操作說明：實作提供者

發行項
04/07/2023

觀察者設計模式需要提供者和一個或多個觀察者之間的分區，其中提供者會監視資料並傳送通知，而觀察者會接收來自提供者的通知 (回呼)。本主題討論如何建立提供者。相關主題為操作說明：實作觀察器，討論如何建立觀察者。

建立提供者

定義提供者負責傳送給觀察者的資料。雖然提供者和它傳送給觀察者的資料可以是單一的型別，但它們通常以不同型別代表。例如，在溫度監控應用程式中，Temperature 結構定義的提供者 (在下一步中以 TemperatureMonitor 代表) 監視並由觀察者訂閱的資料。

using System;

public struct Temperature
{
   private decimal temp;
   private DateTime tempDate;

   public Temperature(decimal temperature, DateTime dateAndTime)
   {
      this.temp = temperature;
      this.tempDate = dateAndTime;
   }

   public decimal Degrees
   { get { return this.temp; } }

   public DateTime Date
   { get { return this.tempDate; } }
}

Public Structure Temperature
    Private temp As Decimal
    Private tempDate As DateTime

    Public Sub New(ByVal temperature As Decimal, ByVal dateAndTime As DateTime)
        Me.temp = temperature
        Me.tempDate = dateAndTime
    End Sub

    Public ReadOnly Property Degrees As Decimal
        Get
            Return Me.temp
        End Get
    End Property

    Public ReadOnly Property [Date] As DateTime
        Get
            Return tempDate
        End Get
    End Property
End Structure

定義資料提供者，這是實作 System.IObservable<T> 介面的型別。提供者的泛型型別引數是提供者傳送給觀察者的型別。下列範例會定義 TemperatureMonitor 類別，這是包含泛型型別引數 Temperature 的建構 System.IObservable<T> 實作。
```
using System;
using System.Collections.Generic;

public class TemperatureMonitor : IObservable<Temperature>
{
```
```
Imports System.Collections.Generic


Public Class TemperatureMonitor : Implements IObservable(Of Temperature)
```

決定提供者儲存對觀察者的參考之方式，讓每個觀察者在適當時被通知。大多數情況下，如泛型 List<T> 物件等集合物件用於此用途。下列範例定義在 TemperatureMonitor 中具現化的私用 List<T> 物件。

using System;
using System.Collections.Generic;

public class TemperatureMonitor : IObservable<Temperature>
{
   List<IObserver<Temperature>> observers;

   public TemperatureMonitor()
   {
      observers = new List<IObserver<Temperature>>();
   }

Imports System.Collections.Generic


Public Class TemperatureMonitor : Implements IObservable(Of Temperature)
    Dim observers As List(Of IObserver(Of Temperature))

    Public Sub New()
        observers = New List(Of IObserver(Of Temperature))
    End Sub

定義提供者可傳回到訂閱者的 IDisposable 實作，讓它們可隨時停止接收通知。下列範例定義巢狀 Unsubscriber 類別，它在類別初始化時將參考傳遞到訂閱者集合及訂閱者。此程式碼讓訂閱者能夠呼叫物件的 IDisposable.Dispose 實作，以將自己從訂閱者集合移除。

private class Unsubscriber : IDisposable
{
   private List<IObserver<Temperature>> _observers;
   private IObserver<Temperature> _observer;

   public Unsubscriber(List<IObserver<Temperature>> observers, IObserver<Temperature> observer)
   {
      this._observers = observers;
      this._observer = observer;
   }

   public void Dispose()
   {
      if (! (_observer == null)) _observers.Remove(_observer);
   }
}

Private Class Unsubscriber : Implements IDisposable
    Private _observers As List(Of IObserver(Of Temperature))
    Private _observer As IObserver(Of Temperature)

    Public Sub New(ByVal observers As List(Of IObserver(Of Temperature)), ByVal observer As IObserver(Of Temperature))
        Me._observers = observers
        Me._observer = observer
    End Sub

    Public Sub Dispose() Implements IDisposable.Dispose
        If _observer IsNot Nothing Then _observers.Remove(_observer)
    End Sub
End Class

