The VisualBrush of WPF only refresh the visual but not the layout

Now we’ll talk about a behavior of WPF VisualBrush. Maybe it is a bug, but let’s view some details and determine whether it is or not.

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The reproduction code

Let’s make a XAML layout to reproduce such an issue.

We have a large Grid which contains an inner Grid and an inner Border. The Grid contains a Rectangle which is as large as the Grid itself and a TextBlock which presents some contents. The Border only shows its background which a VisualBrush of the Grid.

This is the whole XAML file:

<Window x:Class="Walterlv.Demo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Walterlv 的 WindowChrome 示例窗口" Height="450" Width="800"
        WindowStartupLocation="CenterScreen">
    <Grid>
        <Grid x:Name="VisualSource">
            <Rectangle x:Name="VisibleOr" Fill="LightCoral" Visibility="Visible" />
            <TextBlock FontSize="24" TextAlignment="Center" VerticalAlignment="Center">
                <Run Text="I'm walterlv, " />
                <LineBreak />
                <Run Text="I'm reproducing this Visual bug." />
            </TextBlock>
        </Grid>
        <Border>
            <Border.Background>
                <VisualBrush Visual="{Binding Source={x:Reference VisualSource}}" />
            </Border.Background>
        </Border>
    </Grid>
</Window>

This is the code-behind. Notice that it changes the visibility of the Rectangle every 1 second.

using System.Threading.Tasks;
using System.Windows;

namespace Walterlv.Demo
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Loaded += OnLoaded;
        }

        private async void OnLoaded(object sender, RoutedEventArgs e)
        {
            while (true)
            {
                await Task.Delay(1000);
                VisibleOr.Visibility = Visibility.Collapsed;
                await Task.Delay(1000);
                VisibleOr.Visibility = Visibility.Visible;
            }
        }
    }
}

To verify the issue

We know that the VisualBrush shows and stretch the whole Visual so we can predicate only two results:

• If the Rectangle is visible with Visibility property Visible, the Border background which contains the VisualBrush will be exactly the same with the Grid.
• If the Rectangle is invisible with Visibility property Collapsed, the Border background which contains the VisualBrush will stretch the whole content to the Border without any area of the Rectangle.

But it is the real result?

The animation picture below shows the result when the Rectangle is visible as the startup:

The animation picture below shows the result when the Rectangle is invisible as the startup:

Did you notice that?

Only at the very beginning when the program runs it behaves the same as we predicted. But when we change the visibility of the Rectangle the layout never changes.

The issue?

I’ve fired this issue into GitHub and this is the link:

• The VisualBrush only refresh the visual but not the layout when the Visual visibility changes · Issue #1241 · dotnet/wpf

本文会经常更新，请阅读原文： https://blog.walterlv.com/post/visual-brush-refresh-views-only-but-not-layout-en.html ，以避免陈旧错误知识的误导，同时有更好的阅读体验。

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