From: "Richard" <rtsmatrix@hotmail.com>
Newsgroups: borland.public.delphi.graphics
Subject: MDI and background image update (for anybody interested)
Date: Thu, 21 Jun 2001 16:43:41 -0600
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I got some good code that allowed me to get background images drawn on my
MDI application.  My only problem was that if I moved a child window
slightly off screen the vertical and/or horizontal scroll bars would appear
but the background wasn't redrawn to fit.  The same thing happened when you
moved the child window back into the center of the parent and the scroll
bars disappeared. I did some searching into the Windows messages and found
that if I trapped the message WM_WINDOWPOSCHANGED and invalidated the client
area then it would catch this event and my background always redrew
properly. I'm not sure if this is the best way to do it but it works well
for me.

Here is the original code (with many thanks to Peter Below) with my code
changes blocked with // New Code

Richard Speiss

unit Main;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
Dialogs,
  Menus, StdCtrls, ToolWin, ComCtrls;

type
  TForm1 = class(TForm)
    ToolBar1: TToolBar;
    ToolButton1: TToolButton;
    ToolButton2: TToolButton;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure FormResize(Sender: TObject);
  private
    { Private declarations }
    FLogoBitmap: TBitmap;
    FOldProc: Pointer;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

Function ClientWindowProc( wnd: HWND; msg: Cardinal; wparam,
lparam:Integer): Integer; stdcall;
Var
  f: TForm1;
  r: Trect;
Begin
  f:= TForm1( GetWindowLong( wnd, GWL_USERDATA ));
  Case msg of
    WM_ERASEBKGND: Begin
        GetClientrect( wnd, r );
        StretchBlt(wparam, r.left, r.top, r.right, r.bottom,
                   f.FLogoBitmap.Canvas.Handle, 0, 0,
                   f.FLogoBItmap.Width, f.FLogoBitmap.Height,
                   SRCCOPY);
        result := 1;
        Exit;
      End;
    WM_HSCROLL, WM_VSCROLL:
      If LoWord( wparam ) <SB_ENDSCROLL Then
        InvalidateRect( wnd, nil, true );
// New Code
    WM_WINDOWPOSCHANGED:
        InvalidateRect( wnd, nil, true );
// New Code
  End;
  Result := CallWindowProc( f.FOldProc, wnd, msg, wparam, lparam );
End;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FLogoBitmap:= TBitmap.Create;
  FLogoBitmap.LoadFromFile('C:\WINDOWS\Wolken.bmp');
  WindowState := wsMaximized;
  If ClientHandle <0 Then Begin
    SetWindowLong( ClientHandle, GWL_USERDATA, integer(self));
    FOldProc := Pointer(SetWindowLong( ClientHandle, GWL_WNDPROC,
integer(@ClientWindowProc)));
  End;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  FLogobitmap.Free;
end;

// New Code
// This is not needed anymore as the above code handles the window resizing
as well
//procedure TForm1.FormResize(Sender: TObject);
//begin
//  InvalidateRect( ClientHandle, nil, true );
//end;
// New Code