R – TListBox Drag and Drop problems with MultiSelect enabled

delphidrag and dropvcl

I have a TListBox with multiselect and ExtendedSelect both set to true. I need to be able to drag multiple items in the list box to re-arrange them. My problem is what happens when the user clicks on an item that is already selected without holding down the CTRL or SHIFT key.

Case 1: DragMode is set to dmManual
The selection is cleared before the mouse down. This will not allow multiple items to be dragged.

Case 2: DragMode is set to dmAutomatic
The MouseDown event never fires. The selection is not cleared so dragging is OK, but the user cannot clear the selection by clicking on one of the selected items. This really causes a problem if all the items are selected or the next item the user wants to select was part of the current selection.

Note that this problem only happens if you assign something to the DragObject in the OnStartDrag procedure. I think the problem would go away if OnStartDrag would only start after the user moves the mouse. I have Mouse.DragImmediate := false set but I still get the StartDrag fired as soon as I click on an item in the list box.

I am using Delphi 7 for this project but I see the same behavior in Delphi 2007.

Best Answer

I have played with this for a while. And observe the same effects.

I would use Case2 and add a (Select All/Deselect All) button to the list. It even adds extra functionality and solves the most annoying part of the problem.

Related Solutions

WPF Drag & drop from ListBox with SelectionMode Multiple

I've found a very simple way to enable Windows Explorer like drag/drop behaviour when having multiple items selected. The solution replaces the common ListBox with a little derived shim that replaces the ListBoxItem with a more intelligent version. This way, we can encapsulate the click state at the right level and call into the protected selection machinery of the ListBox. Here is the relevant class. For a complete example, see my repo on github.

public class ListBoxEx : ListBox
{
    protected override DependencyObject GetContainerForItemOverride()
    {
        return new ListBoxItemEx();
    }

    class ListBoxItemEx : ListBoxItem
    {
        private bool _deferSelection = false;

        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            if (e.ClickCount == 1 && IsSelected)
            {
                // the user may start a drag by clicking into selected items
                // delay destroying the selection to the Up event
                _deferSelection = true;
            }
            else
            {
                base.OnMouseLeftButtonDown(e);
            }
        }

        protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
        {
            if (_deferSelection)
            {
                try
                {
                    base.OnMouseLeftButtonDown(e);
                }
                finally
                {
                    _deferSelection = false;
                }
            }
            base.OnMouseLeftButtonUp(e);
        }

        protected override void OnMouseLeave(MouseEventArgs e)
        {
            // abort deferred Down
            _deferSelection = false;
            base.OnMouseLeave(e);
        }
    }
}

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