I see iOS crashes in Sentry, seems like caused by clearing the RadCollectionView bound DataSource ObservableCollection, when an item was previously selected

Maui 9.0.70
Telerik 11.0

System.NullReferenceException: Arg_NullReferenceException
  ?, in void RadCollectionViewItemView.OnIsSelectedChanged()
  ?, in static RadCollectionViewItemView()+(BindableObject b, object o, object n) => { }
  ?, in void BindableObject.OnBindablePropertySet(BindableProperty property, object original, object value, bool didChange, bool willFirePropertyChanged)
  ?, in void Element.OnBindablePropertySet(BindableProperty property, object original, object value, bool changed, bool willFirePropertyChanged)
  ?, in void BindableObject.SetValueActual(BindableProperty property, BindablePropertyContext context, object value, bool currentlyApplying, SetValueFlags attributes, SetterSpecificity specificity, bool silent)
  ?, in void BindableObject.SetValueCore(BindableProperty property, object value, SetValueFlags attributes, SetValuePrivateFlags privateAttributes, SetterSpecificity specificity)
  ?, in void BindableObject.SetValue(BindableProperty property, object value)
  ?, in void RadCollectionViewItemView.set_IsSelected(bool value)
  ?, in void RadCollectionView.SelectContainers(IEnumerable items, bool select)
  ?, in void RadCollectionView.SelectionHelperSelectionChanged(object sender, RadSelectionChangedEventArgs e)
  ?, in void <>c__DisplayClass36_0.<SelectionChangerSelectionChanged>b__0(?)+() => { }
  ?, in void ActiveSelectionScope.Dispose()
  ?, in void SelectionHelper.ClearSelection()
  ?, in void RadCollectionView.Telerik.Maui.Controls.ICollectionView.ItemsChanged(NotifyCollectionChangedEventArgs e)
  ?, in void CollectionViewModel.OnDataProviderViewChanged(object sender, ViewChangedEventArgs e)
  ?, in void DataProviderBase.RaiseViewChanged(object sender, List<AddRemoveResult> changes, NotifyCollectionChangedEventArgs args, CollectionChange action, bool affectsAggregates)
  ?, in void LocalDataSourceProvider.ProcessCollectionChanged(NotifyCollectionChangedEventArgs e)
  ?, in void LocalDataSourceProvider.DataView_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
  ?, in void EnumerableDataSourceView.RaiseCollectionChanged(object sender, NotifyCollectionChangedEventArgs args)
  ?, in void EnumerableDataSourceView.ProcessCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
  ?, in void EnumerableDataSourceView.ProcessCollectionChangedOverride(object sender, NotifyCollectionChangedEventArgs args)
  ?, in void EnumerableDataSourceView.ProcessPendingCollectionChange()
  ?, in void DataProviderBase.RaiseViewChanging(object sender, IList changedItems, CollectionChange action)
  ?, in void LocalDataSourceProvider.DataView_CollectionChanging(object sender, NotifyCollectionChangedEventArgs e)
  ?, in void EnumerableDataSourceView.HandleCollectionChangedOverride(object sender, NotifyCollectionChangedEventArgs e)
  ?, in void EnumerableDataSourceView.Telerik.Maui.IWeakEventListener.ReceiveEvent(object sender, object args)
  ?, in void WeakEventHandlerList<NotifyCollectionChangedEventArgs>.ProcessEvent(object sender, object args)
  ?, in void WeakEventHandlerList<NotifyCollectionChangedEventArgs>.OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
  ?, in void ObservableCollection<ViewBundle>.OnCollectionChanged(NotifyCollectionChangedEventArgs)
  ?, in void ExtObservableCollection<ViewBundle>.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
  ?, in void ObservableCollection<ViewBundle>.OnCollectionReset()
  ?, in void ObservableCollection<ViewBundle>.ClearItems()
  ?, in void ExtObservableCollection<ViewBundle>.ClearItems()
  ?, in void Collection<ViewBundle>.Clear()
  ?, in void SearchViewViewModel.Dispose(bool disposing)
  ?, in void PropertyChangedNotifyingBase.Dispose()
  ?, in async void ViewBundle.Dispose(bool disposing)
  ?, in void <>c.<ThrowAsync>b__128_0(?)+(object state) => { }
  ?, in void NSAsyncSynchronizationContextDispatcher.Apply()
  ?, in void __Registrar_Callbacks__.callback_3311_Foundation_NSAsyncSynchronizationContextDispatcher_Apply(IntPtr pobj, IntPtr sel, IntPtr* exception_gchandle)

When having a CollectionView and navigating between different views when tapping on an item from the CollectionView, the item(s) look(s) selected. 

