给MDI主窗口加背景

Question

unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls;type
  TForm1 = class(TForm)
    Image1: TImage;
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
    FClientInstance,FPrevClientProc : TFarProc;
    PROCEDURE ClientWndProc(VAR Message: TMessage);
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementation
{$R *.dfm}PROCEDURE TForm1.ClientWndProc(VAR Message: TMessage);
VAR
  MyDC : hDC;
  Ro, Co : Word;
begin
    with Message do
    case Msg of
        WM_ERASEBKGND:
        begin
            MyDC := TWMEraseBkGnd(Message).DC;
            FOR Ro := 0 TO ClientHeight DIV Image1.Picture.Height DO
                FOR Co := 0 TO ClientWIDTH DIV Image1.Picture.Width DO
                    BitBlt(MyDC,
                           Co*Image1.Picture.Width,
                           Ro*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 case
end;procedure TForm1.FormCreate(Sender: TObject);
begin
    FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
    FClientInstance := MakeObjectInstance(ClientWndProc);
    SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FClientInstance));
end;end.