[Core] Stylesheets on page level now always override parent stylesheets

[csteeg]

Description of Change

To fix issue #5812 I moved the responsibility for appyling a stylesheet to child elements to the Element class.
It would be weird for a stylesheet to get all parent stylesheets of an element and call Apply on them too. I think it's a good thing that StyleSheet.Apply(Element) really only applies it on the given Element? The main issue was that when the ApplyParentStyleSheets was called, the stylesheet of the element itself wasn't applied, so you could never override parent style elements unless you set the stylesheet after the parent was set.

Issues Resolved

• fixes Can't overwrite styles when using CSS #5812

API Changes

None

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

Stylesheets on page level now always override parent stylesheets if they set the same properties. Before this fix this was only happening when the stylesheet was added to the page after the parent was set

Before/After Screenshots

Not applicable

Testing Procedure

Included two new unit test scenario's

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[csteeg]

Hmm, rebase to 4.1 tag seems to include some extra commits .. is that correct or do I need to change something?

[samhouts]

@csteeg Try rebasing again, this time on master. Thanks!

[jfversluis]

/azp run

[azure-pipelines]

No pipelines are associated with this pull request.

[jfversluis]

From the original issue I see two cases defined:

• Styles in the page resources should precede the style in the app
• Styles in a subview should precede the style in the superview

I see cases for the page resource and app scenarios, but not the other one. Would it be useful to add those?

[csteeg]

I added an extra test

[jfversluis]

I see a couple of LINQ queries happening here. Can we do without? It might have impact on the performance here.

[csteeg]

I removed the .concat, but left the .Any(), hope that's ok

[jfversluis]

Build failure is an unrelated (unit) test

[jsuarezruiz]

@csteeg Can you review the conflict?. Thanks!

[csteeg]

@csteeg Can you review the conflict?. Thanks!

Fixed

[csteeg]

Are there actions left that I need to take care off @samhouts and @StephaneDelcroix ?

[samhouts]

@csteeg No, nothing needed from you just yet! Thanks!!

[samhouts]

Build is failing:

"D:\agent\1\s\Xamarin.Forms.sln" (Build target) (1) ->
"D:\agent\1\s\Xamarin.Forms.ControlGallery.Android\Xamarin.Forms.ControlGallery.Android.csproj" (default target) (10) ->
"D:\agent\1\s\Xamarin.Forms.Controls\Xamarin.Forms.Controls.csproj" (default target) (11:2) ->
"D:\agent\1\s\Xamarin.Forms.Build.Tasks\Xamarin.Forms.Build.Tasks.csproj" (default target) (19) ->
"D:\agent\1\s\Xamarin.Forms.Build.Tasks\Xamarin.Forms.Build.Tasks.csproj" (Build target) (19:2) ->
"D:\agent\1\s\Xamarin.Forms.Core\Xamarin.Forms.Core.csproj" (default target) (3:16) ->
  MergedStyle.cs(73,21): error CS1061: 'Element' does not contain a definition for 'ApplyStyleSheetsOnParentSet' and no accessible extension method 'ApplyStyleSheetsOnParentSet' accepting a first argument of type 'Element' could be found (are you missing a using directive or an assembly reference?) [D:\agent\1\s\Xamarin.Forms.Core\Xamarin.Forms.Core.csproj]