请教：有关模式窗体的问题(50分)

function TCustomForm.ShowModal: Integer;
var
  WindowList: Pointer;
  SaveFocusCount: Integer;
  SaveCursor: TCursor;
  SaveCount: Integer;
  ActiveWindow: HWnd;
begin
  CancelDrag;
  if Visible or not Enabled or (fsModal in FFormState) or
    (FormStyle = fsMDIChild) then
    raise EInvalidOperation.Create(SCannotShowModal);
  if GetCapture <> 0 then
    SendMessage(GetCapture, WM_CANCELMODE, 0, 0);
  ReleaseCapture;
  Include(FFormState, fsModal);
  ActiveWindow := GetActiveWindow;
  SaveFocusCount := FocusCount;
  Screen.FSaveFocusedList.Insert(0, Screen.FFocusedForm);
  Screen.FFocusedForm := Self;
  SaveCursor := Screen.Cursor;
  Screen.Cursor := crDefault;
  SaveCount := Screen.FCursorCount;
  WindowList := DisableTaskWindows(0);
  try
    Show;
    try
      SendMessage(Handle, CM_ACTIVATE, 0, 0);
      ModalResult := 0;
      repeat
        Application.HandleMessage;
        if Application.FTerminate then
          ModalResult := mrCancel
        else if ModalResult <> 0 then
          CloseModal;
      until ModalResult <> 0;
      Result := ModalResult;
      SendMessage(Handle, CM_DEACTIVATE, 0, 0);
      if GetActiveWindow <> Handle then
        ActiveWindow := 0;
    finally
      Hide;
    end;
  finally
    if Screen.FCursorCount = SaveCount then
      Screen.Cursor := SaveCursor
    else
      Screen.Cursor := crDefault;
    EnableTaskWindows(WindowList);
    if Screen.FSaveFocusedList.Count > 0 then
    begin
      Screen.FFocusedForm := Screen.FSaveFocusedList.First;
      Screen.FSaveFocusedList.Remove(Screen.FFocusedForm);
    end
    else
      Screen.FFocusedForm := nil;
    if ActiveWindow <> 0 then
      SetActiveWindow(ActiveWindow);
    FocusCount := SaveFocusCount;
    Exclude(FFormState, fsModal);
  end;
end;