The notebook and page widgets provide three different callbacks (pageChangeCallback, pageEnterCallback, and pageLeaveCallback) when a new page is brought to the top. The notebook's pageChangeCallback and a page's pageLeaveCallback both allow, by setting a doit flag to false, an application to stop a new page from being topped. This is useful when some input is still required from the user on the old page. If the previous callbacks have been activated successfully, the page's pageEnterCallback will fire. This notifies an application of the new page being brought on top. By hooking this callback, an application can delay the creation of the children of a page (its widgets) until that page is brought on top for the first time. Delayed pages can be useful when a notebook has many pages or initial open time is critical.