操作說明：實作觀察者

發行項
03/17/2024

觀察者設計模式需要觀察者和提供者之間的分區，其中觀察者會註冊通知，而提供者會監視資料並將通知傳送到一個或多個觀察者。本主題討論如何建立觀察者。相關主題如何：實作提供者會介紹如何建立提供者。

建立觀察者

定義觀察者，這是實作 System.IObserver<T> 介面的型別。例如，下列程式碼定義名為 TemperatureReporter 的型別，它是包含 Temperature 泛型型別引數的建構 System.IObserver<T> 實作。
```
public class TemperatureReporter : IObserver<Temperature>
```
```
Public Class TemperatureReporter : Implements IObserver(Of Temperature)
```

如果觀察者可以在提供者呼叫其 IObserver<T>.OnCompleted 實作之前停止接收通知，請定義會保留由提供者的 IObservable<T>.Subscribe 方法傳回之 IDisposable 實作的私用變數。您也應該定義訂閱方法，它會呼叫提供者的 Subscribe 方法並儲存傳回的 IDisposable 物件。例如，下列的程式碼定義名為 unsubscriber 的私用變數，並定義呼叫提供者的 Subscribe 方法之 Subscribe 方法，然後將傳回的物件指派到 unsubscriber 變數。

public class TemperatureReporter : IObserver<Temperature>
{
   private IDisposable unsubscriber;
   private bool first = true;
   private Temperature last;

   public virtual void Subscribe(IObservable<Temperature> provider)
   {
      unsubscriber = provider.Subscribe(this);
   }

Public Class TemperatureReporter : Implements IObserver(Of Temperature)

    Private unsubscriber As IDisposable
    Private first As Boolean = True
    Private last As Temperature

    Public Overridable Sub Subscribe(ByVal provider As IObservable(Of Temperature))
        unsubscriber = provider.Subscribe(Me)
    End Sub

定義讓觀察者能夠在提供者呼叫其 IObserver<T>.OnCompleted 實作之前停止接收通知的方法 (如果需要此功能)。下列範例定義 Unsubscribe 方法。
```
public virtual void Unsubscribe()
{
   unsubscriber.Dispose();
}
```
```
Public Overridable Sub Unsubscribe()
    unsubscriber.Dispose()
End Sub
```

提供 IObserver<T> 介面定義的三個方法之實作：IObserver<T>.OnNext，IObserver<T>.OnError 和 IObserver<T>.OnCompleted。視提供者和應用程式需求而定，OnError 和 OnCompleted 方法可以是虛設常式實作。請注意，OnError 方法不應將傳遞的 Exception 物件當作例外狀況處理，而 OnCompleted 方法可以自由呼叫提供者的 IDisposable.Dispose 實作。下列範例顯示 TemperatureReporter 類別的 IObserver<T> 實作。

public virtual void OnCompleted()
{
   Console.WriteLine("Additional temperature data will not be transmitted.");
}

public virtual void OnError(Exception error)
{
   // Do nothing.
}

public virtual void OnNext(Temperature value)
{
   Console.WriteLine("The temperature is {0}°C at {1:g}", value.Degrees, value.Date);
   if (first)
   {
      last = value;
      first = false;
   }
   else
   {
      Console.WriteLine("   Change: {0}° in {1:g}", value.Degrees - last.Degrees,
                                                    value.Date.ToUniversalTime() - last.Date.ToUniversalTime());
   }
}

Public Overridable Sub OnCompleted() Implements System.IObserver(Of Temperature).OnCompleted
    Console.WriteLine("Additional temperature data will not be transmitted.")
End Sub

Public Overridable Sub OnError(ByVal [error] As System.Exception) Implements System.IObserver(Of Temperature).OnError
    ' Do nothing.
End Sub

Public Overridable Sub OnNext(ByVal value As Temperature) Implements System.IObserver(Of Temperature).OnNext
    Console.WriteLine("The temperature is {0}°C at {1:g}", value.Degrees, value.Date)
    If first Then
        last = value
        first = False
    Else
        Console.WriteLine("   Change: {0}° in {1:g}", value.Degrees - last.Degrees,
                                                      value.Date.ToUniversalTime - last.Date.ToUniversalTime)
    End If
End Sub

範例

以下範例包含 TemperatureReporter 類別的完整原始程式碼，它提供溫度監視應用程式的 IObserver<T> 實作。

public class TemperatureReporter : IObserver<Temperature>
{
   private IDisposable unsubscriber;
   private bool first = true;
   private Temperature last;

   public virtual void Subscribe(IObservable<Temperature> provider)
   {
      unsubscriber = provider.Subscribe(this);
   }

   public virtual void Unsubscribe()
   {
      unsubscriber.Dispose();
   }

   public virtual void OnCompleted()
   {
      Console.WriteLine("Additional temperature data will not be transmitted.");
   }

   public virtual void OnError(Exception error)
   {
      // Do nothing.
   }

   public virtual void OnNext(Temperature value)
   {
      Console.WriteLine("The temperature is {0}°C at {1:g}", value.Degrees, value.Date);
      if (first)
      {
         last = value;
         first = false;
      }
      else
      {
         Console.WriteLine("   Change: {0}° in {1:g}", value.Degrees - last.Degrees,
                                                       value.Date.ToUniversalTime() - last.Date.ToUniversalTime());
      }
   }
}

Public Class TemperatureReporter : Implements IObserver(Of Temperature)

    Private unsubscriber As IDisposable
    Private first As Boolean = True
    Private last As Temperature

    Public Overridable Sub Subscribe(ByVal provider As IObservable(Of Temperature))
        unsubscriber = provider.Subscribe(Me)
    End Sub

    Public Overridable Sub Unsubscribe()
        unsubscriber.Dispose()
    End Sub

    Public Overridable Sub OnCompleted() Implements System.IObserver(Of Temperature).OnCompleted
        Console.WriteLine("Additional temperature data will not be transmitted.")
    End Sub

    Public Overridable Sub OnError(ByVal [error] As System.Exception) Implements System.IObserver(Of Temperature).OnError
        ' Do nothing.
    End Sub

    Public Overridable Sub OnNext(ByVal value As Temperature) Implements System.IObserver(Of Temperature).OnNext
        Console.WriteLine("The temperature is {0}°C at {1:g}", value.Degrees, value.Date)
        If first Then
            last = value
            first = False
        Else
            Console.WriteLine("   Change: {0}° in {1:g}", value.Degrees - last.Degrees,
                                                          value.Date.ToUniversalTime - last.Date.ToUniversalTime)
        End If
    End Sub
End Class

共用方式為

操作說明：實作觀察者

建立觀察者

範例

另請參閱

其他資源