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。
答案 0 :(得分:7)
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!
答案 1 :(得分:4)
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...