Question

我正在尝试MVVM模式的基本级别，并且在ICommand CanExecute发生了变化。我有如下XAML绑定：

    <ListBox ItemsSource="{Binding Contact.Addresses}"  x:Name="AddressCollections" Height="152" SelectedValue="{Binding SelectedAddress}"
             DockPanel.Dock="Top" />
    <Button Content="Add" Command="{Binding AddAddressCommand}" DockPanel.Dock="Top" />
    <Button Content="Remove" Command="{Binding DeleteAddressCommand}" DockPanel.Dock="Bottom" />

命令：

Public Class DeleteCommand
Implements ICommand

Private method As Object
Private methodname As String

Public Sub New(ByVal Controlname As String, ByVal mee As Object)
    methodname = Controlname
    method = mee
End Sub

Public Function CanExecute(parameter As Object) As Boolean Implements ICommand.CanExecute
    Select Case methodname
        Case "Address"
            Return TryCast(method, ModelView.Contacts.ContactMV).CanDeleteAddress()
        Case "Numbers"
            Return TryCast(method, ModelView.Contacts.ContactMV).CanDeleteNumbers
        Case Else : Return False
    End Select
End Function

Public Event CanExecuteChanged(sender As Object, e As EventArgs) Implements ICommand.CanExecuteChanged

Public Sub Execute(parameter As Object) Implements ICommand.Execute
    Select Case methodname
        Case "Address"
            TryCast(method, ModelView.Contacts.ContactMV).DeleteAddress()
        Case "Numbers"
            TryCast(method, ModelView.Contacts.ContactMV).DeleteNumbers()
        Case Else

    End Select
End Sub
End Class

我的模型视图：

Public Class ContactMV

Property Contact As Model.Contacts.ContactMod
Property AddAddressCommand As New Commands.AddCommand("Address", Me)
Property DeleteAddressCommand As New Commands.DeleteCommand("Address", Me)
Property SelectedAddress As Model.Contacts.AddressModel
Public Sub AddAddress()
    If Contact.Addresses.Count = 0 Then
        Contact.Addresses.Add(New Model.Contacts.AddressModel(Contact.Primary.ID, True))
    Else
        Contact.Addresses.Add(New Model.Contacts.AddressModel(Contact.Primary.ID, False))
    End If

End Sub
Public Sub DeleteAddress()
    If IsNothing(SelectedAddress) = False Then
        Try
            Contact.Addresses.Remove(SelectedAddress)
        Catch ex As Exception
            MsgBox("Address not found")
        End Try
    End If
End Sub
Public Function CanDeleteAddress()
    If IsNothing(SelectedAddress) Then
        Return False
    Else
        Return Contact.Addresses.Contains(SelectedAddress)
    End If
End Function
End Class

问题是Canexecutechanged仅在启动时触发，我实际上只想在选择列表框中的某些内容时启用删除按钮，并且我想通过MVVM-ICommand绑定方法完成它。你能否解释我哪里出错或错过理解ICommand的实施。

谢谢。

更新了我使用的Relay iCommand代码：

    Public Class RelayCommand
        Implements ICommand
        ''' <summary>
        ''' A command whose sole purpose is to relay its functionality to other objects by invoking delegates. The default return value for the CanExecute method is 'true'.
        ''' </summary>
        ''' <remarks></remarks>

#Region "Declarations"
        Private ReadOnly _CanExecute As Func(Of Boolean)
        Private ReadOnly _Execute As Action
#End Region

#Region "Constructors"
        Public Sub New(ByVal execute As Action)
            Me.New(execute, Nothing)
        End Sub

        Public Sub New(ByVal execute As Action, ByVal canExecute As Func(Of Boolean))
            If execute Is Nothing Then
                Throw New ArgumentNullException("execute")
            End If
            _Execute = execute
            _CanExecute = canExecute
        End Sub
#End Region

#Region "ICommand"
        Public Custom Event CanExecuteChanged As EventHandler Implements System.Windows.Input.ICommand.CanExecuteChanged

            AddHandler(ByVal value As EventHandler)
                If _CanExecute IsNot Nothing Then
                    AddHandler CommandManager.RequerySuggested, value
                End If
            End AddHandler
            RemoveHandler(ByVal value As EventHandler)

                If _CanExecute IsNot Nothing Then
                    RemoveHandler CommandManager.RequerySuggested, value
                End If
            End RemoveHandler

            RaiseEvent(ByVal sender As Object, ByVal e As System.EventArgs)
                'This is the RaiseEvent block
                'CommandManager.InvalidateRequerySuggested()
            End RaiseEvent
        End Event

        Public Function CanExecute(ByVal parameter As Object) As Boolean Implements System.Windows.Input.ICommand.CanExecute
            If _CanExecute Is Nothing Then
                Return True
            Else
                Return _CanExecute.Invoke
            End If
        End Function

        Public Sub Execute(ByVal parameter As Object) Implements System.Windows.Input.ICommand.Execute
            _Execute.Invoke()
        End Sub
#End Region
    End Class

大多数代码都是副本，但我理解通过以下注释工作。

Answer 1

正如RaulOtaño所指出的那样，你可以提高CanExecuteChanged。但是，并非所有MVVM框架都提供RaiseCanExecuteChanged方法。值得注意的是，必须在UI线程上调用实际事件CanExecuteChanged。因此，如果您期望从模型中的某个线程进行回调，则需要将其调用回UI线程，如下所示：

    public void RaiseCanExecuteChanged()
    {
        if (CanExecuteChanged != null)
        {
            Application.Current.Dispatcher.Invoke((Action)(() => { CanExecuteChanged(this, EventArgs.Empty); }));
        }
    }

我非常建议不要调用CommandManager.InvalidateRequerySuggested()，因为尽管这在功能上有效，并且适用于小型应用程序，但它是不分青红皂白的，并且可能会重新查询每个命令！在一个包含大量命令的大型系统中，这可能非常慢！

Answer 2

您必须在ICommand实现中使用RiseCanExecuteChanged等方法来触发事件CanExecuteChanged。然后，每次更改列表中的选定项目时，在视图模型中，您都可以从所需的命令执行RaiseCanExecuteChanged。我建议您使用任何通用命令，例如GalaSoft MVVMLite库的RelayCommand或DelegateCommand的任何实现。希望这会有所帮助...

ICommand CanExecuteChanged没有更新

2 个答案: