WPF MVVM未注册的处理程序

我试图注销处理程序在委托命令，因为增加内存大小，所以想到使用弱引用，我希望这可能是代码，如果没有，请亲切电话是什么方式实现它和WPF MVVM未注册的处理程序

重要的是我很急切地想知道如下的代码确实有解释，什么这么多“的网站N'number有这个代码，可惜没有解释。

Imports System 
Imports System.Collections.Generic 
Imports System.Windows 
Imports System.Windows.Input 

Namespace Base 
    ''' <summary> 
    '''  This class allows delegating the commanding logic to methods passed as parameters, 
    '''  and enables a View to bind commands to objects that are not part of the element tree. 
    ''' </summary> 
    Public Class DelegateCommand 
     Implements ICommand 

#Region "Constructors" 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action) 
      Me.New(executeMethod, Nothing, False) 
     End Sub 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action, ByVal canExecuteMethod As Func(Of Boolean)) 
      Me.New(executeMethod, canExecuteMethod, False) 
     End Sub 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action, ByVal canExecuteMethod As Func(Of Boolean), ByVal isAutomaticRequeryDisabled As Boolean) 
      If executeMethod Is Nothing Then 
       Throw New ArgumentNullException("executeMethod") 
      End If 

      _executeMethod = executeMethod 
      _canExecuteMethod = canExecuteMethod 
      _isAutomaticRequeryDisabled = isAutomaticRequeryDisabled 
     End Sub 

#End Region 

#Region "Public Methods" 

     ''' <summary> 
     '''  Method to determine if the command can be executed 
     ''' </summary> 
     Public Function CanExecute() As Boolean 
      If _canExecuteMethod IsNot Nothing Then 
       Return _canExecuteMethod() 
      End If 
      Return True 
     End Function 

     ''' <summary> 
     '''  Execution of the command 
     ''' </summary> 
     Public Sub Execute() 
      If _executeMethod IsNot Nothing Then 
       _executeMethod() 
      End If 
     End Sub 

     ''' <summary> 
     '''  Property to enable or disable CommandManager's automatic requery on this command 
     ''' </summary> 
     Public Property IsAutomaticRequeryDisabled() As Boolean 
      Get 
       Return _isAutomaticRequeryDisabled 
      End Get 
      Set(ByVal value As Boolean) 
       If _isAutomaticRequeryDisabled <> value Then 
        If value Then 
         CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers) 
        Else 
         CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers) 
        End If 
        _isAutomaticRequeryDisabled = value 
       End If 
      End Set 
     End Property 

     ''' <summary> 
     '''  Raises the CanExecuteChaged event 
     ''' </summary> 
     Public Sub RaiseCanExecuteChanged() 
      OnCanExecuteChanged() 
     End Sub 

     ''' <summary> 
     '''  Protected virtual method to raise CanExecuteChanged event 
     ''' </summary> 
     Protected Overridable Sub OnCanExecuteChanged() 
      CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers) 
     End Sub 

#End Region 

#Region "ICommand Members" 

     ''' <summary> 
     '''  ICommand.CanExecuteChanged implementation 
     ''' </summary> 
     Public Custom Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged 
      AddHandler(ByVal value As EventHandler) 
       If Not _isAutomaticRequeryDisabled Then 
        AddHandler CommandManager.RequerySuggested, value 
       End If 
       CommandManagerHelper.AddWeakReferenceHandler(_canExecuteChangedHandlers, value, 2) 
      End AddHandler 
      RemoveHandler(ByVal value As EventHandler) 
       If Not _isAutomaticRequeryDisabled Then 
        RemoveHandler CommandManager.RequerySuggested, value 
       End If 
       CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value) 
      End RemoveHandler 
      RaiseEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) 
      End RaiseEvent 
     End Event 

     Private Function ICommand_CanExecute(ByVal parameter As Object) As Boolean Implements ICommand.CanExecute 
      Return CanExecute() 
     End Function 

