[ios] ListView.SelectedIndex set during item insertion is offset

[Xiaoy312]

Current behavior

If you attempt to set the selection during list mutation, the set selection will be offset on iOS.

Expected behavior

^ should not happen.

How to reproduce it (as minimally and precisely as possible)

[TestMethod]
[RunsOnUIThread]
public async Task When_SelectionChanged_Midst_ListMutation_AsdAsd3()
{
	var source = new ObservableCollection<int> { 1, 11, 12, 2 };
	var SUT = new ListView
	{
		ItemsSource = source,
		SelectionMode = ListViewSelectionMode.Single,
	};

	await UITestHelper.Load(SUT);

	Assert.AreEqual(-1, SUT.SelectedIndex, "[pre-insert]SUT.SelectedIndex should be -1");
	Assert.IsNull(SUT.SelectedItem, "[pre-insert]SUT.SelectedItem should be null");

	(int Index, object Item) snapshot = (int.MinValue, default);
	SUT.ContainerContentChanging += (s, e) =>
	{
		if (e.Item as int? == 111)
		{
			e.ItemContainer.IsSelected = true;
			snapshot = (SUT.SelectedIndex, SUT.SelectedItem);
		}
	};
	source.Insert(index: 2, 111);
	await UITestHelper.WaitForIdle();

	Assert.AreEqual(2, snapshot.Index, "[mid-insert]snapshot.Index should be 2");  // actual: 2
	Assert.AreEqual(111, snapshot.Item, "[mid-insert]snapshot.Item should be 111"); // actual: 111
	Assert.AreEqual(2, SUT.SelectedIndex, "[post-insert]SUT.SelectedIndex should be 2"); // actual: 3
	Assert.AreEqual(111, SUT.SelectedItem, "[post-insert]SUT.SelectedItem should be 111"); // actual: 12
}

Workaround

No response

Works on UWP/WinUI

Yes

Environment

Uno.WinUI / Uno.WinUI.WebAssembly / Uno.WinUI.Skia

NuGet package version(s)

No response

Affected platforms

iOS

IDE

Visual Studio 2022

IDE version

No response

Relevant plugins

No response

Anything else we need to know?

note that on windows, LV::ContainerContentChanging will only get invoked once per app-launch (or until container cache gets purged...), so the test only work once...

[Xiaoy312]

slightly updated the test a bit

code

[TestMethod]
[RunsOnUIThread]
public async Task When_SelectionChanged_Midst_ListMutation_AsdAsd3()
{
	var states = new List<(int Index, int? Item, string Context)>();
	var source = new ObservableCollection<int>{ 1, 11, 12, 2, 3 };
	var SUT = new ListView
	{
		ItemsSource = source,
		SelectionMode = ListViewSelectionMode.Single,
	};
	SUT.SelectedIndex = 3;

	void SaveState(string context) => states.Add((SUT.SelectedIndex, SUT.SelectedItem as int?, context));

	await UITestHelper.Load(SUT);
	SaveState("0 pre-insert");

	SUT.ContainerContentChanging += (s, e) =>
	{
		SaveState("1 mid-insert before re-selection");
		if (e.Item as int? == 111)
		{
			e.ItemContainer.IsSelected = true;
		}
		SaveState("2 mid-insert after re-selection");
	};
	source.Insert(index: 2, 111);
	await UITestHelper.WaitForIdle();
	SaveState("3 post-insert");

	Assert.AreEqual(4, states.Count, "4 snapshots are expected.");
	Assert.AreEqual((3, 2), (states[0].Index, states[0].Item), states[0].Context); // selection should be value=2 at index=3
	Assert.AreEqual((4, 2), (states[1].Index, states[1].Item), states[1].Context); // selection should still be value=2 at index=4 (at this point, the index should've been already updated)
	Assert.AreEqual((2, 111), (states[2].Index, states[2].Item), states[2].Context); // selection should now be value=111 at index=2
	Assert.AreEqual((2, 111), (states[3].Index, states[3].Item), states[3].Context); // selection should still be value=111 at index=2
}

observation:

win,skia: states // new List<(int Index, int? Item, string Context)>();
Count = 4
    [0]: (3, 2, "0 pre-insert")
+   [1]: (4, 2, "1 mid-insert before re-selection")
    [2]: (2, 111, "2 mid-insert after re-selection")
+   [3]: (2, 111, "3 post-insert")

ios: states // new List<(int Index, int? Item, string Context)>();
Count = 4
    [0]: (3, 2, "0 pre-insert")
-   [1]: (3, 2, "1 mid-insert before re-selection")
    [2]: (2, 111, "2 mid-insert after re-selection")
-   [3]: (3, 12, "3 post-insert")

^ notice that on every other platform the offset correction occurs before ContainerContentChanging,
^ but on iOS, it happen afterward.