Fix: Enable seamless forward and backward transition in VirtualizingCarouselPanel

[LuckyGeorge1975]

What does the pull request do?

The calculation of the 'forward' flag has been changed. The change only applies when using more than 2 items in the carousel. If using only 2 items the flag is calculated as before.

What is the current behavior?

Currently the flag changes from forward to backward when transitioning from the last element to the first element in the item list and the flag changes from backward to forward when transitioning from the first item to the last item. The transition is not like a carousel but more like a linear stack.

What is the updated/expected behavior with this PR?

The transition direction does not change when transitioning from the first or last item when using more than 2 items. The transition is now seamless.

How was the solution implemented (if it's not obvious)?

When using more than 2 items it is checked if the _transitionFromIndex is the last one and the _realizedIndex is the first or if the _transitionFromIndex is the first and the _realizedIndex is the last.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes #14115

[LuckyGeorge1975]

@dotnet-policy-service agree

[LuckyGeorge1975]

There is currently no way to calculate the correct transition direction when using only 2 items. So the fix only applies when using more than 2 items in a carousel.

Description
When using only two items the panel does not know if the transition should be forward or backward when transitioning from index 1 to index 0.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043332-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[jp2masa]

Maybe the carousel should be reworked to work with deltas? I think that's the only way to get a circular behavior.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0043334-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[LuckyGeorge1975]

@jp2masa

Thought about it. But how would you calculate the delta direction if the user changes SelectedItem or SelectedIndex via DataBinding or programmatically? I'm currently thinking about a change to calculate the "shortest" distance between _transitionFromIndex and _realizedIndex. Imagine a carousel with 10 items and you want to transition from 2 to 9 - in that case the transition should go backward and not forward.

But this will not solve the issue when using only 2 items as you have no idea what the user really wants. The only way to make this work correctly is to make SelectedItem and SelectedIndex readonly and only allow transitions with the Carousel.Next() and Carousel.Previous() methods. But that change would cripple the control.

[jp2masa]

The only way to make this work correctly is to make SelectedItem and SelectedIndex readonly and only allow transitions with the Carousel.Next() and Carousel.Previous() methods. But that change would cripple the control.

In my opinion, the carousel, being a circular control, should have readonly selection properties and some kind of Move(int delta) public method.

Anyway, I don't make decisions here, so someone from the core team will have to confirm what approach is preferred for the future.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044014-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]