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Program won't accept TMessage without Show, then Hide

Posted on 2008-10-20
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Last Modified: 2012-05-05
Hi. I'm making a system tray application that is not supposed to be visible, ever. It needs to detect when the user is about to log off, and pop up a message. However, I'm having issues getting a clean solution to intercepting the windows messages, as I have to show the form and then hide it. If I don't do this, then the messages never get received.
unit Unit2;
 

interface
 

uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;
 

type

  TForm2 = class(TForm)

    Button1: TButton;

  procedure EndSMsg(var T : TMessage);message  WM_QUERYENDSESSION;

    procedure Button1Click(Sender: TObject);

    procedure FormCreate(Sender: TObject);

  private

    { Private declarations }

  public

    { Public declarations }

  end;
 

var

  Form2: TForm2;
 
 

implementation
 

{$R *.dfm}
 
 

procedure TForm2.Button1Click(Sender: TObject);

begin

  hide;

end;
 

procedure TForm2.EndSMsg(var T : TMessage);

begin

  T.result := 1;

  if T.lparam = 0 then

    begin

    ShowMessage('System is shutting down');

    end

  else if (DWORD(T.lparam) and ENDSESSION_LOGOFF) =

                                 ENDSESSION_LOGOFF then

    begin

    ShowMessage('User is logging off');

    end;

end;
 

procedure TForm2.FormCreate(Sender: TObject);

begin

  show;

  hide;

end;
 

end.

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0
Comment
Question by:the_modder
  • 4
  • 2
  • 2
8 Comments
 
LVL 13

Expert Comment

by:ThievingSix
ID: 22755764
For something like this you wouldn't use a form at all.

Like so:



program Project1;
 

uses

  Windows, Messages, Dialogs;
 

var

  Msg : TMsg;

begin

  While GetMessage(Msg,0,0,0) Do

    begin

    If Msg.message = WM_QUERYENDSESSION Then

      begin

      If Msg.lParam = 0 Then

        begin

        ShowMessage('System is shutting down');

        Break;

      end

      Else If (Msg.lParam And ENDSESSION_LOGOFF) = ENDSESSION_LOGOFF Then

        begin

        ShowMessage('User is logging off');

        Break;

      end;

    end;

  end;

end.

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0
 
LVL 21

Expert Comment

by:ziolko
ID: 22756016
use Application.OnMessage event handler, the way you do this message wil lbe intercepted only if TFOrm2 exists

ziolko.
0
 
LVL 1

Author Comment

by:the_modder
ID: 22760129
ThievingSix:
The attached code doesn't seem to work. Nothing gets triggered.

ziolko: I think that the message never gets sent to my window because the application never registers itself since its invisible (application.showmainform = false)
program Project1;
 

uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls, ShellAPI, INIFiles,

  Unit1 in 'Unit1.pas' {Form1};
 

{$R *.res}
 

var

  Msg : TMsg;

begin

  Application.Initialize;

  Application.CreateForm(TForm1, Form1);

  application.showmainform := false;

  Application.Run;

  While GetMessage(Msg,0,0,0) Do

    begin

    If Msg.message = WM_QUERYENDSESSION Then

      begin

      If Msg.lParam = 0 Then

        begin

        ShowMessage('System is shutting down');

        Break;

      end

      Else If (Msg.lParam And ENDSESSION_LOGOFF) = ENDSESSION_LOGOFF Then

        begin

        ShowMessage('User is logging off');

        Break;

      end;

    end;

  end;

end.

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0
 
LVL 13

Assisted Solution

by:ThievingSix
ThievingSix earned 20 total points
ID: 22763417
Comment out line 16: "Application.Run". If you need to use a form in the application do something like below.
program Project1;
 

uses

  Windows,

  Messages,

  SysUtils,

  Variants,

  Classes,

  Graphics,

  Controls,

  Forms,

  Dialogs,

  StdCtrls,

  ShellAPI,

  INIFiles,

  Unit2 in 'Unit2.pas' {Form2};
 

{$R *.res}
 

var

  Msg : TMsg;

begin

  Application.Initialize;

  Application.CreateForm(TForm2, Form2);

  While GetMessage(Msg,0,0,0) Do

    begin

    TranslateMessage(Msg);

    DispatchMessage(Msg);

    If Msg.message = WM_QUERYENDSESSION Then

      begin

      If Msg.lParam = 0 Then

        begin

        ShowMessage('System is shutting down');

        Break;

      end

      Else If (Msg.lParam And ENDSESSION_LOGOFF) = ENDSESSION_LOGOFF Then

        begin

        ShowMessage('User is logging off');

        Break;

      end;

    end;

  end;

end.

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LVL 1

Author Comment

by:the_modder
ID: 22764552
Even this code doesn't work. It does not detect a user logoff :(
    program Project1;

 

uses

  Windows,

  Messages,

  SysUtils,

  Variants,

  Classes,

  Graphics,

  Controls,

  Forms,

  Dialogs,

  StdCtrls,

  ShellAPI,

  INIFiles;

 

{$R *.res}

 

var

  Msg : TMsg;

begin

  While GetMessage(Msg,0,0,0) Do

    begin

    TranslateMessage(Msg);

    DispatchMessage(Msg);

    If Msg.message = WM_QUERYENDSESSION Then

      begin

      If Msg.lParam = 0 Then

        begin

        ShowMessage('System is shutting down');

        Break;

      end

      Else If (Msg.lParam And ENDSESSION_LOGOFF) = ENDSESSION_LOGOFF Then

        begin

        ShowMessage('User is logging off');

        Break;

      end;

    end;

  end;

end.

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0
 
LVL 21

Accepted Solution

by:
ziolko earned 80 total points
ID: 22764979
>>I think that the message never gets sent to my window because the application never registers itself since its invisible (application.showmainform = false)

showing main form doesn't matter once aplication object is created it's ready to process windows messages, WM_QUERYENDSESSION is sent to all top level windows so it must reach Appplication object.

your code snippets with while GetMessage() do ... are not a good idea because once GetMessage() returns false (no messages in queue) loop is exited and never again checks for incoming messages.

Like I wrote before write OnMessage event handler and assign it to Application.OnMessage or use TApplicationEvents component and it's OnMessage event.

ziolko.
0
 
LVL 1

Author Comment

by:the_modder
ID: 22774331
ziolko: I have followed your advice and added a TApplicationEvents component. However, it still does not detect a user log off. If I don't comment out the Msg.lparam and the following sections, when I right click on the desktop, it seems that the application just keeps on opening up message boxes.
procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;

  var Handled: Boolean);

begin

  //if Msg.lparam = 0 then

  //  begin

  //  ShowMessage('System is shutting down');

  //  end

  //else

  if (DWORD(Msg.lparam) and ENDSESSION_LOGOFF) =

                                 ENDSESSION_LOGOFF then

    begin

    ShowMessage('User is logging off');

    end;
 

end;

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0
 
LVL 1

Author Closing Comment

by:the_modder
ID: 31507735
Thanks guys. After hours of bashing my head into the wall, I figured out that another one of my procedures was grabbing the messages. However, ziolko's solution was better, as I learned something new from it, which I will use later in my projects.
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