[iOS] Change how ItemsViewController.CheckEmptySource checks for item size

[GalaxiaGuy]

Description of Change

This commit

Xamarin.Forms/Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewController.cs

Line 20 in aeec186

bool _isEmpty = true;

caused the iOS CarouselView native items to have the wrong size under some specific circumstances.

Stepping through , there was an incorrect call to ItemsViewLayout.ConstrainTo(CollectionView.Bounds.Size);. This changes it to use ConstrainToItemsView() instead (which uses the ItemsView size if it has been calculated and CollectionView.Bounds.Size otherwise, which seems to match the existing comment).

Issues Resolved

• fixes [Bug] [iOS] Changing CollectionView.ItemSizingStrategy that contains a CarouselView causes the CarouselView to size its cells incorrectly. #15287

API Changes

None

Platforms Affected

iOS

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Issue 15287 has been added to the app. Scroll up and down the collection view and see that none of the items are incorrectly sized.

PR Checklist

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

[jfversluis]

/azp run

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[YevhenVolynets]

@jfversluis After reverting that issue exists again. I use collection view inside Carousel view. Items have incorrect size. It seems that cell use all of all Carousel View width. I can see only the end of cell. But after when I do "Pull to refresh", I see normal size.
Before Pull to Refresh

After Pull to Refresh

It was working in 5.0.0.2478 but doesn't work in 5.0.0.2515