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

Answer 1

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 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

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.

Answer 4

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