How do I use DoMouseWheel to scroll a line at a time?

Question

I've written a grid control and would like to add support for the mouse wheel to it. I thought it would be as simple as overriding the DoMouseWheel virtual method, but there is a bit of a problem with it.

You can set the number of lines to scroll at a time in Control Panel and the default there is three. And this makes perfect sense when scrolling through a document or web page, but on a grid, I think the expectation is rather to scroll a line at a time. But it seems that Delphi's wheel support will call DoMouseWheel three times for every notch that I scroll, meaning that I can only scroll to each third line in the grid (or whatever that global setting is).

How do I go about scrolling a single line at a time for every turn of the mouse wheel?

Update: The short answer here is to simply set Result to True after scrolling - then it doesn't scroll three times, but only once.

Answer 1

Just copy the code from the TCustomGrid class, which overrides both DoMouseWheelDown() and DoMouseWheelUp() to scroll exactly one line at a time.

Answer 2

In general, it is not a very good idea to fight against the system defaults and/or the user preferences. In this case means that you should respect whatever the system or the user has decided to set in the scrolling time.

Having said so, if you really believe that the multiscroll effect is totally wrong and misleading for the kind of component you want to drive, you might envision a way to get rid of this. You could try to set some timer and ignore all but one of the mouseWheel events that happen in a given lapse of time (in the range of milliseconds). One thing you should do is to set a configuration option in your program, to let the user to turn off this behaviour.

Answer 3

In my case, I used the JVDBGrid component, but I think this work for DbGrid too. You may overwrite the following functions: OnMouseWheelDown and OnMouseWheelUp.

E.g.:

Type declaration:

type
  TMyGrid = class(TJvExDBGrid);

Implementation

procedure TFExample.JvDBGrid1MouseWheelDown(Sender: TObject; Shift: TShiftState; MousePos: TPoint; var Handled: Boolean);
begin

  Handled := TMyGrid(Sender).DataLink.DataSet.MoveBy(1) <> 0;

end;

procedure TFExample.JvDBGrid1MouseWheelUp(Sender: TObject; Shift: TShiftState; MousePos: TPoint; var Handled: Boolean);
begin

  Handled := TMyGrid(Sender).DataLink.DataSet.MoveBy(-1) <> 0;

end;