当 TextBox 获得焦点时，为什么 WPF 网格的可见性会延迟？

Question

在以下 XAML 中，editPanel 始终可见。仅当按 F5 键开始长时间操作时，overlayGrid 才可见。视觉效果是，如果过程很长，则 editPanel 会变灰。

<Window.InputBindings>
    <KeyBinding Key="F5" Command="{Binding Path=RefreshCommand}"/>
</Window.InputBindings>

<Grid>

    <StackPanel Name="editPanel">
        <TextBox>set focus here to see the problem.</TextBox>
        <CheckBox>set focus here to remove the problem.</CheckBox>
        <TextBlock Text="{Binding Path=Worker.Message}"/>
    </StackPanel>

    <Grid Name="overlayGrid" 
          Visibility="{Binding Path=Worker.Visibility}"
          Background="Gray" Opacity="0.5">

        <TextBox Text="{Binding Path=Worker.Message}" FontWeight="Bold" 
                 HorizontalAlignment="Center" VerticalAlignment="Center" 
                 />

    </Grid>

</Grid>

除了 TextBox 具有焦点时之外，overlayGrid 完全按照预期显示。如果文本框具有焦点，则在您看到overlayGrid 快速闪烁之前会发生长时间操作。就好像代码是：执行长时间操作，显示overlayGrid，折叠overlayGrid。

执行长操作并更改overlayGrid可见性的ViewModel代码如下：

Sub Refresh()

    Me.Worker.Message = String.Format("Refresh started at {0}..", 
                                      Date.Now.ToString("hh:mm:ss.fff")
    )

    Me.Worker.Visibility = Visibility.Visible

    ' Give the UI a chance to update itself.
    System.Windows.Forms.Application.DoEvents()

    Console.WriteLine("Debug: " + Me.Worker.Message)

    ' Fake the long operation.
    System.Threading.Thread.Sleep(1000)

    Me.Worker.Message = String.Format("Refresh completed at {0}.", 
                                      Date.Now.ToString("hh:mm:ss.fff")
    )

    Me.Worker.Visibility = Visibility.Collapsed

    Console.WriteLine("Debug: " + Me.Worker.Message)

End Sub

为什么当TextBox获得焦点时overlayGrid的实际可见性会延迟？我该如何解决这个问题？

Answer 1

AFAIK，即使在 WinForms 中也不鼓励使用 System.Windows.Forms.Application.DoEvents() 。您当然不应该在 WPF 中使用它，即使它有效（显然，它不起作用）。

您应该做的是在后台线程上运行长操作，然后使用 Dispatcher.Invoke() 在 UI 上设置结果。像这样的东西：

Sub Refresh()

    Me.Worker.Message = String.Format("Refresh started at {0}..",
                                      Date.Now.ToString("hh:mm:ss.fff")
    )

    Me.Worker.Visibility = Visibility.Visible

    Console.WriteLine("Debug: " + Me.Worker.Message)

    Task.Factory.StartNew(
        Function()
            ' Fake the long operation.
            System.Threading.Thread.Sleep(10000)

            Dispatcher.Invoke(
                Function()
                    Me.Worker.Message = String.Format("Refresh completed at {0}.",
                                                      Date.Now.ToString("hh:mm:ss.fff")
                    )

                    Me.Worker.Visibility = Visibility.Collapsed

                    Console.WriteLine("Debug: " + Me.Worker.Message)

                End Function)

        End Function)

End Sub