在编程中，装饰器模式是一种非常强大的设计模式，它允许动态地给对象添加新的功能。然而，当涉及到多线程或异步操作时，传统的装饰器模式可能会遇到一些挑战。本文将介绍一种称为“反向装饰器”的模式，它允许在多线程和异步环境中更有效地使用装饰器模式。
反向装饰器模式是一种特殊的装饰器模式，它将装饰器的顺序颠倒过来，从而允许在多线程或异步环境中更灵活地使用装饰器。这种模式的核心思想是将装饰器的执行顺序反转，使得最后一个装饰器首先执行，然后依次向前执行，直到第一个装饰器。
为了更好地理解反向装饰器模式，将构建一个多线程的应用，该应用将一个图片文件转换为Base64字符串。这个示例将展示如何在VB.NET中实现反向装饰器模式。
首先，定义一个接口IConvertFileToBase64，它包含一个名为RunMe的方法。这个方法将接受一个值类ConvertFileToBase64Vals作为参数，该值类包含文件路径、错误对象和Base64字符串等属性。
Public Interface IConvertFileToBase64
Sub RunMe(ByVal dataObj As ConvertFileToBase64Vals)
End Interface
Public Class ConvertFileToBase64Vals
Public Property ErrObj As ErrorObj
Public Property FilePathAndName As String
Public Property Base64String As String
End Class
接下来，实现第一个动作GetFilePath。这个动作将使用一个文件对话框来获取用户选择的文件路径，并将其存储在ConvertFileToBase64Vals对象中。如果发生错误，将错误信息存储在错误对象中。
Public Class GetFilePath Implements IConvertFileToBase64
Private _runMeNext As IConvertFileToBase64
Public Sub New(ByVal runMeNext As IConvertFileToBase64)
_runMeNext = runMeNext
End Sub
Public Sub RunMe(dataObj As ConvertFileToBase64Vals) Implements IConvertFileToBase64.RunMe
If Not dataObj.ErrObj.HasError Then
Try
Dim OpenFileDialog1 As New OpenFileDialog()
If OpenFileDialog1.ShowDialog() = System.Windows.Forms.DialogResult.OK Then
dataObj.FilePathAndName = OpenFileDialog1.FileName
End If
Catch ex As Exception
dataObj.ErrObj.HasError = True
dataObj.ErrObj.Message = "GetFilePath: " & ex.Message
End Try
If Not IsNothing(_runMeNext) Then
_runMeNext.RunMe(dataObj)
End If
End If
End Sub
End Class
接下来，实现其他两个动作：ConvertToBase64和LongRunningTask。这两个动作将分别负责将文件内容转换为Base64字符串，并执行一个长时间运行的任务。
为了在多线程环境中执行这些动作，实现了一个名为MoveToNewThread的动作。这个动作使用了一个后台工作线程（BackgroundWorker）来执行任务，并在任务完成后调用一个回调方法。
Public Class MoveToNewThread Implements IConvertFileToBase64
Private WithEvents _bgw As BackgroundWorker
Private _nextSub As IConvertFileToBase64
Private _callMeWhenDone As Action(Of ConvertFileToBase64Vals)
Public Sub New(ByRef callMeWhenDone As Action(Of ConvertFileToBase64Vals), ByRef nextSub As IConvertFileToBase64)
_callMeWhenDone = callMeWhenDone
_nextSub = nextSub
If IsNothing(_bgw) Then
_bgw = New BackgroundWorker()
_bgw.WorkerReportsProgress = True
_bgw.WorkerSupportsCancellation = True
End If
End Sub
Private Sub bgw_DoWork(ByVal sender As System.Object, ByVal e As DoWorkEventArgs) Handles _bgw.DoWork
Dim locDataObj As ConvertFileToBase64Vals = TryCast(e.Argument, ConvertFileToBase64Vals)
If IsNothing(locDataObj) Then
Throw New System.Exception("GetServerInfo: No locDataObj passed. Ending Execution.")
Exit Sub
End If
e.Result = locDataObj
If Not IsNothing(_nextSub) Then
_nextSub.RunMe(locDataObj)
End If
End Sub
Private Sub bgw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs) Handles _bgw.RunWorkerCompleted
Dim dataObj As ConvertFileToBase64Vals = TryCast(e.Result, ConvertFileToBase64Vals)
_callMeWhenDone(dataObj)
End Sub
Public Sub RunMe(dataObj As ConvertFileToBase64Vals) Implements IConvertFileToBase64.RunMe
If Not dataObj.ErrObj.HasError AndAlso Not _bgw.IsBusy Then
_bgw.RunWorkerAsync(dataObj)
ElseIf Not IsNothing(_nextSub) Then
_nextSub.RunMe(dataObj)
End If
End Sub
End Class
最后，需要按照反向的顺序组合这些动作。首先创建一个LongRunningTask实例，然后创建一个ConvertToBase64String实例，接着创建一个MoveToNewThread实例，并传递一个回调方法和一个GetFilePath实例。
Dim runMe As IConvertFileToBase64 = Nothing
runMe = New LongRunningTask(runMe)
runMe = New ConvertToBase64String(runMe)
runMe = New MoveToNewThread(AddressOf btnConvert_Click_FinishUp, runMe)
runMe = New GetFilePath(runMe)
Dim dataObj As New ConvertFileToBase64Vals()
runMe.RunMe(dataObj)