Capturing application messages

Copyright © 2000 Ernesto De Spirito

Sometimes we need to capture Windows messages at an application level. One way is by a hook, and another way is by using the Message event of the Application object which is triggered every time the application gets a Windows message. We are going to follow the second path. There are two ways to set an event handler for this event: one is using an ApplicationEvents object that comes in Delphi 5 (simply double click its OnMessage combo box in the Object Inspector) and the other is doing it by hand:

1. In the private section of your main form add the following declaration:

   procedure ApplicationMessage(var Msg: TMsg; var Handled: Boolean);

2. Assign the OnMessage property in the Create event of the form for example:

   procedure TForm1.FormCreate(Sender: TObject);
   begin
     Application.OnMessage := ApplicationMessage;
   end;

3. When you want to stop capturing messages all you have to do is set OnMessage to nil:

   procedure TForm1.FormDestroy(Sender: TObject);
   begin
     Application.OnMessage := nil;
   end;

Finally you should implement the procedure. For example, we are going to trap the keyboard messages WM_KEYUP and WM_KEYDOWN to convert the decimal point of the numeric keypad to a comma (this is useful in Spanish applications).

procedure TForm1.ApplicationMessage(var Msg: TMsg;
  var Handled: Boolean);
begin
  case Msg.Message of
  WM_KEYDOWN, WM_KEYUP:
    case Msg.wParam of
    // Replace the period in the numeric keypad (key code = 110)
    // with a comma (key code = 188).
    110: Msg.wParam := 188;
    end;
  end;
end;

Now you can drop an Edit control on your form and test it.