为什么没有触发WM_ERASEBKGND消息

[Jerffsan]

procedure TForm1.FormCreate(Sender: TObject);
begin
FClientInstance := MakeObjectInstance(ClientWndProc);
FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FClientInstance));
end;

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;

窗体的背景设为picture,但是编译后缺没有达到预期目标,跟进去发现根本
没有触发WM_ERASEBKGND消息，请问怎么解决？

[Jerffsan]

怎么没有人回答呢？

[Jerffsan]

期待高手指点！