fix(iOS): styles resolution

[ajpinedam]

GitHub Issue (If applicable): closes #19825

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Style resolution only works from immediate parents. Any style at the Page level or above the direct parent is not being recognized.

What is the new behavior?

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• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
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• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
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Other information

Internal Issue (If applicable):

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Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

[ajpinedam]

First of all, the issue reported is happening when within a ScrollViewer

Reason:

When, doing the resource resolution, we had this code:

while (candidate is not null)
{
    if (candidate is FrameworkElement fe)
    {
        if (fe.TryGetResources() is { IsEmpty: false }) // It's legal (if pointless) on UWP to set Resources to null from user code, so check
        {
            yield return fe.Resources;
        }
        candidate = fe.Parent as FrameworkElement; // [Marked Line] Issue is here
    }
    else
    {
        candidate = VisualTreeHelper.GetParent(candidate) as DependencyObject;
    }
}

On the marked line, we expect the Parent to be a FrameworkElement. When this is not the case, the candidate becomes null, forcing the code to exit the while loop, and preventing us from going higher up in the visual tree.

In our case specifically, for the ScrollViewer, this has a NativeScrollContentPresenter as one of its Children. The NativeScrollContentPresenter is a DependencyObject but not a FrameworkElement.

So for the following XAML tree:

  <Page.Resources>
    <Style TargetType="Button">
      <Setter Property="Background" Value="Red"/>
    </Style>
  </Page.Resources>

  <ScrollViewer>
    <ScrollViewer.Resources>
      <Style TargetType="Button">
        <Setter Property="Background" Value="Red"/>
      </Style>
    </ScrollViewer.Resources>
    <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center" utu:SafeArea.Insets="VisibleBounds">
      <Button Content="Test Content"/>
    </StackPanel>
  </ScrollViewer>
</Page>

When we reached the point of navigating from the StackPanel up to the ScrollViewer, we stopped the style resolution at the NativeScrollContentPresenter and never reached the ScrollViewer or the Page.

The solution proposed is to remove the casting to FrameworkElement

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[unodevops]

⚠️⚠️ The build 159879 has failed on Uno.UI - CI.

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⚠️⚠️ The build 159904 has failed on Uno.UI - CI.

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[unodevops]

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[jeromelaban]

@Mergifyio backport release/stable/5.6

[mergify]

backport release/stable/5.6

✅ Backports have been created

• #19862 fix(iOS): styles resolution (backport #19841) has been created for branch release/stable/5.6