Delphi: How to use ShowWindow properly on external application [duplicate] - delphi

Question

This question already has an answer here:
How can I make the second instance of my program pass control back to the first instance?
(1 answer)
Closed 8 years ago.
See also:
How can I tell if another instance of my program is already running?
i use the following code before starting my application, to check if another instance
of it is already started:
var _PreviousHandle : THandle;
begin
_PreviousHandle := FindWindow('TfrmMainForm',nil);
if _PreviousHandle <> 0 then
begin
ShowMessage('Application "" is already running!');
SetForegroundWindow(_PreviousHandle);
ShowWindow(_PreviousHandle, SW_SHOW);
Application.Terminate;
Exit;
end;
...
However, if it has started, i need to show that application. The problem is after it is shown in this way the minimize button no longer works, and when i click the icon in the taskbar, it "unminimizes" and the animation that is shown is as if it was minimized. Am i missing something? is there a proper way to activate and show external application while it's minimized?

Answer 1

Here is a complete project, which keeps running only one instance of the application, and which should bring already running instance window to front.
You can download a testing project or try the code, which follows:
Project1.dpr
program Project1;
uses
Forms,
Windows,
Unit1 in 'Unit1.pas' {Form1};
{$R *.res}
var
Mutex: THandle;
const
AppID = '{0AEEDBAF-2643-4576-83B1-8C9422726E98}';
begin
MessageID := RegisterWindowMessage(AppID);
Mutex := CreateMutex(nil, False, AppID);
if (Mutex <> 0) and (GetLastError = ERROR_ALREADY_EXISTS) then
begin
PostMessage(HWND_BROADCAST, MessageID, 0, 0);
Exit;
end;
Application.Initialize;
Application.MainFormOnTaskbar := True;
Application.CreateForm(TForm1, Form1);
Application.Run;
end.
Unit1.pas
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StrUtils, StdCtrls;
type
TForm1 = class(TForm)
private
function ForceForegroundWindow(WndHandle: HWND): Boolean;
function ForceRestoreWindow(WndHandle: HWND; Immediate: Boolean): Boolean;
protected
procedure WndProc(var AMessage: TMessage); override;
end;
var
Form1: TForm1;
MessageID: UINT;
implementation
{$R *.dfm}
{ TForm1 }
function TForm1.ForceForegroundWindow(WndHandle: HWND): Boolean;
var
CurrThreadID: DWORD;
ForeThreadID: DWORD;
begin
Result := True;
if (GetForegroundWindow <> WndHandle) then
begin
CurrThreadID := GetWindowThreadProcessId(WndHandle, nil);
ForeThreadID := GetWindowThreadProcessId(GetForegroundWindow, nil);
if (ForeThreadID <> CurrThreadID) then
begin
AttachThreadInput(ForeThreadID, CurrThreadID, True);
Result := SetForegroundWindow(WndHandle);
AttachThreadInput(ForeThreadID, CurrThreadID, False);
if Result then
Result := SetForegroundWindow(WndHandle);
end
else
Result := SetForegroundWindow(WndHandle);
end;
end;
function TForm1.ForceRestoreWindow(WndHandle: HWND;
Immediate: Boolean): Boolean;
var
WindowPlacement: TWindowPlacement;
begin
Result := False;
if Immediate then
begin
WindowPlacement.length := SizeOf(WindowPlacement);
if GetWindowPlacement(WndHandle, #WindowPlacement) then
begin
if (WindowPlacement.flags and WPF_RESTORETOMAXIMIZED) <> 0 then
WindowPlacement.showCmd := SW_MAXIMIZE
else
WindowPlacement.showCmd := SW_RESTORE;
Result := SetWindowPlacement(WndHandle, #WindowPlacement);
end;
end
else
Result := SendMessage(WndHandle, WM_SYSCOMMAND, SC_RESTORE, 0) = 0;
end;
procedure TForm1.WndProc(var AMessage: TMessage);
begin
inherited;
if AMessage.Msg = MessageID then
begin
if IsIconic(Handle) then
ForceRestoreWindow(Handle, True);
ForceForegroundWindow(Application.Handle);
end;
end;
end.
Tested on OS versions:
Windows 8.1 64-bit
Windows 7 SP1 64-bit Home Premium
Windows XP SP 3 32-bit Professional
Known issues and limitations:
The MainFormOnTaskbar is not taken into account at all; it must be set to True at this time

Answer 2

You're asking your Main form to show, but it may occur the application hidden window itself is minimized when you minimize the application to the task bar, in case of MainFormOnTaskBar being false.
Don't call the ShowWindow method from the oustide. IMHO it's better if you pass a message to the application and respond from inside, calling the Application.Restore` method, which performs the proper ShowWindow calls among other things.

Answer 3

This is a very common problem with VCL apps, and has been asked and answered many many times in the Borland/CodeGear/Embarcadero forums over the years. Using ShowWindow() in this manner does not work for VCL windows very well because of the way the MainForm interacts with the TApplication object at runtime, especially in different versions of Delphi. What you should do instead is have the second instance send a custom message to the first instance, and then let the first instance restore itself as needed when it receives the message, such as by setting its MainForm.WindowState property, or calling Application.Restore(), etc, and let the VCL work out the details for you, like #jachguate suggested.

Answer 4

The following works well for me. I'm not 100% certain I have fully understood the question though, so do let me know if I've got it wrong.
var
_PreviousHandle: HWND;
WindowPlacement: TWindowPlacement;
....
WindowPlacement.length := SizeOf(WindowPlacement);
GetWindowPlacement(_PreviousHandle, WindowPlacement);
if WindowPlacement.flags and WPF_RESTORETOMAXIMIZED<>0 then
WindowPlacement.showCmd := SW_MAXIMIZE
else
WindowPlacement.showCmd := SW_RESTORE;
SetWindowPlacement(_PreviousHandle, WindowPlacement);
SetForegroundWindow(_PreviousHandle);
Note that the correct type for _PreviousHandle is HWND and not THandle.