WPF MVVM did not register handlers

Question

I’m trying to unregister the handlers in the Delegate command due to the increase in memory, so I thought about using a weak link, I hope this can be code, and if not, please, to achieve this and

Importantly, I really want to know what the explanation code below does , so many "N'number sites" have this code, unfortunately, without explanation.

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 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 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

Answer 1

Suppose the event CanExecuteChangedwas implemented as follows:

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

CanExecuteChanged, AddHandler . , .

, . CanExecuteChanged, Enabled/Disabled. :

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

button2 , command , button1 . , _handlers , handler2. handler2 button2, button2 , , . .

:

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

, . CommandManager.RequerySuggested. , CanExecuteChanged, RequerySuggested. , RequerySuggested , , , .

, CommandManager.RequerySuggested .

, ( ), - RaiseCanExecuteChanged , CommandManager.