[iOS] Prevent crash when adding item to source of CollectionView in tab

[hartez]

Description of Change

If

• a CollectionView is on a tab that has never been displayed
• the CollectionView's ItemsSource implements INotifyCollectionChanged
• an item is added to that ItemsSource

on iOS 13 a layout error will be raised and the CollectionView will be left in a weird state. Pre-13, it'll just crash.

These changes detect that scenario and avoid the crash.

Issues Resolved

• fixes [Bug][iOS] If CollectionView in other Tab gets changed before it's displayed, it stays invisible #7700

API Changes

None

Platforms Affected

• iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Automated test.

PR Checklist

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

[hartez]

Adding a DNM for now - this also needs to handle the same edge case for grouping.

[jfversluis]

"Xamarin.Forms.Platform.iOS\CollectionView\GroupableItemsViewController.cs(33,68): Error CS1503: Argument 2: cannot convert from 'UIKit.UICollectionView' to 'UIKit.UICollectionViewController'"
Review

and some small conflicts. Code in itself seems to look good

[samhouts]

/Users/runner/runners/2.158.0/work/1/s/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7700.cs(49,20): error CS0117: 'FlagTestHelpers' does not contain a definition for 'SetCollectionViewTestFlag' [/Users/runner/runners/2.158.0/work/1/s/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj]
/Users/runner/runners/2.158.0/work/1/s/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7993.xaml.cs(31,54): error CS0117: 'CollectionView' does not contain a definition for 'CollectionViewExperimental' [/Users/runner/runners/2.158.0/work/1/s/Xamarin.Forms.Controls/Xamarin.Forms.Controls.csproj]

[PureWeen]

Looks good!

Add some quick instructions to that first page on 7700? then merge?