Fix SelectedItem in Picker set to null when bound ItemsSource raises CollectionChanged reset event (#2032)

[ajphall]

Description of Change

Fixes SelectedItem in Picker set to null when bound ItemsSource raises CollectionChanged reset event by storing the previously selected item before refreshing the picker items. After the refresh the previously selected item attempts to be reselected.

Issues Resolved

• fixes ItemsSource CollectionChanged (Reset) Clears Picker SelectedItem #2032

API Changes

None

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Unit test added as part of the PR.

PR Checklist

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

[dnfadmin]

All CLA requirements met.

[jfversluis]

Now that we're so close to the sunsetting of Xamarin.Forms unfortunately we won't be able to take this in anymore, we're really sorry about that. Nevertheless, thank you so much for your time and effort that you have put into this PR.

Please have a look at the evolution of Xamarin.Forms, .NET MAUI. A lot of development has been going on there. Hopefully this issue was already fixed in that codebase. If not, feel free to port this over to there.

Again, thank you so much for being a contributor and Xamarin.Forms user!