Tpaintbox控件的Refresh等三个步骤特点、区别

procedure TControl.Update;
begin
  if Parent <> nil then Parent.Update;
end;

procedure TControl.Refresh;
begin
  Repaint;
end;

procedure TControl.Repaint;
var
  DC: HDC;
begin
  if (Visible or (csDesigning in ComponentState) and
    not (csNoDesignVisible in ControlStyle)) and (Parent <> nil) and
    Parent.HandleAllocated then
    if csOpaque in ControlStyle then
    begin
      DC := GetDC(Parent.Handle);
      try
        IntersectClipRect(DC, Left, Top, Left + Width, Top + Height);
        Parent.PaintControls(DC, Self);
      finally
        ReleaseDC(Parent.Handle, DC);
      end;
    end else
    begin
      Invalidate;
      Update;
    end;
end;