實作 IObservable<T>.Subscribe 方法。該方法傳遞參考到 System.IObserver<T> 介面，並應該儲存在步驟 3 中為該目的而設計的物件中。這個方法應再傳回在步驟 4 中開發的 IDisposable 實作。下列範例顯示 TemperatureMonitor 類別中 Subscribe 方法的實作。

public IDisposable Subscribe(IObserver<Temperature> observer)
{
   if (! observers.Contains(observer))
      observers.Add(observer);

   return new Unsubscriber(observers, observer);
}

Public Function Subscribe(ByVal observer As System.IObserver(Of Temperature)) As System.IDisposable Implements System.IObservable(Of Temperature).Subscribe
    If Not observers.Contains(observer) Then
        observers.Add(observer)
    End If
    Return New Unsubscriber(observers, observer)
End Function

藉由呼叫觀察者的 IObserver<T>.OnNext、IObserver<T>.OnError 和 IObserver<T>.OnCompleted 實作，以適時通知它們。在某些情況下，當錯誤發生時提供者可能不會呼叫 OnError 方法。例如，下列 GetTemperature 方法模擬此監視器每五秒鐘讀取一次溫度資料，並通知觀察者自上次讀取之後溫度是否已變更至少 1 度。如果裝置未報告溫度 (即如果其值為 Null)，提供者會通知觀察者傳輸已完成。請注意，除了呼叫每個觀察者的 OnCompleted 方法，GetTemperature 方法可清除 List<T> 集合。在此情況下，提供者沒有呼叫其觀察者的 OnError 方法。

public void GetTemperature()
{
   // Create an array of sample data to mimic a temperature device.
   Nullable<Decimal>[] temps = {14.6m, 14.65m, 14.7m, 14.9m, 14.9m, 15.2m, 15.25m, 15.2m,
                                15.4m, 15.45m, null };
   // Store the previous temperature, so notification is only sent after at least .1 change.
   Nullable<Decimal> previous = null;
   bool start = true;

   foreach (var temp in temps) {
      System.Threading.Thread.Sleep(2500);
      if (temp.HasValue) {
         if (start || (Math.Abs(temp.Value - previous.Value) >= 0.1m )) {
            Temperature tempData = new Temperature(temp.Value, DateTime.Now);
            foreach (var observer in observers)
               observer.OnNext(tempData);
            previous = temp;
            if (start) start = false;
         }
      }
      else {
         foreach (var observer in observers.ToArray())
            if (observer != null) observer.OnCompleted();

         observers.Clear();
         break;
      }
   }
}

Public Sub GetTemperature()
    ' Create an array of sample data to mimic a temperature device.
    Dim temps() As Nullable(Of Decimal) = {14.6D, 14.65D, 14.7D, 14.9D, 14.9D, 15.2D, 15.25D, 15.2D,
                                          15.4D, 15.45D, Nothing}
    ' Store the previous temperature, so notification is only sent after at least .1 change.
    Dim previous As Nullable(Of Decimal)
    Dim start As Boolean = True

    For Each temp In temps
        System.Threading.Thread.Sleep(2500)

        If temp.HasValue Then
            If start OrElse Math.Abs(temp.Value - previous.Value) >= 0.1 Then
                Dim tempData As New Temperature(temp.Value, Date.Now)
                For Each observer In observers
                    observer.OnNext(tempData)
                Next
                previous = temp
                If start Then start = False
            End If
        Else
            For Each observer In observers.ToArray()
                If observer IsNot Nothing Then observer.OnCompleted()
            Next
            observers.Clear()
            Exit For
        End If
    Next
End Sub

