Title: Preventing multiple application instances (final answer ;) )
Question: How to prevent second lunch of Your application and activate the previous instance instead
Answer:
Yes, I know about all articles about the same qustion ;)
But I hadn't seen this method description.
My decision gives You ability either prevent previous instance lunching and (optional) activating previous instance instead.
It's based on memory mapped files and works fine in Win9x/NT/2000.
simply place in .dpr file this lines (after 'begin')
if CheckAppAlreadyRunning ('MyApplicationSign')
then EXIT;
(here 'MyApplicationSign' - unique ID for this application)
and inlude in Your project unit with following function:
{$IFDEF VER120} { Delphi 4.0}
{$DEFINE D4_}
{$ENDIF}
{$IFDEF VER130} { Delphi 5.0}
{$DEFINE D4_}
{$ENDIF}
const
CheckAppRunningMFile : THandle = 0;
function CheckAppAlreadyRunning (const AppSign : string;
BringToFront : boolean {$IFDEF D4_}= true{$ENDIF}; DestroyOnExit : boolean {$IFDEF D4_}= false{$ENDIF}) : boolean;
// prevent double-starting of application. AppSign
// - unique identifier of application (this function
// will make MMF with this name)
// If BringToFront then send to prev instance msg RESTORE.
// If DestroyOnExit, then mapview will be closed (only for presence check,
// will not prevent starting apps with this AppSign).
var
p : Pointer;
AppWnd, TopWnd: HWND;
begin
Result := false;
if Result then ; // Kill compiler's stupid hint !!!
CheckAppRunningMFile := CreateFileMapping (
THandle ($FFFFFFFF), // use paging file
nil, // no security attributes
PAGE_READWRITE, // read/write access
0, // size: high 32 bits
4096, // size: low 32 bits
PChar(AppSign) // name of map object
);
if CheckAppRunningMFile = 0 then raise Exception.Create (
'CheckAppAlreadyRunning: can''t create file mapping');
try { close CheckAppRunningMFile if DestroyOnExit}
Result := GetLastError() = ERROR_ALREADY_EXISTS;
p := MapViewOfFile (
CheckAppRunningMFile, // object to map view of
FILE_MAP_WRITE, // read/write access
0, // high offset: map from
0, // low offset: beginning
0 // default: map entire file
);
if p = nil then raise Exception.Create (
'CheckAppAlreadyRunning: can''t create map view of file.');
try { p must die}
if Result then begin // already running
if BringToFront then begin
Move (p^, AppWnd, SizeOf (AppWnd));
if AppWnd 0 then begin
if IsIconic (AppWnd)
then SendMessage (AppWnd, WM_SYSCOMMAND, SC_RESTORE, 0);
TopWnd := GetLastActivePopup (AppWnd);
if (TopWnd 0) and (TopWnd AppWnd)
and IsWindowVisible (TopWnd)
and IsWindowEnabled (TopWnd)
then SetForegroundWindow (TopWnd);
end;
end;
end
else begin // first instance
AppWnd := Application.Handle;
Move (AppWnd, p^, SizeOf (AppWnd));
end;
finally UnmapViewOfFile (p);
end; { of try p must die}
finally if DestroyOnExit and (CheckAppRunningMFile 0) then begin
CloseHandle (CheckAppRunningMFile);
CheckAppRunningMFile := 0;
end
end; { of try 'close CheckAppRunningMFile if DestroyOnExit'}
end; { of function CheckAppAlreadyRunning
--------------------------------------------------------------}
initialization
finalization
if CheckAppRunningMFile 0
then CloseHandle (CheckAppRunningMFile);