     Private Sub ICommand_Execute(ByVal parameter As Object) Implements ICommand.Execute 
      Execute() 
     End Sub 

#End Region 

#Region "Data" 

     Private ReadOnly _executeMethod As Action = Nothing 
     Private ReadOnly _canExecuteMethod As Func(Of Boolean) = Nothing 
     Private _isAutomaticRequeryDisabled As Boolean = False 
     Private _canExecuteChangedHandlers As List(Of WeakReference) 

#End Region 
    End Class 

    ''' <summary> 
    '''  This class allows delegating the commanding logic to methods passed as parameters, 
    '''  and enables a View to bind commands to objects that are not part of the element tree. 
    ''' </summary> 
    ''' <typeparam name="T">Type of the parameter passed to the delegates</typeparam> 
    Public Class DelegateCommand(Of T) 
     Implements ICommand 

#Region "Constructors" 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action(Of T)) 
      Me.New(executeMethod, Nothing, False) 
     End Sub 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action(Of T), ByVal canExecuteMethod As Func(Of T, Boolean)) 
      Me.New(executeMethod, canExecuteMethod, False) 
     End Sub 

     ''' <summary> 
     '''  Constructor 
     ''' </summary> 
     Public Sub New(ByVal executeMethod As Action(Of T), ByVal canExecuteMethod As Func(Of T, Boolean), ByVal isAutomaticRequeryDisabled As Boolean) 
      If executeMethod Is Nothing Then 
       Throw New ArgumentNullException("executeMethod") 
      End If 

      _executeMethod = executeMethod 
      _canExecuteMethod = canExecuteMethod 
      _isAutomaticRequeryDisabled = isAutomaticRequeryDisabled 
     End Sub 

#End Region 

#Region "Public Methods" 

     ''' <summary> 
     '''  Method to determine if the command can be executed 
     ''' </summary> 
     Public Function CanExecute(ByVal parameter As T) As Boolean 
      If _canExecuteMethod IsNot Nothing Then 
       Return _canExecuteMethod(parameter) 
      End If 
      Return True 
     End Function 

     ''' <summary> 
     '''  Execution of the command 
     ''' </summary> 
     Public Sub Execute(ByVal parameter As T) 
      If _executeMethod IsNot Nothing Then 
       _executeMethod(parameter) 
      End If 
     End Sub 

     ''' <summary> 
     '''  Raises the CanExecuteChaged event 
     ''' </summary> 
     Public Sub RaiseCanExecuteChanged() 
      OnCanExecuteChanged() 
     End Sub 

     ''' <summary> 
     '''  Protected virtual method to raise CanExecuteChanged event 
     ''' </summary> 
     Protected Overridable Sub OnCanExecuteChanged() 
      CommandManagerHelper.CallWeakReferenceHandlers(_canExecuteChangedHandlers) 
     End Sub 

     ''' <summary> 
     '''  Property to enable or disable CommandManager's automatic requery on this command 
     ''' </summary> 
     Public Property IsAutomaticRequeryDisabled() As Boolean 
      Get 
       Return _isAutomaticRequeryDisabled 
      End Get 
      Set(ByVal value As Boolean) 
       If _isAutomaticRequeryDisabled <> value Then 
        If value Then 
         CommandManagerHelper.RemoveHandlersFromRequerySuggested(_canExecuteChangedHandlers) 
        Else 
         CommandManagerHelper.AddHandlersToRequerySuggested(_canExecuteChangedHandlers) 
        End If 
        _isAutomaticRequeryDisabled = value 
       End If 
      End Set 
     End Property 

#End Region 

#Region "ICommand Members" 

