Dim the main application form when a dialog box is displayed

Dialog windows you use to display critical information to the user are, in most cases, displayed modally. A modal form is one where the application can't continue to run until the modal (information) form is closed. Delphi's ShowMessage, InputBox and MessageDlg, for example, display a modal form to the user waiting for some action. Your custom dialogs are displayed using the ShowModal method of a form. To emphasize the importance of a modal form and the information it presents, you could gray out the main form of the application when the modal form is activated. Here's how to add a dim-out effect to your main form when modal forms are waiting for the user input.

Have a Main Form of your application Add a new form to the project. This will be the "dimmer" form. Drop the TApplicationEvents component on the main form. Handle the OnModalBegin and OnModalEnd events. The TDimmerForm is created at application startup - and since it is not the main form, it will not be displayed initially. The Display procedure aligns the dimmer form above the main form and gets displayed before any modal form is shown. This happens in the TApplicationEvent's OnModalBegin event. OnModalEnd ensures that dimmer form is hidden until needed next time.

procedure TDimmerForm.FormCreate(Sender: TObject) ;
  AlphaBlend := true;
  AlphaBlendValue := 128;
  BorderStyle := bsNone;

The above is the OnCreate event handler for the dimmer form. By using the AlphaBlend property and the AlphaBlendValue you can make the form translucent. As it will be displayed over the main form, we want it to create the dim-out effect. BorderStyle ensures this form has no border, no caption bar, no title buttons. The Display procedure aligns the dimmer form above the main form:

procedure TDimmerForm.Display;
  with Self do
    Left := MainForm.Left;
    Top := MainForm.Top;
    Width := MainForm.Width;
    Height := MainForm.Height;

Finally, handle the OnModalBegin and OnModalEnd events of the TApplicationEvents component on the main form:

//TApplicationEvents OnModalBegin
procedure TMainForm.ApplicationEvents1ModalBegin(Sender: TObject) ;
//TApplicationEvents OnModalEnd
procedure TMainForm.ApplicationEvents1ModalEnd(Sender: TObject) ;

That's it!

In MDI (multiple document interface) Delphi applications, more than one form can be opened within a single parent window - main form. MDI application are ideal candidates for the dimmer form :)

Author: Unknown
Contributor: topellina
Added: 2013-09-06
Last updated: 2013-09-06