如何：实现监视程序

项目
05/10/2023

观察程序设计模式要求区分观察程序（注册获取通知）和提供程序（监视数据并将通知发送到一个或多个观察程序）。本主题介绍了如何创建观察程序。相关主题如何：实现提供程序中介绍了如何创建提供程序。

创建观察程序的具体步骤

定义观察程序，即实现 System.IObserver<T> 接口的类型。例如，下面的代码定义了 TemperatureReporter 类型，即使用 Temperature 泛型类型参数的构造 System.IObserver<T> 实现。
```
public class TemperatureReporter : IObserver<Temperature>
```
```
Public Class TemperatureReporter : Implements IObserver(Of Temperature)
```

如果观察程序可以在提供程序调用 IObserver<T>.OnCompleted 实现前停止接收通知，请定义专用变量，用于保留提供程序 IObservable<T>.Subscribe 方法返回的 IDisposable 实现。还应定义订阅方法，用于调用提供程序的 Subscribe 方法，并存储返回的 IDisposable 对象。例如，下面的代码定义 unsubscriber 专用变量，并定义 Subscribe 方法，用于调用提供程序的 Subscribe 方法，并将返回的对象分配给 unsubscriber 变量。

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

定义一个方法，以便观察程序可以在提供程序调用 IObserver<T>.OnCompleted 实现前停止接收通知（如果需要此功能的话）。下面的示例定义了 Unsubscribe 方法。
```
public virtual void Unsubscribe()
{
   unsubscriber.Dispose();
}
```
```
Public Overridable Sub Unsubscribe()
    unsubscriber.Dispose()
End Sub
```

实现 IObserver<T> 接口定义的三个方法：IObserver<T>.OnNext、IObserver<T>.OnError 和 IObserver<T>.OnCompleted。根据提供程序和应用需求，OnError 和 OnCompleted 方法可以是存根实现。请注意，OnError 方法不得将传入的 Exception 对象处理为异常，OnCompleted 方法可以随意调用提供程序的 IDisposable.Dispose 实现。下面的示例展示了 TemperatureReporter 类的 IObserver<T> 实现。

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

示例

下面的示例包含 TemperatureReporter 类的完整源代码，提供了温度监视应用的 IObserver<T> 实现。

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

如何：实现监视程序

创建观察程序的具体步骤

示例

请参阅

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