     ''' <summary> 
     '''  ICommand.CanExecuteChanged implementation 
     ''' </summary> 
     Public Custom Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged 
      AddHandler(ByVal value As EventHandler) 
       If Not _isAutomaticRequeryDisabled Then 
        AddHandler CommandManager.RequerySuggested, value 
       End If 
       CommandManagerHelper.AddWeakReferenceHandler(_canExecuteChangedHandlers, value, 2) 
      End AddHandler 
      RemoveHandler(ByVal value As EventHandler) 
       If Not _isAutomaticRequeryDisabled Then 
        RemoveHandler CommandManager.RequerySuggested, value 
       End If 
       CommandManagerHelper.RemoveWeakReferenceHandler(_canExecuteChangedHandlers, value) 
      End RemoveHandler 
      RaiseEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) 
      End RaiseEvent 
     End Event 

     Private Function ICommand_CanExecute(ByVal parameter As Object) As Boolean Implements ICommand.CanExecute 
      ' if T is of value type and the parameter is not 
      ' set yet, then return false if CanExecute delegate 
      ' exists, else return true 
      If parameter Is Nothing AndAlso GetType(T).IsValueType Then 
       Return (_canExecuteMethod Is Nothing) 
      End If 
      Return CanExecute(DirectCast(parameter, T)) 
     End Function 

     Private Sub ICommand_Execute(ByVal parameter As Object) Implements ICommand.Execute 
      Execute(DirectCast(parameter, T)) 
     End Sub 

#End Region 

#Region "Data" 

     Private ReadOnly _executeMethod As Action(Of T) = Nothing 
     Private ReadOnly _canExecuteMethod As Func(Of T, Boolean) = Nothing 
     Private _isAutomaticRequeryDisabled As Boolean = False 
     Private _canExecuteChangedHandlers As List(Of WeakReference) 

#End Region 
    End Class 

    ''' <summary> 
    '''  This class contains methods for the CommandManager that help avoid memory leaks by 
    '''  using weak references. 
    ''' </summary> 
    Friend Class CommandManagerHelper 
     Friend Shared Sub CallWeakReferenceHandlers(ByVal handlers As List(Of WeakReference)) 
      If handlers IsNot Nothing Then 
       ' Take a snapshot of the handlers before we call out to them since the handlers 
       ' could cause the array to me modified while we are reading it. 

       Dim callees(handlers.Count - 1) As EventHandler 
       Dim count As Integer = 0 

       For i As Integer = handlers.Count - 1 To 0 Step -1 
        Dim reference As WeakReference = handlers(i) 
        Dim handler As EventHandler = TryCast(reference.Target, EventHandler) 
        If handler Is Nothing Then 
         ' Clean up old handlers that have been collected 
         handlers.RemoveAt(i) 
        Else 
         callees(count) = handler 
         count += 1 
        End If 
       Next i 

       ' Call the handlers that we snapshotted 
       For i As Integer = 0 To count - 1 
        Dim handler As EventHandler = callees(i) 
        handler(Nothing, EventArgs.Empty) 
       Next i 
      End If 
     End Sub 

     Friend Shared Sub AddHandlersToRequerySuggested(ByVal handlers As List(Of WeakReference)) 
      If handlers IsNot Nothing Then 
       For Each handlerRef As WeakReference In handlers 
        Dim handler As EventHandler = TryCast(handlerRef.Target, EventHandler) 
        If handler IsNot Nothing Then 
         AddHandler CommandManager.RequerySuggested, handler 
        End If 
       Next handlerRef 
      End If 
     End Sub 

     Friend Shared Sub RemoveHandlersFromRequerySuggested(ByVal handlers As List(Of WeakReference)) 
      If handlers IsNot Nothing Then 
       For Each handlerRef As WeakReference In handlers 
        Dim handler As EventHandler = TryCast(handlerRef.Target, EventHandler) 
        If handler IsNot Nothing Then 
         RemoveHandler CommandManager.RequerySuggested, handler 
        End If 
       Next handlerRef 
      End If 
     End Sub 

     Friend Shared Sub AddWeakReferenceHandler(ByRef handlers As List(Of WeakReference), ByVal handler As EventHandler) 
      AddWeakReferenceHandler(handlers, handler, -1) 
     End Sub 

