Rewrite SelectionModel

[grokys]

What does the pull request do?

SelectionModel as ported from WinUI has a number of problems:

• Didn't handle binding SelectedIndex (Selection Model Issues #4496)
• Combining flat and nested selection led to unergonomic API (IndexPath for flat selection)
• Nested selection didn't work very well anyway due to the way TreeView works (don't know what the child items are until the TreeViewItem is materialized, so can't select non-expanded nodes)
• API was complex with lots of convenience methods so extracted interface was large
• Was not generic so lots of casts needed in ViewModel layer
• Did not preserve SelectedIndex/SelectedItem; instead returned first selection
• AnchorIndex didn't behave like in normal controls
• Didn't allow setting SelectedItem
• SelectRange/SelectAll selected multiple items even when SingleSelect = true
• Didn't work well when making selections before Source assigned, so BeginInit/EndInit logic had to be hacked into control (needs to be possible to set SelectedIndex/SelectedItem before Items is assigned in XAML)

This PR first reverts the changes introduced by the previous SelectionModel attempt and then rewrites SelectionModel to:

• Only handle list (flat) selections, so indexes are plain ints
• Slims down the interface
• Makes SelectionModel generic so that it can be used in ViewModels without casts everywhere
• Preserves SelectedIndex
• Makes AnchorIndex work as expected
• Allows setting SelectedItem
• Disable range selection when SingleSelect = true
• Makes setting selection before Source is assigned work better, so BeginInit can simply delay the Source assigment until initialization is finished

Note that after this change, TreeView no longer has a selection model: it will remain reverted to its pre-SelectionModel v1 state.

It's still a WIP, left to do:

• Use the list of selected objects in SelectedItems to repopulate selection after Reset (needs to be a way to turn it off for large collections)
• Correctly select newly added items with IsSelected set
• Re-apply use of Selection to samples (were changed back to SelectedItems when I reverted the previous SelectionModel)
• Various other little bugs

Breaking changes

Yep, it's a breaking change :) Not so much from 0.9.x but the SelectionModel API has changed from the previous version shipped in the 0.10 previews.

Fixed issues

Fixes #4496

[robloo]

A lot of these changes make sense. Removing IndexPath; however, cripples the usefulness of SelectionModel and as you stated, doesn't allow it to work in a hierarchy (TreeView). Removing IndexPath doesn't seem like a good idea to future-proof this API. IndexPath with a single int index isn't that complicated.

[grokys]

Removing IndexPath; however, cripples the usefulness of SelectionModel and as you stated, doesn't allow it to work in a hierarchy (TreeView)

The idea will be to add a TreeSelectionModel at a later date: I'm working on it over at https://github.com/grokys/AvaloniaSelectionModel but it's a bit more involved so might take a while.

IndexPath with a single int index isn't that complicated.

Heh, it's a lot more complicated than it first seems!

[jmacato]

code style 👍 tested controlcatalog and found nothing wrong 👍 would need a deeper dive testing @danwalmsley

[Kermalis]

Does this fix #4472 or #4048? More unlikely to fix #4461

[grokys]

@Kermalis I wasn't targeting those issues specifically, as this PR was holding up a release, however it's possible it may have fixed some things. I plan to look into those issues again when I have a moment.