Author: Jonas Bilinkevicius

How can I switch focus to an existing instance of an application instead of 
creating a new instance (HPrevInst)?

Answer:
1   
2   {usual stuff at the top of the project source file}
3   
4   var
5     hwnd: Word;
6   begin
7     if hPrevInst = 0 then
8     begin
9       Application.CreateForm(TForm1, Form1);
10      Application.Run;
11    end
12    else
13    begin
14      hwnd := FindWindow('TForm1', nil);
15      if (not IsWindowVisible(hwnd)) then
16      begin
17        ShowWindow(hwnd, sw_ShowNormal);
18        PostMessage(hwnd, wm_User, 0, 0);
19      end
20      else
21        SetWindowPos(hwnd, HWND_TOP, 0, 0, 0, 0, SWP_NOSIZE or SWP_NOMOVE);
22    end;
23  end.


in the form's PAS file add a message response function for the wm_User message.
24  
25  {in the form declaration}
26  public
27  
28  procedure WMUser(var msg: TMessage); message wm_User;
29  
30  {in the implementation section}
31  
32  procedure TForm1.WMUser(var msg: TMessage);
33  begin
34    Application.Restore;
35  end;