//工程引用此单元就能防止同时出现多个实例
unit MultInst;

interface

uses Windows ,Messages, SysUtils, Classes, Forms;

implementation

const
  STR_UNIQUE    = '{2BE6D96E-827F-4BF9-B33E-8740412CDE96}';
  MI_ACTIVEAPP  = 1;  {激活应用程序}
  MI_GETHANDLE  = 2;  {取得句柄}

var
  iMessageID    : Integer;
  OldWProc      : TFNWndProc;
  MutHandle     : THandle;
  BSMRecipients : DWORD;

function NewWndProc(Handle: HWND; Msg: Integer; wParam, lParam: Longint): Longint; stdcall;
begin
  Result := 0;

  if Msg = iMessageID then
  begin
    case wParam of
      MI_ACTIVEAPP: {激活应用程序}
        if lParam<>0 then
        begin
          {收到消息的激活前一个实例}
          {为什么要在另一个程序中激活?}
          {因为在同一个进程中SetForegroundWindow并不能把窗体提到最前}
          if IsIconic(lParam) then
            OpenIcon(lParam)
          else
            SetForegroundWindow(lParam);
          Application.Terminate; {终止本实例}
        end;
      MI_GETHANDLE: {取得程序句柄}
        begin
          PostMessage(HWND(lParam), iMessageID, MI_ACTIVEAPP, Application.Handle);
        end;
    end;
  end
  else
    Result := CallWindowProc(OldWProc, Handle, Msg, wParam, lParam);
end;

procedure InitInstance;
begin
  {取代应用程序的消息处理}
  OldWProc := TFNWndProc(SetWindowLong(Application.Handle, GWL_WNDPROC, Longint(@NewWndProc)));

  {打开互斥对象}
  MutHandle := OpenMutex(MUTEX_ALL_ACCESS, False, STR_UNIQUE);
  if MutHandle = 0 then
  begin
    {建立互斥对象}
    MutHandle := CreateMutex(nil, False, STR_UNIQUE);
  end
  else
  begin
    Application.ShowMainForm  :=  False;

    {已经有程序实例,广播消息取得实例句柄}
    BSMRecipients := BSM_APPLICATIONS;
    BroadCastSystemMessage(BSF_IGNORECURRENTTASK or BSF_POSTMESSAGE, @BSMRecipients, iMessageID, MI_GETHANDLE,Application.Handle);
  end;
end;

initialization
  {注册消息}
  iMessageID  := RegisterWindowMessage(STR_UNIQUE);
  InitInstance;

finalization
  {还原消息处理过程}
  if OldWProc <> Nil then
    SetWindowLong(Application.Handle, GWL_WNDPROC, LongInt(OldWProc));

  {关闭互斥对象}
  if MutHandle <> 0 then
    CloseHandle(MutHandle);
end.