Sync scrolling of two TListViews

Here is a quick solution, how to synchronize scrolling of two TListViews. All you have to do is to write a code for OnAdvancedCustomDraw event for both lists.

Suppose, you have two lists of TListView type — fList and fNewList. Write code for first one (fListAdvancedCustomDraw event):

[code language=”delphi”]
procedure TMainForm.fListAdvancedCustomDraw(Sender: TCustomListView; const ARect: TRect; Stage: TCustomDrawStage; var DefaultDraw: Boolean);
var
dY: Integer;
begin
if(fList.TopItem <> nil) and (fNewList.Items.Count > fList.TopItem.Index) then
begin
dY := fNewList.TopItem.DisplayRect(drBounds).Bottom – fNewList.TopItem.DisplayRect(drBounds).Top;
dY := (fList.TopItem.Index – fNewList.TopItem.Index) * dY;

fNewList.Scroll(0, dY);
end;
end;
[/code]

Code for second one (fNewListAdvancedCustomDraw event) is quite similar:

[code language=”delphi”]
procedure TMainForm.fNewListAdvancedCustomDraw(Sender: TCustomListView; const ARect: TRect; Stage: TCustomDrawStage; var DefaultDraw: Boolean);
var
dY: Integer;
begin
if(fNewList.TopItem <> nil) and (fList.Items.Count > fNewList.TopItem.Index) then
begin
dY := fList.TopItem.DisplayRect(drBounds).Bottom – fList.TopItem.DisplayRect(drBounds).Top;
dY := (fNewList.TopItem.Index – fList.TopItem.Index) * dY;

fList.Scroll(0, dY);
end;
end;
[/code]

And that’s it.

Of course, there are penalities for being lazy ass! :] Performance is killing you and everyone around as you’re using drawing event to catch scrolling (madness! :]) and you can actually clearly see, when your lists are refreshed. But since I needed a really quick solution, without playing around window messages and aligning, this was the best I could get. It was enough for me.

If you’re looking for harder to good, but much more performance full, then you have to look around for subclassing and catching window’s messages, as there are not specific events published for catching scroll of an TListView. I suppose, you can start with StackOverflow articles: TListView scroll event and Catching ListView BeginScroll and EndScroll Events.