ICommand CanExecuteChanged not updating

问题

I am trying for MVVM pattern basic level and got struck at ICommand CanExecute changed. I have XAML binding as follows:

    <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" />

Commands:

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

My ModelView:

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

The problem is that the Canexecutechanged is firing only at start, I actually want to get the delete button enabled only when something in the listbox is selected, and I want to get it done by MVVM - ICommand binding method. Could you please explain where i went wrong or miss understood the ICommand implementation.

Thank you.

Updated Relay iCommand code I use:

    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

Most of the code is a copy, but I understood the working by below comments.

回答1:

You must have in your ICommand implementation some method like RiseCanExecuteChanged that fires the event CanExecuteChanged. Then every time that the selected item in the list changed, in your view model you execute the RaiseCanExecuteChanged from the command you want. Any way I suggest you to use any generic command, like the RelayCommand of GalaSoft MVVMLite library, or any implementation of DelegateCommand. Hope this helps...

回答2:

As Raul Otaño has pointed out, you can raise the CanExecuteChanged. However, not all MVVM frameworks provide a RaiseCanExecuteChanged method. It's also worth noting that the actual event CanExecuteChanged must be called on the UI thread. So, if you're expecting a callback from some thread in your model, you need to invoke it back to the UI thread, like this:

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

I would very much recommend against calling CommandManager.InvalidateRequerySuggested() because although this works functionally, and is ok for small applications, it is indiscriminate and will potentially re-query every command! In a large system with lots of commands, this can be very very slow!

来源：https://stackoverflow.com/questions/19657463/icommand-canexecutechanged-not-updating