mdi 背景

Question

procedure TMainForm.CreateWnd;
begin
  inherited CreateWnd;
  // Turn the ClientWndProc method into a valid window procedure
  FNewClientProc := MakeObjectInstance(ClientWndProc);
  // Get a pointer to the original window procedure
  FOldClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
  // Set ClientWndProc as the new window procedure
  SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FNewClientProc));
end;procedure TMainForm.DrawCentered;
{ This procedure centers the image on the form's client area }
var
  CR: TRect;
begin
  GetWindowRect(ClientHandle, CR);
   with imgMain do
     BitBlt(FDrawDC, ((CR.Right - CR.Left) - Picture.Width) div 2,
            ((CR.Bottom - CR.Top) - Picture.Height) div 2,
            Picture.Graphic.Width, Picture.Graphic.Height,
            Picture.Bitmap.Canvas.Handle, 0, 0, SRCCOPY);
end;

Answer 1

...
  private
    foldclientproc,fnewclientproc:tfarproc;
    fdrawdc:hdc;
    procedure drawtiled;
    procedure clientwndproc(var message:tmessage);
    procedure createwnd;override;
    { Private declarations }
  public
...
procedure tmain.createwnd;
begin
   inherited createwnd;
   fnewclientproc:=makeobjectinstance(clientwndproc);
   foldclientproc:=pointer(getwindowlong(clienthandle,gwl_wndproc));
   setwindowlong(clienthandle,gwl_wndproc,longint(fnewclientproc));
end;procedure tmain.clientwndproc(var message:tmessage);
begin
   case message.Msg of
      wm_erasebkgnd:
      begin
         callwindowproc(foldclientproc,clienthandle,message.msg,message.wparam,message.lparam);
         fdrawdc:=twmerasebkgnd(message).dc;
         drawtiled;
         message.Result:=1;
      end;
      wm_vscroll,wm_hscroll:
      begin
         message.Result:=callwindowproc(foldclientproc,clienthandle,message.msg,message.wparam,message.lparam);
         invalidaterect(clienthandle,nil,true);
      end;
      else
        message.Result:=callwindowproc(foldclientproc,clienthandle,message.msg,message.wparam,message.lparam);
   end;
end;procedure tmain.drawtiled;
var
  row,col:integer;
  cr,ir:trect;
  numrows,numcols:integer;
begin
   getwindowrect(clienthandle,cr);
   ir:=image1.clientrect;
   numrows:=cr.bottom div ir.bottom;
   numcols:=cr.right div ir.right;
   with image1 do
     for row:=0 to 4 do
        for col:=0 to 7 do
            bitblt(fdrawdc,col*picture.width,row*picture.height,picture.width,picture.height,picture.bitmap.canvas.handle,0,0,srccopy);
end;