Author: Jonas Bilinkevicius

How to activate the previous instance of an application

Answer:

Place the following at the start of your project file:
1   
2   begin
3     if HPrevInst < > 0 then
4     begin
5       ActivatePreviousInstance;
6       Exit;
7     end;
8     { ... }
9   
10  and include the following unit:
11  
12  unit PrevInst;
13  
14  interface
15  
16  uses
17    WinProcs, WinTypes, SysUtils;
18  
19  type
20    PHWnd = ^HWnd;
21  
22  {function EnumFunc(Wnd: HWnd; TargetWindow: PHWnd): Bool; export;
23  procedure ActivatePreviousInstance;}
24  
25  implementation
26  
27  function EnumFunc(Wnd: HWnd; TargetWindow: PHWnd): Bool;
28  var
29    ClassName: array[0..30] of char;
30  begin
31    Result := True;
32    if GetWindowWord(Wnd, GWW_HINSTANCE) = HPrevInst then
33    begin
34      GetClassName(Wnd, ClassName, 30);
35      if StrIComp(ClassName, 'TApplication') = 0 then
36      begin
37        TargetWindow^ := Wnd;
38        Result := False;
39      end;
40    end;
41  end;
42  
43  procedure ActivatePreviousInstance;
44  var
45    PrevInstWnd: HWnd;
46  begin
47    PrevInstWnd := 0;
48    EnumWindows(@EnumFunc, Longint(@PrevInstWnd));
49    if PrevInstWnd <> 0 then
50      if IsIconic(PrevInstWnd) then
51        ShowWindow(PrevInstWnd, SW_RESTORE)
52      else
53        BringWindowToTop(PrevInstWnd);
54  end;
55  
56  end.