问题
Is there any Out Of the Box Vaadin 10 (and higher) event similar to window.onbeforeunload in JavaScript? I've tried to use onDetach() or beforeLeave(), but it only works inside UI, and when user reloads the page or closes the page it's not working.
回答1:
You can use the approach described in https://vaadin.com/forum/thread/17523194/unsaved-changes-detect-page-exit-or-reload that was already suggested in a comment.
At the same time, I'd urge you to be really careful with beforeunload events since they are in some situations fired even though the user is actually not navigating away from the page.
The most common case is if the user clicks a link that starts a download. In that case the browser will fire the event immediately when the user clicks the link. Slightly later when the browser receives the response headers, it will discover that it's a download and not a new HTML page to display. The end result is then that beforeunload has been fired but the previous page is still kept running.
If you want to use the event for cleanup, then the best approach today is probably a combination of the unload event and then using the new-ish Beacon API for notifying the server that the user has actually navigated away. Integrating this into a Vaadin application will require slightly more JavaScript, but it has the benefit that it will actually work.
来源：https://stackoverflow.com/questions/56651887/vaadin-onbeforeunload-event