How to update ViewModel collection after re-ordering of ListView in Xamarin.Forms?

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:ListViewXamarin;assembly=ListViewXamarin"
             xmlns:syncfusion="clr-namespace:Syncfusion.ListView.XForms;assembly=Syncfusion.SfListView.XForms" xmlns:data="clr-namespace:Syncfusion.DataSource;assembly=Syncfusion.DataSource.Portable"
             x:Class="ListViewXamarin.MainPage">
    <ContentPage.Behaviors>
        <local:Behavior/>
    </ContentPage.Behaviors>
    <StackLayout >
        <syncfusion:SfListView x:Name="ToDoListView" ItemSize="60" IsStickyHeader="True" ItemsSource="{Binding ToDoList}" DragStartMode="OnHold,OnDragIndicator" SelectionMode="None">
            <syncfusion:SfListView.DataSource>
                <data:DataSource>
                    <data:DataSource.GroupDescriptors>
                        <data:GroupDescriptor PropertyName="CategoryName" />
                    </data:DataSource.GroupDescriptors>
                </data:DataSource>
            </syncfusion:SfListView.DataSource>
      ...
            <syncfusion:SfListView.ItemTemplate>
                <DataTemplate>
                    <Frame HasShadow="True" Padding="0">
                        ...
                    </Frame>
                </DataTemplate>
            </syncfusion:SfListView.ItemTemplate>
        </syncfusion:SfListView>
        <Button x:Name="saveButton" Text="Save" BackgroundColor="#949cdf" HeightRequest="50" Command="{Binding SaveOrderCommand}"/>
    </StackLayout>
</ContentPage>

public class Behavior : Behavior<ContentPage>
{
    SfListView ListView;
    Button SaveButton;
 
    protected override void OnAttachedTo(ContentPage bindable)
    {
        ListView = bindable.FindByName<SfListView>("ToDoListView");
        SaveButton = bindable.FindByName<Button>("saveButton");
        SaveButton.Clicked += SaveButton_Clicked;
        base.OnAttachedTo(bindable);
    }
 
    private void SaveButton_Clicked(object sender, EventArgs e)
    {
        var newList = new List<ToDoItem>();
        foreach (var item in ListView.DataSource.DisplayItems)
        {
            if (!(item is GroupResult))
                newList.Add((ToDoItem)item);
        }
        (ListView.BindingContext as ViewModel).ToDoList = new ObservableCollection<ToDoItem>(newList);
        App.Current.MainPage.DisplayAlert("Items order has been updated", "", "Close");
    }
}

namespace ListViewXamarin
{
    public class Behavior : Behavior<ContentPage>
    {
        SfListView ListView;
 
        protected override void OnAttachedTo(ContentPage bindable)
        {
            ListView = bindable.FindByName<SfListView>("ToDoListView");
            ListView.ItemDragging += ListView_ItemDragging;
            base.OnAttachedTo(bindable);
        }
 
        private void ListView_ItemDragging(object sender, ItemDraggingEventArgs e)
        {
            if (e.Action == DragAction.Drop)
            {
                int dropIndex = e.NewIndex;
                var dropItem = ListView.DataSource.DisplayItems[dropIndex];
                var dropedGroup = GetGroup(dropItem, e);
 
                if ((e.ItemData as ToDoItem).CategoryName != dropedGroup.Key.ToString())
                {
                    (e.ItemData as ToDoItem).CategoryName = dropedGroup.Key.ToString();
                }
            }
        }
 
        public GroupResult GetGroup(object itemData, ItemDraggingEventArgs args)
        {
            GroupResult itemGroup = null;
 
            foreach (var item in this.ListView.DataSource.DisplayItems)
            {
                if (itemData is GroupResult)
                {
                    if (args.OldIndex > args.NewIndex && item == itemData) break;
                    if (item is GroupResult) itemGroup = item as GroupResult;
                    if (args.OldIndex < args.NewIndex && item == itemData) break;
                }
                else
                {
                    if (item == itemData) break;
                    if (item is GroupResult) itemGroup = item as GroupResult;
                }
            }
            return itemGroup;
        }
    }
}