It seems the behavior is due to the mouse over and selected visual states.

In MAUI CollectionView when scrolling an item into view, the exact position of the item after the scroll has completed can be specified with the position argument of the ScrollTo methods.

Please provide such option for Telerik MAUI CollectionView inside the ScrollItemIntoView method.

Add support for sticky group headers (iOS, Android)

This feature is available in the ListView control. E.g. On iOS using the built-in CollectionView it can be added this way:

public class CollectionViewPlatformHandler : CollectionViewHandler { /// <inheritdoc /> protected override ItemsViewLayout SelectLayout() { var layout = base.SelectLayout(); if (ItemsView.IsGrouped && layout is UICollectionViewFlowLayout flowLayout) { // Enable sticky section headers. flowLayout.SectionHeadersPinToVisibleBounds = true; } return layout; } }

Add support for scrollable index titles (iOS).

This feature is available in the ListView control. E.g. On iOS using the built-in CollectionView it can be added this way:

    public class CollectionViewPlatformHandler : CollectionViewHandler
    {
        /// <inheritdoc />
        protected override UIView CreatePlatformView()
        {
            var platformView = base.CreatePlatformView();
            if (ItemsView.IsGrouped && ItemsView.ItemsSource is IEnumerable<IGrouping<string, object>> groups && platformView.Subviews[0] is UICollectionView collectionView)
            {
                // Enable index titles.
                collectionView.DataSource = new GroupedCollectionViewDataSource(collectionView.DataSource, groups.Select(q => q.Key));
            }

            return platformView;
        }
        
        private class GroupedCollectionViewDataSource(IUICollectionViewDataSource datasource, IEnumerable<string> sections) : UICollectionViewDataSource
        {
            public override nint NumberOfSections(UICollectionView collectionView) => datasource.NumberOfSections(collectionView);
            public override nint GetItemsCount(UICollectionView collectionView, nint section) => datasource.GetItemsCount(collectionView, section);
            public override UICollectionViewCell GetCell(UICollectionView collectionView, NSIndexPath indexPath) => datasource.GetCell(collectionView, indexPath);
            public override UICollectionReusableView GetViewForSupplementaryElement(UICollectionView collectionView, NSString elementKind, NSIndexPath indexPath) => datasource.GetViewForSupplementaryElement(collectionView, elementKind, indexPath);
            public override string[] GetIndexTitles(UICollectionView collectionView) => sections.ToArray();
        }
    }

Note: This is available out-of-the-box on iOS 14 and above for a native UICollectionView. See documentation.

https://developer.apple.com/documentation/uikit/uicollectionviewdatasource/2851455-indextitlesforcollectionview

When collapsing all groups, the footer still keeps bottom position and does not align with the groups. 

In RadListView, the footer is aligned to the groups when they are collapsed. Please provide such option for the CollectionView. 

Provide a built it way to notify the parent group when the items in the nested groups are changed. 

For example, there is no PropertyChanged or CollectionChanged notification that we can hook in to in order to detect updates in parent group headers when the count changes in a subgroup. Only the leaf group headers get updated, but not the parents.

1. Bind the CheckedItems collection of the TreeView to the CollectionView ItemsSource.

2. there aren't any items in the TreeView initially.  Add items to it

3. Start checking the checkboxes

4. The items are added to the CollectionView, still they are not visible in the control.

The content inside the swipe template is missing when using MAUI 9.0.40 and Telerik MAUI 10.0.0

Workaround:

Use MAUI 9.0.30 and Telerik MAUI 10.0.0