WPF MVVM取消注册处理程序

发布时间：2020-12-17 07:13:45 所属栏目：百科来源：网络整理

导读：我试图在Delegate命令中取消注册处理程序,因为增加了内存大小,所以想到使用弱引用,我希望这可能是代码,如果没有,请告知tel实现它的方法是什么, 重要的是,我非常渴望知道下面给出的代码做了什么解释, 很多’N’个网站都有这个代码,遗憾的是没有解释. Imports

我试图在Delegate命令中取消注册处理程序,因为增加了内存大小,所以想到使用弱引用,我希望这可能是代码,如果没有,请告知tel实现它的方法是什么,

重要的是,我非常渴望知道下面给出的代码做了什么解释,
很多’N’个网站都有这个代码,遗憾的是没有解释.

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,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,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,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,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

解决方法

假设CanExecuteChanged事件以这种方式实现：

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

当对象侦听CanExecuteChanged时,其处理程序将传递给AddHandler并添加到列表中.这基本上是标准事件的实现方式.

假设2个按钮都绑定到同一个命令.它们都监听CanExecuteChanged,以便更新其启用/禁用状态.现在你有这样的参考：

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

如果从屏幕上删除了button2,则命令仍在内存中,因为button1仍然绑定到它.这意味着_handlers仍然在内存中,其中包含handler2. handler2引用button2,所以button2必须保留在内存中,即使它不应该,因为它不再需要在屏幕上.这就是浪费内存问题可能来自的地方.

但是,事件可以通过这种方式实现：

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

在此实现中,该命令不存储处理程序列表,因此上述情况不会发生.而是将处理程序传递给CommandManager.RequerySuggested.换句话说,当一个按钮请求监听CanExecuteChanged时,它实际上将监听RequerySuggested.这通常是可以接受的,因为RequerySuggested经常被提升,并且处理程序很简单,不会通过经常调用来影响性能.

这里也没有内存问题需要担心,因为CommandManager.RequerySuggested使用弱引用.

只有当你想要使用RaiseCanExecuteChanged来调用它们而不是依赖于CommandManager时,才需要处理程序列表(或者对处理程序的弱引用列表).

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!