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Frequently Asked Questions

Displaying a bitmap on the client area of an MDI parent form.

Question:

How do I display a bitmap to the client area of an MDI parent form?

Answer:

Here are the necessary steps to add wallpaper to a MDI parent

form:





Create a new project

Set the form's FormStyle to fsMDIForm

Drop an image on the form and select a bitmap into it.

Find the { Private Declarations } comment in the form's

   definition and add these lines right after it:



    FClientInstance : TFarProc;

    FPrevClientProc : TFarProc;

    procedure ClientWndProc(var Message: TMessage);



Find the "implementation" line and the {$R *.DFM} line that

   follows it. After that line, enter this code:



procedure TMainForm.ClientWndProc(var Message: TMessage);

var

  Dc : hDC;

  Row : Integer;

  Col : Integer;

begin

  with Message do

    case Msg of

      WM_ERASEBKGND:

      begin

        Dc := TWMEraseBkGnd(Message).Dc;

        for Row := 0 to ClientHeight div Image1.Picture.Height do

          for Col := 0 to ClientWidth div Image1.Picture.Width do

            BitBlt(Dc,

               Col * Image1.Picture.Width,

               Row * Image1.Picture.Height,

               Image1.Picture.Width,

               Image1.Picture.Height,

               Image1.Picture.Bitmap.Canvas.Handle,

               0,

               0,

               SRCCOPY);

          Result := 1;

      end;

      else

        Result := CallWindowProc(FPrevClientProc,

                                 ClientHandle,

                                 Msg,

                                 wParam,

                                 lParam);

  end;

end;



In the OnCreate method for the form, type the following lines

of code:



   FClientInstance := MakeObjectInstance(ClientWndProc);

   FPrevClientProc := Pointer(GetWindowLong(ClientHandle,

                              GWL_WNDPROC));

   SetWindowLong(ClientHandle,

                 GWL_WNDPROC, LongInt(FClientInstance));



Add a new form to your project and set its FormStyle property to

fsMDIChild.







Now you have a working MDI project with "wallpaper" where the image

bitmap is tiled to cover the MDI form's client area.

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