Drag and drop for an ExpandableListView (MvvmCross)

ExpandableListViewGroupDragHelper.cs

public class ExpandableListViewGroupDragHelper : IDisposable
{
    private class DragState
    {
        public int PreviousY { get; set; }
        public int SelectedGroupPosition { get; set; }
    }

    public class DragListEventArgs : EventArgs
    {
        public int PreviousPosition { get; private set; }
        public int CurrentPosition { get; private set; }

        public DragListEventArgs(int previousPosition, int currentPosition)
        {
            this.PreviousPosition = previousPosition;
            this.CurrentPosition = currentPosition;
        }
    }

    public float DragRatio { get; set; }

    private DragState _dragState;

    private readonly WeakReference<ExpandableListView> _list;

    public event EventHandler<DragListEventArgs> DragListPositionChanged;

    private readonly Func<ClipDescription, bool> _acceptsClipData;

    protected virtual void OnDragListPositionChanged(DragListEventArgs e)
    {
        EventHandler<DragListEventArgs> handler = this.DragListPositionChanged;
        if (handler != null) handler(this, e);
    }

    public ExpandableListViewGroupDragHelper(ExpandableListView listView,
        Func<ClipDescription, bool> acceptsClipData)
    {
        this._list = new WeakReference<ExpandableListView>(listView);
        this._acceptsClipData = acceptsClipData;

        this.DragRatio = 1.0f;

        listView.Drag += this.ListOnDrag;
    }

    ~ExpandableListViewGroupDragHelper()
    {
        this.Dispose(false);
    }

    public void Dispose()
    {
        this.Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool disposeManaged)
    {
        if (disposeManaged)
        {
            ExpandableListView list;
            if (this._list.TryGetTarget(out list))
                list.Drag -= this.ListOnDrag;
        }
    }

    private void ListOnDrag(object sender, View.DragEventArgs dragEventArgs)
    {
        ExpandableListView list;
        if (!this._list.TryGetTarget(out list))
            return;

        var action = dragEventArgs.Event.Action;

        switch (action)
        {
            case DragAction.Started:
                if (!this._acceptsClipData(dragEventArgs.Event.ClipDescription))
                {
                    dragEventArgs.Handled = false;
                    return;
                }

                long packedPosition = dragEventArgs.Event.LocalState.ToNetObject<long>();

                if (ExpandableListView.GetPackedPositionType(packedPosition) == PackedPositionType.Group)
                {
                    var groupPosition = ExpandableListView.GetPackedPositionGroup(packedPosition);

                    for (int i = 0; i < list.ExpandableListAdapter.GroupCount; i++)
                        list.CollapseGroup(i);

                    this._dragState = new DragState();
                    this._dragState.SelectedGroupPosition = groupPosition;

                    dragEventArgs.Handled = true;
                }
                break;
            case DragAction.Location:
                float y = dragEventArgs.Event.GetY();
                int speed = (int)((y - this._dragState.PreviousY) * this.DragRatio);

                // Let's try swapping items?
                var listPosition = list.PointToPosition((int)dragEventArgs.Event.GetX(), (int)y);
                if (listPosition != AdapterView.InvalidPosition &&
                    listPosition != this._dragState.SelectedGroupPosition)
                {
                    var p1 = this._dragState.SelectedGroupPosition;
                    var p2 = listPosition;

                    this.OnDragListPositionChanged(new DragListEventArgs(p1, p2));

                    this._dragState.SelectedGroupPosition = p2;
                }

                if (list.LastVisiblePosition < list.Count && speed > 0)
                    list.SmoothScrollBy(speed, 1);
                if (list.FirstVisiblePosition > 0 && speed < 0)
                    list.SmoothScrollBy(speed, 1);

                this._dragState.PreviousY = (int)y;

                dragEventArgs.Handled = true;
                break;
            case DragAction.Ended:
                list.SmoothScrollToPositionFromTop(1, 0, 200);
                dragEventArgs.Handled = true;
                break;

            case DragAction.Entered:
            case DragAction.Exited:
            case DragAction.Drop:
                dragEventArgs.Handled = true;
                break;
        }
    }
}

Fragment.cs

public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    // ...
    ((ExpandableListView)this.List).ItemLongClick += (sender, args) =>
    {
        View v = (View)sender;
    
        if (ExpandableListView.GetPackedPositionType(args.Id) == PackedPositionType.Group)
        {
            long packedPos = ((ExpandableListView)args.Parent).GetExpandableListPosition(args.Position);
    
            var pos = this.List.GetFlatListPosition(packedPos);
            View groupView = this.List.GetChildAt(pos);
    
            ClipData.Item item = new ClipData.Item(this.ClipId);
            ClipData dragData = new ClipData(this.ClipId, new string[] {ClipDescription.MimetypeTextPlain},
                item);
    
            v.StartDrag(dragData, new View.DragShadowBuilder(groupView), packedPos.ToJavaObject(), 0);
    
            args.Handled = true;
        }
    };
    // ...
}

public override void OnDestroy()
{
    base.OnDestroy();

    if (this._dragHelper != null)
    {
        this._dragHelper.DragListPositionChanged -= this.DragHelperOnDragListPositionChanged;
        this._dragHelper.Dispose();
        this._dragHelper = null;
    }
}

protected override void DragHelperOnDragListPositionChanged(object sender,
                ExpandableListViewGroupDragHelper.DragListEventArgs e)
{
    LayerSelectionViewModel viewModel = ViewModel as LayerSelectionViewModel;
    if (viewModel != null)
        viewModel.ReorderItems(viewModel.AreaItems, e.PreviousPosition, e.CurrentPosition); // DO YOUR MAGIC HERE
}