Work out how to handle lazily-evaluated ItemsControl.Items

[grokys]

If a lazily-evaluated IEnumerable is assigned to ItemsControl.Items then stuff breaks, as the sequence gets re-enumerated every time the control accesses the items.

Some possible solutions:

1. Have two separate properties as in WPF: Items and ItemsSource. This feels a little messy to me, having two properties to do essentially the same thing, and making one read-only in certain situations etc as in WPF.
2. Change the type of Items from IEnumerable to ICollection.
3. Throw an exception if value doesn't implement ICollection or IList etc.

@SuperJMN thoughts? Any other ideas?

[danwalmsley]

In WPF the Items type is ItemsCollection.

ItemCollection : CollectionView, IList, ICollection, 

[grokys]

The Items collection in WPF is a different thing: the equivalent in WPF is ItemsSource which is an IEnumerable.

[kekekeks]

ItemsControl needs to work with finite enumerables anyway, so we might as well use IList there

[grokys]

We now have ItemsSourceView in the tree (ported from WinUI). We should use this in ItemsControl to handle this issue. Unfortunately this will result in breaking changes (templates will need to bind to the ItemsSourceView instead of Items) so going to fix this as part of other breaking changes in #3388.