Keep SelectingItemsControl selection values until ItemsSource is set

[MrJul]

What does the pull request do?

This PR keeps the values for SelectedItem, SelectedIndex and SelectedValue from SelectingItemsControl unchanged until an ItemsSource is set.

Note that this PR does not change the behavior if the ItemsSource property is set to a valid collection which is only populated later.

What is the current behavior?

Setting a selection property such as SelectedItem before the ItemsSource is set immediately resets it to its default value, because the item/index/value doesn't exist inside the list.

There's already code in place to avoid this issue when XAML initialization takes place. However, it's frequent to have the ItemsSource available only after initialization has completed (for example through a relative binding).

What is the updated/expected behavior with this PR?

The selection values are kept until ItemsSource is set.

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

For SelectedItem and SelectedIndex, which are part of the ISelectionModel, Avalonia simply waits to set SelectionModel.ItemsSource until we have a proper ItemsControl.ItemsSource (commits 1 and 2). There's already code in place to automatically set the correct selection properties as soon as the items source is set.

For SelectedValue, this isn't the case. We need to retrieve the matching SelectedItem right before setting the source (commits 3 and 4).

Notes

Copying the comments I left in SelectingItemsControl:

InternalSelectionModel keeps the SelectedIndex and SelectedItem values before the ItemsSource is set. However, SelectedValue isn't part of that model, so we have to set the SelectedItem from SelectedValue manually now that we have a source.

While this works, this is messy: we effectively have "lazy selection initialization" in 3 places:

• UpdateState (all selection properties, for BeginInit/EndInit)
• InternalSelectionModel (SelectedIndex/SelectedItem)
• SelectedItemsControl (SelectedValue)

There's the opportunity to have a single place responsible for this logic.

Fixed issues

• Fixes ComboBox.SelectedItem lost on control generated in a template #14654
• Fixes ComboBox Binding in DataGrid, selecteditem did not display in combobox #18437
• Fixes Datagrid ComboBox SelectedItem Show Error #18615

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0056011-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[timunie]

Hey @MrJul ,

If a control is unloaded or DataContext changes, does this also prevent clearing selection in case ItemsSource is changed before SelectedItem?

[MrJul]

If a control is unloaded or DataContext changes, does this also prevent clearing selection in case ItemsSource is changed before SelectedItem?

No, unfortunately this PR only prevents clearing the selection before ItemsSource is set the very first time, nothing more.

[mjo151]

After updating from 11.2.7 to 11.2.8, I noticed a change in behavior and I believe it's caused by the changes in this issue. The TabControl.SelectionChanged event is no longer raised when the tab control first loads. Is that an intended change in behavior or a bug introduced in 11.2.8?

[timunie]

I believe this is by desgin