Wpf 在后台线程上填充面板时未触发CanExcuteOutDevenHandler

WPF窗口的CommandBinding上绑定的按钮：

XAML

CanExecuteRouteEvent的处理程序定义如下：

Private Sub hndlCmdRenderData(ByVal sender As Object, ByVal e As ExecutedRoutedEventArgs)
       '...
End Sub
'(3)
Private Sub hndlCmdCanRenderData(ByVal sender As Object, ByVal e As CanExecuteRoutedEventArgs)
        e.CanExecute = pnlData.Children.Count > 1
End Sub

这意味着，如果主面板包含数据，例如另一个UIElement，则该按钮处于活动状态。面板的UiElement填充在背景线程上

Private Sub renderData()
        _clearPanel()'clears pnlData (1)
        m_UielementToLoad = New ctlPDFViewer()'the UiElement, a global variable
        Dim rep As New clsPDFRenderer(Me.Dispatcher)'the class performing the data stuff in background thread
        AddHandler rep.ScreenDone, AddressOf _threadScreenRendered'Address of the sub invoked in thread
        m_ActiveScreenThread = New Threading.Thread(AddressOf rep.Post)
        m_ActiveScreenThread.Start()
 End Sub

后台线程末尾调用的子线程定义如下：

Private Sub hndlCmdRenderData(ByVal sender As Object, ByVal e As ExecutedRoutedEventArgs)
       '...
End Sub
'(3)
Private Sub hndlCmdCanRenderData(ByVal sender As Object, ByVal e As CanExecuteRoutedEventArgs)
        e.CanExecute = pnlData.Children.Count > 1
End Sub

私有子线程ScreenRendered（发送者作为对象，屏幕作为字符串）
CType（m_UielementToLoad，ctlPDFViewer）.SetDocumentData（screen）'全局UIElement由后台线程的数据填充
pnlData.Children.Add（m_UielementToLoad）'UIElement作为子元素添加到面板（2）
pnlData.Children（0）.SetValue（DockPanel.DockProperty，Dock.Left）
端接头

如果面板数据已填充，则按钮处于活动状态。如果我启动子“renderData（）”并执行第（1）行（清除面板），按钮将立即变为非活动状态-CanExecute处理程序（3）将被触发，线程将启动，子“threadScreenRendered（..”）将在最后被调用。面板被填满（2），但CanExecute处理程序（3）未被触发，按钮保持非活动状态。 仅在用鼠标单击一次后，按钮才会激活。

我看不出第（1）行和第（2）行之间有什么不对劲或区别

Private Sub renderData()
        _clearPanel()'clears pnlData (1)
        m_UielementToLoad = New ctlPDFViewer()'the UiElement, a global variable
        Dim rep As New clsPDFRenderer(Me.Dispatcher)'the class performing the data stuff in background thread
        AddHandler rep.ScreenDone, AddressOf _threadScreenRendered'Address of the sub invoked in thread
        m_ActiveScreenThread = New Threading.Thread(AddressOf rep.Post)
        m_ActiveScreenThread.Start()
 End Sub