範例

下列範例包含定義 IObservable<T> 溫度監視應用程式實作的完整原始程式碼。它包含 Temperature 結構，這是傳送給觀察者的資料，以及 TemperatureMonitor 類別，這是 IObservable<T> 實作。

using System.Threading;
using System;
using System.Collections.Generic;

public class TemperatureMonitor : IObservable<Temperature>
{
   List<IObserver<Temperature>> observers;

   public TemperatureMonitor()
   {
      observers = new List<IObserver<Temperature>>();
   }

   private class Unsubscriber : IDisposable
   {
      private List<IObserver<Temperature>> _observers;
      private IObserver<Temperature> _observer;

      public Unsubscriber(List<IObserver<Temperature>> observers, IObserver<Temperature> observer)
      {
         this._observers = observers;
         this._observer = observer;
      }

      public void Dispose()
      {
         if (! (_observer == null)) _observers.Remove(_observer);
      }
   }

   public IDisposable Subscribe(IObserver<Temperature> observer)
   {
      if (! observers.Contains(observer))
         observers.Add(observer);

      return new Unsubscriber(observers, observer);
   }

   public void GetTemperature()
   {
      // Create an array of sample data to mimic a temperature device.
      Nullable<Decimal>[] temps = {14.6m, 14.65m, 14.7m, 14.9m, 14.9m, 15.2m, 15.25m, 15.2m,
                                   15.4m, 15.45m, null };
      // Store the previous temperature, so notification is only sent after at least .1 change.
      Nullable<Decimal> previous = null;
      bool start = true;

      foreach (var temp in temps) {
         System.Threading.Thread.Sleep(2500);
         if (temp.HasValue) {
            if (start || (Math.Abs(temp.Value - previous.Value) >= 0.1m )) {
               Temperature tempData = new Temperature(temp.Value, DateTime.Now);
               foreach (var observer in observers)
                  observer.OnNext(tempData);
               previous = temp;
               if (start) start = false;
            }
         }
         else {
            foreach (var observer in observers.ToArray())
               if (observer != null) observer.OnCompleted();

            observers.Clear();
            break;
         }
      }
   }
}

Imports System.Threading
Imports System.Collections.Generic


Public Class TemperatureMonitor : Implements IObservable(Of Temperature)
    Dim observers As List(Of IObserver(Of Temperature))

    Public Sub New()
        observers = New List(Of IObserver(Of Temperature))
    End Sub

    Private Class Unsubscriber : Implements IDisposable
        Private _observers As List(Of IObserver(Of Temperature))
        Private _observer As IObserver(Of Temperature)

        Public Sub New(ByVal observers As List(Of IObserver(Of Temperature)), ByVal observer As IObserver(Of Temperature))
            Me._observers = observers
            Me._observer = observer
        End Sub

        Public Sub Dispose() Implements IDisposable.Dispose
            If _observer IsNot Nothing Then _observers.Remove(_observer)
        End Sub
    End Class

    Public Function Subscribe(ByVal observer As System.IObserver(Of Temperature)) As System.IDisposable Implements System.IObservable(Of Temperature).Subscribe
        If Not observers.Contains(observer) Then
            observers.Add(observer)
        End If
        Return New Unsubscriber(observers, observer)
    End Function

    Public Sub GetTemperature()
        ' Create an array of sample data to mimic a temperature device.
        Dim temps() As Nullable(Of Decimal) = {14.6D, 14.65D, 14.7D, 14.9D, 14.9D, 15.2D, 15.25D, 15.2D,
                                              15.4D, 15.45D, Nothing}
        ' Store the previous temperature, so notification is only sent after at least .1 change.
        Dim previous As Nullable(Of Decimal)
        Dim start As Boolean = True

        For Each temp In temps
            System.Threading.Thread.Sleep(2500)

            If temp.HasValue Then
                If start OrElse Math.Abs(temp.Value - previous.Value) >= 0.1 Then
                    Dim tempData As New Temperature(temp.Value, Date.Now)
                    For Each observer In observers
                        observer.OnNext(tempData)
                    Next
                    previous = temp
                    If start Then start = False
                End If
            Else
                For Each observer In observers.ToArray()
                    If observer IsNot Nothing Then observer.OnCompleted()
                Next
                observers.Clear()
                Exit For
            End If
        Next
    End Sub
End Class

共用方式為

操作說明：實作提供者

建立提供者

範例

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其他資源