[GH-10729] Added Current Page in Shell

[pictos]

Description of Change

This PR provides an easy way to access the PresentedPage in Shell applications.

Issues Resolved

• fixes [Enhancement] Provide an easy way to get the Current Page at Shell #10729

API Changes

Added:

• Page Shell.CurrentPage { get; }
• Shell : IPageContainer<Page>

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Create an AppShell, and access the Shell.Current.CurrentPage, its value should be the presented page. Also, I added some unit tests.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[StephaneDelcroix]

@PureWeen opinion on this ?

[gresolio]

Definitely a very useful enhancement, it would be nice to have access to the PresentedPage and stop using workarounds like this: (Shell.Current?.CurrentItem?.CurrentItem as IShellSectionController)?.PresentedPage

[pictos]

@PureWeen, something else fixed that issue, the code works without the Device.BeginInvokeOnMainThread, also all the core Shell tests are green.

[PureWeen]

Test run is here

https://dev.azure.com/xamarin/public/_releaseProgress?_a=release-pipeline-progress&releaseId=2176