procedure TForm1.ListBox1MouseMove(Sender: TObject; Shift: TShiftState; 
                                   X, Y: Integer);
var
  P : TPoint;
begin
  {Grab the position of the mouse and put it into a TPoint}
  P.X := X;
  P.Y := Y;
  {Set the ItemIndex of the list box to the Item under the current mouse
   coordinates}
  ListBox1.ItemIndex := ListBox1.ItemAtPos(P, True);
end;

The ItemAtPos method of TListBox makes all of this work. As the mouse passes over the list box, the event handler picks up the coordinates. ItemAtPos then passes back the index of the list under the current mouse position. Doing the assignment to the ItemIndex property changes the current selection in the list box—pretty neat.

Well, that was the quick and dirty solution, but there's more to it. It is possible that you could use this functionality in more than one project. In that case, you'd have to create a new component. Now while the solution is simple, and in fact, uses pretty much the same code as before, finding the right method to override in the ancestral tree isn't as readily apparent as you might think. This is because the handler code for a mouse move isn't defined at the TListBox level. It's actually defined about four parent levels up at the TControl level. So visibility is a bit of an issue.

Furthermore, for those of you who have written event handlers for components, the natural tendency (and usually the correct one) would be to write a Windows message handler for WM_MOUSEMOVE. Well, you can, but in order to get the coordinates, you need to get the high- and low-order bits from the lParam parameter of the message record to resolve the coordinate values. It's just a bit more coding.

As I mentioned earlier, you can use code similar to the previous listing for the new descendant class. And you can do this with TControl's dynamic method MouseMove. MouseMove's declaration is almost exactly like the OnMouseMove event handler. It's just missing the Sender parameter. The other parameters Shift, X, and Y are all there. How convenient! So with MouseMove, you've found the method to override.

Before I give you the code listing, you might wonder why I chose to descend from TCustomListBox, and not TListBox. The reason for this is simple: TListBox surfaces a property called MultiSelect that allows a user to select multiple items. Within the scope of this solution, that property is meaningless. Why? Because as the mouse moves over the list, the ItemIndex property constantly changes. You just can't select multiple items in this case. With that in mind, descending from TCustomListBox makes more sense because you can leave out the MultiSelect property yet retain all the other functionality of TListBox. Here's the code listing: