How to move a WPF UIElement from the visual tree to a FixedPage?

问题

My MVVM application is using on-screen visual objects to render screen content to a printed document. My View has a ContentControl that uses DataTemplate resources to determine what to display, but when I try to add that content to a FixedPage object, I get an ArgumentException with the message:

Specified Visual is already a child of another Visual or the root of a CompositionTarget.

The (simplified) XAML code for the view that handles the printing looks like this:

<DockPanel>
    <!-- Print button -->
    <Button DockPanel.Dock="Top"
            DataContext="{Binding Path=PrintCommand}"
            Click="PrintButton_Click" />
    <!-- Print container. -->
    <Border BorderBrush="Black" BorderThickness="1" DockPanel.Dock="Top" >
        <!-- The visual content of this is what needs adding to the FixedPage.  -->
        <ContentControl x:Name="printContentControl"
                        Content="{Binding Path=PrintMe}" />
    </Border>
</DockPanel>

... and the code behind for the print button click handler that has the problem looks like this:

private void PrintButton_Click(object sender, RoutedEventArgs e)
{
    // Get a reference to the Visual objects to be printed.
    var content = VisualTreeHelper.GetChild(printContentControl, 0);
    Debug.Assert((content as UIElement) != null, "Print content is not a UIElement");

#region This doesn't work.  See below for other things that also don't work.
    printContentControl.Content = null;
    // The line below doesn't help, but tried it in case the framework
    // needed a poke to get it to update the visual tree.
    UpdateLayout();
#endregion

    // Make sure that all the data binding, layout, etc. has completed
    // before the visual object is printed by queuing the print job at
    // a later priority.
    Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new Action(delegate()
    {
        // Create a fixed page to hold the content to print.
        FixedPage page = new FixedPage();

        // Add the content to print to the page.
#region Here be dragons: 'System.ArgumentException'
        page.Children.Add(content as UIElement);
#endregion

        // Send the document back to the View model for actual printing.
        var command = ((sender as Button).DataContext as ICommand);
        command.Execute(page);
    }));
}

I've tried the following things to solve the problem, all with no success:

Setting the Content member of the ContentControl to null (as in the code above).
Setting this.Content=null. Screen goes blank, but still get the exception.
Using RemoveLogicalChild() on various controls. None of which worked.
Using RemoveVisualChild() on the printContentControl and the content object. Neither of those were children of the current object.
Using DataTriggers in the XAML to replace the template when the Content or DataContext of the ContentControl is `{x:Null}'. No effect.

It doesn't matter if the appearance of the screen is messed up, as the ViewModel changes the View when its print command is executed.

How can I move the printContentControl's visual content (or the entire control) from the View's visual tree to the ViewModel's FixedPage object?

Dirty work-around

By creating a wrapper class for the content to be printed, I can make the functionality work, but it doesn't answer the original question.

Wrapper class

public class PrintContentControl : UserControl
{
    internal UIElement GetPrintContent()
    {
        UIElement printContent = Content as UIElement;
        Content = null;
        return printContent;
    }
}

Updated XAML using the wrapper to hold the printable content

<Border BorderBrush="Black" BorderThickness="1" DockPanel.Dock="Top" >
    <c:PrintContentControl x:Name="printContent">
        <ContentControl ContentTemplate="{StaticResource ReportTemplate}"
                        Content="{Binding Path=PrintMe}" />
    </c:PrintContentControl>
</Border>

Updated code behind to get the printable content from the wrapper

private void PrintButton_Click(object sender, RoutedEventArgs e)
{
    // Use the wrapper to get the Visual objects to be printed.
    UIElement content = printContent.GetPrintContent();
    Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new Action(delegate()
    {
        // Works fine now.
        FixedPage page = new FixedPage();
        page.Children.Add(content as UIElement);
        var command = ((sender as Button).DataContext as ICommand);
        command.Execute(page);
    }));
}

来源：https://stackoverflow.com/questions/41999906/how-to-move-a-wpf-uielement-from-the-visual-tree-to-a-fixedpage

标签

wpf

visual-tree