IObserver<T> 介面

定義

命名空間:

System

組件:

System.Runtime.dll

組件:

mscorlib.dll

組件:

netstandard.dll

提供接收推播通知的機制。

generic <typename T>
public interface class IObserver

public interface IObserver<in T>

type IObserver<'T> = interface

Public Interface IObserver(Of In T)

類型參數

T

提供通知資訊的物件。

這是反變數的型別參數。 也就是說，您可以使用您指定的類型，或衍生程度較低的任何類型。 如需共變數與反變數的詳細資訊，請參閱泛型中的共變數與反變數。

衍生

Microsoft.Extensions.DiagnosticAdapter.DiagnosticSourceAdapter

System.Web.Hosting.LowPhysicalMemoryObserver

System.Web.Hosting.RecycleLimitObserver

範例

下列範例說明觀察者設計模式。 它會定義 Location 包含緯度和經度資訊的類別。

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

類別提供 實 LocationReporter 作 IObserver<T> 。 它會顯示主控台目前位置的相關資訊。 其建構函式包含參數 name ，可讓 LocationReporter 實例在其字串輸出中識別本身。 它也包含 Subscribe 方法，它會包裝提供者方法的 Subscribe 呼叫。 這可讓 方法將傳回的 IDisposable 參考指派給私用變數。 類別 LocationReporter 也包含 Unsubscribe 方法，它會呼叫 IDisposable.Dispose 方法所 IObservable<T>.Subscribe 傳回之 物件的 方法。 下列程式碼會 LocationReporter 定義 類別。

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

類別提供 實 LocationTracker 作 IObservable<T> 。 其 TrackLocation 方法會傳遞可為 Null Location 的物件，其中包含緯度和經度資料。 Location如果值不是 null ，則 TrackLocation 方法會呼叫 OnNext 每個觀察者的 方法。

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

Location如果值為 null ，此方法會 TrackLocation 具現化 LocationNotFoundException 物件，如下列範例所示。 然後它會呼叫每個觀察者的 OnError 方法，並傳遞物件 LocationNotFoundException 。 請注意， LocationNotFoundException 衍生自 Exception ，但不會新增任何新成員。

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

exception LocationUnknownException

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

觀察者會藉由呼叫 其 IObservable<T>.Subscribe 方法，向觀察者物件指派參考給私用泛型 List<T> 物件，以註冊以接收物件的通知 TrackLocation 。 方法會傳 Unsubscriber 回 物件，這是 IDisposable 可讓觀察者停止接收通知的實作。 類別 LocationTracker 也包含 EndTransmission 方法。 當沒有進一步的位置資料可用時，方法會呼叫每個觀察者的 OnCompleted 方法，然後清除觀察者的內部清單。

下列程式碼接著會具現化提供者和觀察者。

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.

備註

IObserver<T>和 IObservable<T> 介面提供推播式通知的一般化機制，也稱為觀察者設計模式。 介面 IObservable<T> 代表 (提供者) 傳送通知的類別; IObserver<T> 介面代表 (觀察者) 接收通知的類別。 T 表示提供通知資訊的類別。

實 IObserver<T> 作會透過將本身的實例傳遞至提供者 IObservable<T>.Subscribe 的 方法，排列從提供者 (IObservable<T> 實作) 接收通知。 這個方法會傳 IDisposable 回物件，這個物件可用來在提供者完成傳送通知之前取消訂閱觀察者。

介面 IObserver<T> 會定義觀察者必須實作的下列三種方法：

• OnNext方法，提供者通常會呼叫這個方法，以提供新的資料或狀態資訊給觀察者。

• 方法 OnError ，通常由提供者呼叫，以指出資料無法使用、無法存取或損毀，或提供者發生其他錯誤狀況。

• 方法 OnCompleted ，通常由提供者呼叫，以指出它已完成傳送通知給觀察者。

方法

OnCompleted()	通知觀察者，提供者已完成傳送推播通知。
OnError(Exception)	通知觀察器此提供者發生錯誤狀況。
OnNext(T)	提供觀察者新的資料。

另請參閱

• IObservable<T>