How to synchronize the scrolling of 2 TTreeviews?

Question

I have 2 TTreeviews. Both of them have the same number of items. I'd like to be able to synchronize their scrollbars... If I move one of them, the other moves also... For the horizontal, it works as I expect... For the vertical, it works if I use the arrows of the scrollbar, but it doesn't if I drag the thumb or if I use the mouse wheel... Here is a sample I've written to illustrate my issue: ``` unit main; interface uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, ComCtrls, StrUtils; type TForm1 = class(TForm) tv1: TTreeView; tv2: TTreeView; procedure FormCreate(Sender: TObject); procedure FormDestroy(Sender: TObject); private { Private declarations } originalTv1WindowProc : TWndMethod; originalTv2WindowProc : TWndMethod; procedure Tv1WindowProc (var Msg : TMessage); procedure Tv2WindowProc (var Msg : TMessage); public { Public declarations } end; var Form1: TForm1; implementation {$R *.dfm} procedure TForm1.FormCreate(Sender: TObject); var i:…

Accepted Answer

First, an interesting test: uncheck "enable runtime themes" in the project options and you'll see both of the treeviews will scroll synchronously. This shows us that the default window procedure for a treeview control is implemented differently in different versions of comctl32.dll. It would seem, the implementation in comctl32 v6 is particularly different when scrolling vertical.

Anyway, it appears that, for vertical scrolling only, the control looks for the thumb position and then adjusts the window contents accordingly. When you route a `WM_VSCROLL` to the adjacent treeview, it looks its thumb's position and as it is not changed, decides there's nothing to do (we only changed the thumb position of the one we are dragging).

So, to make it work, adjust the treeview's thumb position before sending the `WM_VSCROLL`. The modified procedure for tv1 would look like this:

```
procedure TForm1.Tv1WindowProc(var Msg: TMessage);
begin
  originalTv1WindowProc(Msg);

if Msg.Msg = WM_VSCROLL then begin
    if Msg.WParamLo = SB_THUMBTRACK then
      SetScrollPos(tv2.Handle, SB_VERT, Msg.WParamHi, False);
  end;

if ((Msg.Msg = WM_VSCROLL)
   or (Msg.Msg = WM_HSCROLL)
   or (Msg.msg = WM_Mousewheel)) then
  begin
//    tv2.Perform(Msg.Msg, Msg.wparam, Msg.lparam);
    originalTv2WindowProc(Msg);
  end;
end;
```

How to synchronize the scrolling of 2 TTreeviews?

Solution

Problem