(Of T) topic for the complete example.
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