WM_PARENTNOTIFY的作用（不完全）

VCL里源码如下：

procedure TWinControl.WMParentNotify(var Message: TWMParentNotify);
begin
  with Message do
    if (Event <> WM_CREATE) and (Event <> WM_DESTROY) or
      not DoControlMsg(Message.ChildWnd, Message) then inherited;
end;

function DoControlMsg(ControlHandle: HWnd; var Message): Boolean;
var
  Control: TWinControl;
begin
  DoControlMsg := False;
  Control := FindControl(ControlHandle);
  if Control <> nil then
    with TMessage(Message) do
    begin
      Result := Control.Perform(Msg + CN_BASE, WParam, LParam);
      DoControlMsg := True;
    end;
end;