     Friend Shared Sub AddWeakReferenceHandler(ByRef handlers As List(Of WeakReference), ByVal handler As EventHandler, ByVal defaultListSize As Integer) 
      If handlers Is Nothing Then 
       handlers = (If(defaultListSize > 0, New List(Of WeakReference)(defaultListSize), New List(Of WeakReference)())) 
      End If 

      handlers.Add(New WeakReference(handler)) 
     End Sub 

     Friend Shared Sub RemoveWeakReferenceHandler(ByVal handlers As List(Of WeakReference), ByVal handler As EventHandler) 
      If handlers IsNot Nothing Then 
       For i As Integer = handlers.Count - 1 To 0 Step -1 
        Dim reference As WeakReference = handlers(i) 
        Dim existingHandler As EventHandler = TryCast(reference.Target, EventHandler) 
        If (existingHandler Is Nothing) OrElse (existingHandler Is handler) Then 
         ' Clean up old handlers that have been collected 
         ' in addition to the handler that is to be removed. 
         handlers.RemoveAt(i) 
        End If 
       Next i 
      End If 
     End Sub 
    End Class 
End Namespace

来源

2013-12-17 Selva

假设CanExecuteChanged事件这种方式来实现：

Private _handlers As New List(Of EventHandler)() 

Public Custom Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged 
    AddHandler(ByVal value as EventHandler) 
     _handlers.Add(value) 
    End AddHandler 
    RemoveHandler(ByVal value As EventHandler) 
     _handlers.Remove(value) 
    End RemoveHandler 
    RaiseEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) 
    End RaiseEvent 
End Event

当一个对象听CanExecuteChanged，其处理程序传递给AddHandler并添加到列表中。这基本上是如何实施标准事件的。

假设2个按钮都绑定到相同的命令。他们都听CanExecuteChanged，以便他们可以更新其启用/禁用状态。现在，你必须引用这样的：

handler1 -> button1 
handler2 -> button2 
button1 -> command -> _handlers {handler1, handler2} 
button2 -> command -> _handlers {handler1, handler2}

如果button2从屏幕上消失，command仍然在内存中，因为button1仍然绑定到它。这意味着_handlers还在内存中，其中包含handler2。 handler2参考button2，所以button2必须保留在内存中，即使它不应该是因为它不再需要在屏幕上。这是浪费的内存问题可以来自哪里。

事件可以实现这种方式，虽然：

Public Custom Event CanExecuteChanged As EventHandler Implements ICommand.CanExecuteChanged 
    AddHandler(ByVal value as EventHandler) 
     AddHandler CommandManager.RequerySuggested, value 
    End AddHandler 
    RemoveHandler(ByVal value As EventHandler) 
     RemoveHandler CommandManager.RequerySuggested, value 
    End RemoveHandler 
    RaiseEvent(ByVal sender As System.Object, ByVal e As System.EventArgs) 
    End RaiseEvent 
End Event

在此实现，该命令不存储处理程序的列表，所以上述情况是不可能发生的。处理程序被传递给CommandManager.RequerySuggested。换句话说，当一个按钮请求收听CanExecuteChanged时，它实际上正在收听RequerySuggested。这通常是可以接受的，因为经常引发RequerySuggested，处理程序很简单，不会经常被调用而影响性能。

由于CommandManager.RequerySuggested使用弱引用，所以在这里也没有需要担心的内存问题。

您只需要一个处理程序列表（或处理程序的弱引用列表）就是如果您想使用RaiseCanExecuteChanged来调用它们而不是依靠CommandManager。

来源

2013-12-19 17:26:08 nmclean

@nmclean .. thanx很多..我没有得到关于“RaiseCanExecuteChanged”和“CommandManager”的最后两行。请您解释一下。 – Selva

@Selva在我的代码中，处理程序只会在引发'RequerySuggested'时从'CommandManager'中调用。要从命令本身调用它们，您还需要像原始代码一样在命令中存储它们的列表，并在'CanExecuteChanged'的'RaiseEvent'块中调用它们中的每一个。 – nmclean

WPF MVVM未注册的处理程序

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