WPF datagrid and the tab key

Question

Another datagrid keybindings question

I have a datagrid. It has selection mode set to FullRow and KeyboardNavigation.TabNavigation="Once" which I was hoping would get my desired result but it doesn't.

When the tab key is pressed when the datagrid has focus it will tab over each column in the grid one by one. So if I tab into the grid which has 4 columns, I will have to press tab 4 times to go to the next tabindex.

What I want is for the tab key to tab right out of the datagrid on first press and give focus to the next tabindex... if that makes sense.

I have tried overriding the tab key in the keydown event handler like so.

class BetterDataGrid : DataGrid
{
  ..............
  protected override void OnKeyDown(System.Windows.Input.KeyEventArgs e)
  {
    ..............
    if (e.Key == Key.Tab)
    {
        Console.WriteLine("TAB");
        MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
    }
    .........
  }

It does write "TAB" to the console but the tab still keeps it's default behavior. Not sure if this is the right way to go the next tabindex, but then this should make the tab key do nothing but write to the console or cause an exception.
Makes me think it's impossible to override the tab key behavior.

Hoping for some helpful input.
As always, thanks in advance.

Answer 1

I wanted this for my line-of-business software, and the only way I have found to solve it is by codebehind, using the PreviewKeyDown, GotKeyboardFocus and LostKeyboardFocus events of the datagrid. I have put these eventhandlers in a WPF decorator, to avoid repeating it for every single DataGrid. It would probably be possible to subclass the DataGrid, but I haven't tried that.

The code for the handlers are as follows (DataGrid is x:Name="grid" for this sample code):

        private IInputElement lastDataGridFocus = null;
    private int selectedcolumnindex = 0;

    void grid_GotKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
    {
        if (grid.Items.Count > 0 && (e.NewFocus is DataGrid || (e.NewFocus is DataGridCell && !(e.OldFocus is DataGridCell))))
        {
            DataGridCell cell = null;

            if (lastDataGridFocus != null)
            {
                FocusManager.SetFocusedElement(grid, lastDataGridFocus);
                lastDataGridFocus = null;
                e.Handled = true;
                return;
            }

            if (grid.SelectedCells.Count == 0)
            {
                DataGridRow rowContainer = (DataGridRow)grid.ItemContainerGenerator.ContainerFromIndex(0);
                if (rowContainer != null)
                {
                    DataGridCellsPresenter presenter = GetVisualChild<DataGridCellsPresenter>(rowContainer);
                    cell = (DataGridCell)presenter.ItemContainerGenerator.ContainerFromIndex((selectedcolumnindex < 0) ? 0 : selectedcolumnindex);
                }
            }
            else
            {
                DataGridCellInfo selectedDataGridCellInfo = (grid.SelectedCells[0] as DataGridCellInfo?).Value;
                DataGridRow rowContainer = (DataGridRow)grid.ItemContainerGenerator.ContainerFromItem(selectedDataGridCellInfo.Item);
                if (rowContainer != null)
                {
                    DataGridCellsPresenter presenter = GetVisualChild<DataGridCellsPresenter>(rowContainer);
                    cell = (DataGridCell)presenter.ItemContainerGenerator.ContainerFromIndex((selectedcolumnindex < 0) ? 0 : selectedcolumnindex);
                }
            }
            if (null != cell)
            {
                FocusManager.SetFocusedElement(grid, cell as IInputElement);
                e.Handled = true;
            }
        }
    }

    void grid_LostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
    {
        if (!(e.NewFocus is DataGridCell))
        {
            if (grid.CurrentCell != null)
            {
                selectedcolumnindex = grid.Columns.IndexOf(grid.CurrentCell.Column);
            }
        }
    }

    void grid_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        if (Keyboard.Modifiers == ModifierKeys.Shift && e.Key == Key.Tab)
        {
            lastDataGridFocus = Keyboard.FocusedElement;
            grid.MoveFocus(new TraversalRequest(FocusNavigationDirection.Previous));
            e.Handled = true;
        }
        else if (Keyboard.Modifiers == ModifierKeys.None && e.Key == Key.Tab)
        {
            lastDataGridFocus = Keyboard.FocusedElement;
            grid.MoveFocus(new TraversalRequest(FocusNavigationDirection.Last));
            (Keyboard.FocusedElement as FrameworkElement).MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
            e.Handled = true;
        }
    }

With this code you can navigate inside the grid using the cursor keys, and the tab key and shift-tab key gets you out of the datagrid. If you tab out of the grid and come back to the grid, you also get to the same cell that you left. This is what my users and I want, and this is IMHO what the DataGrid control should provide as default behaviour.