UWP ScrollViewer headers are loaded twice

[nseveno]

Hi,

In a UWP application, the controls inside the ScrollViewer’s LeftHeader, TopHeader, TopLeftHeader are loaded twice.

Here is the XAML :

<ScrollViewer Background="LightGray"
                  HorizontalScrollMode="Auto"
                  HorizontalScrollBarVisibility="Auto"
                  VerticalScrollMode="Auto"
                  VerticalScrollBarVisibility="Auto">
        <ScrollViewer.TopHeader>
            <Rectangle x:Name="_topRectangle"
                       Fill="DarkGray"
                       Height="32"
                       Width="10000"
                       Loaded="_topRectangle_Loaded"
                       Unloaded="_topRectangle_Unloaded" />
        </ScrollViewer.TopHeader>
        <ScrollViewer.LeftHeader>
            <Rectangle Fill="DarkGray"
                       Width="32"
                       Height="10000"
                       Loaded="_leftRectangle_Loaded"
                       Unloaded="_leftRectangle_Unloaded"/>
        </ScrollViewer.LeftHeader>
        <ScrollViewer.TopLeftHeader>
            <Rectangle Fill="Gray"
                       Height="32"
                       Width="32"
                       Loaded="_topLeftRectangle_Loaded"
                       Unloaded="_topLeftRectangle_Unloaded" />
        </ScrollViewer.TopLeftHeader>
        <TextBlock x:Name="_textBlock"
                   Loaded="_textBlock_Loaded"
                   Unloaded="_textBlock_Unloaded"
                   Text="Test"
                   Width="10000"
                   Height="10000"
                   FontSize="48"
                   Foreground="Blue"
                   HorizontalAlignment="Left"
                   VerticalAlignment="Top" />
    </ScrollViewer>

And for the C# code, I just call Debug.WriteLine in each Loaded and Unloaded events.

Here is the output :

_textBlock_Loaded

  _leftRectangle_Loaded
  _topRectangle_Loaded
  _topLeftRectangle_Loaded

  _leftRectangle_Loaded
  _topRectangle_Loaded
  _topLeftRectangle_Loaded

  _topLeftRectangle_Unloaded
  _topRectangle_Unloaded
  _leftRectangle_Unloaded

For each control, the events are raised in the following order : Loaded, Loaded, Unloaded. And if I remove the ScrollViewer from the visual tree, the Unloaded event is raised once again.

This causes problems in a real application. I have some user controls inside the headers and they don’t like to be double-loaded. The ScrollViewer headers should be loaded only once.

Also I tried to put a Win2D CanvasVirtualControl in the TopHeader property. It does not work, nothing is rendered. I think it is also linked to this double-loading behavior…

[nseveno]

Here is a minimal repro project :

ReproEventsBug.zip

[jevansaks]

Loaded/Unloaded has been painful for a while but it's difficult to fix in a backwards compatible way. We'll be able to address it more completely in WinUI 3.0+. Since Windows 1809 I think we have support for the "IsLoaded" property. @ddalp do we also have the changed notification for this property, or a writeup of the recommended way to use this property to solve the duplicate notifications issue?

[nseveno]

Thanks for the answer.

I can't use the IsLoaded property since I target the 1803 platform.
But a counter incremented in Loaded and decremented in Unloaded does the trick.

I'll wait for the new "flexible" ScrollViewer. It seems to be a lot more customizable...

[github-actions]

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