Hello,

Thank you for the sample application you have provided.

The reason the "Qty" column from the top layout is updated only when performing operations like sorting is because the XamGrid gets all property values when sorting and in this case the QtyDelivered property returns the latest calculation result.

In order to keep the QtyDelivered property updated when adding new child rows, I can suggest you a couple of approaches:

1. Handle the RowAdded event of the XamGrid by checking if a new row has been added to the child layout. If so, then update the QtyDelivered property of the parent row manually.

private void grid_RowAdded(object sender, RowEventArgs e)
{
    var grid = sender as XamGrid;
    var addedRow = e.Row;

    if (addedRow.ColumnLayout == grid.ColumnLayouts["Deliveries"])
    {
        var parentRow = addedRow.ParentRow;
        var parentRowItem = (parentRow.Data as Item);
        parentRowItem.QtyDelivered = parentRowItem.Deliveries.Sum(D => D.Qty);
    }
}

2. Use an ObservableCollection instead of a List for storing the Delivery items. This way you can hook for the CollectionChanged event of the collection and update the QtyDelivered property with a private set within the Item class context.

this._Deliveries = new ObservableCollection<Delivery>();
this._Deliveries.CollectionChanged += (sender, e) =>
{
    if (e.Action == NotifyCollectionChangedAction.Add)
    {
        QtyDelivered = this.Deliveries.Sum(D => D.Qty);
    }
};

I would suggest going with the second approach, since it keeps the MVVM pattern and the ObservableCollection itself is more suitable when it comes to storing data for visual representation.
I have modified your sample with the second approach from above.

If you have any questions on the matter, please let me know.