How to ensure only one instance of application runs at once?

unit PrevInst;

interface

Uses
  Windows, SysUtils, Messages;

function ActivatePrevInstance(const MainFormClass, ATitle: string): Boolean;

(*
Usage:
  if ActivatePrevInstance(TMainForm.ClassName, '') then
    Exit;
*)

implementation

function StrPAlloc(const S: string): PChar;
begin
  Result := StrPCopy(StrAlloc(Length(S) + 1), S);
end;

function GetWindowParent(Wnd: HWND): HWND;
begin
  Result := GetWindowLong(Wnd, GWL_HWNDPARENT);
end;

function WindowClassName(Wnd: HWND): string;
var Buffer: array [0..255] of Char;
begin
  SetString(Result, Buffer, GetClassName(Wnd, Buffer, SizeOf(Buffer) - 1));
end;

function WindowsEnum(Handle: HWND; Param: Longint): BOOL; export; stdcall;
begin
  if WindowClassName(Handle) = 'TAppBuilder' then
  begin
    Result := False;
    PLongint(Param)^ := 1;
  end
  else
    Result := True;
end;

procedure ActivateWindow(Wnd: HWND);
begin
  if Wnd <> 0 then
  begin
    ShowWindow(Wnd, SW_SHOWNOACTIVATE);   //ShowWinNoAnimate(Wnd, SW_SHOW);
    SetForegroundWindow(Wnd);
  end;
end;

function FindPrevInstance(const MainFormClass, ATitle: string): HWnd;
var
  BufClass, BufTitle: PChar;
begin
  Result := 0;
  if (MainFormClass = '') and (ATitle = '') then Exit;
  BufClass := nil; BufTitle := nil;
  if (MainFormClass <> '') then
    BufClass := StrPAlloc(MainFormClass);
  if (ATitle <> '') then
    BufTitle := StrPAlloc(ATitle);
  try
    Result := FindWindow(BufClass, BufTitle);
  finally
    StrDispose(BufTitle);
    StrDispose(BufClass);
  end;
end; //FindPrevInstance

function ActivatePrevInstance(const MainFormClass, ATitle: string): Boolean;
var
  PrevWnd, PopupWnd, ParentWnd: HWnd;
  IsDelphi: Longint;
begin
  Result := False;
  PrevWnd := FindPrevInstance(MainFormClass, ATitle);
  if PrevWnd <> 0 then begin
    ParentWnd := GetWindowParent(PrevWnd);
    while (ParentWnd <> GetDesktopWindow) and (ParentWnd <> 0) do begin
      PrevWnd := ParentWnd;
      ParentWnd := GetWindowParent(PrevWnd);
    end;
    if WindowClassName(PrevWnd) = 'TApplication' then begin
      IsDelphi := 0;
      EnumThreadWindows(GetWindowTask(PrevWnd), @WindowsEnum, LPARAM(@IsDelphi));
      if Boolean(IsDelphi) then
        Exit;
      if IsIconic(PrevWnd) then  { application is minimized }
      begin
        SendMessage(PrevWnd, WM_SYSCOMMAND, SC_RESTORE, 0);
        Result := True;
        Exit;
      end
      else
        ShowWindow(PrevWnd, SW_SHOWNOACTIVATE); {ShowWinNoAnimate(PrevWnd, SW_SHOWNOACTIVATE);}
    end
    else
      ActivateWindow(PrevWnd);
    PopupWnd := GetLastActivePopup(PrevWnd);
    if (PrevWnd <> PopupWnd) and IsWindowVisible(PopupWnd) and IsWindowEnabled(PopupWnd) then
      SetForegroundWindow(PopupWnd)
    else
      ActivateWindow(PopupWnd);
    Result := True;
  end;
end; //ActivatePrevInstance

end.

program Project1;

uses
  Forms,
  Unit1 in 'Unit1.pas' {Form1},
  PrevInst in 'PrevInst.pas';

{$R *.res}

begin
  if ActivatePrevInstance(TForm1.ClassName, '') then
    Exit;
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.