See attached self contained example.
Subclassing QQuickView::event and returning false on an event of type QEvent::Close doesn't prevent the application to quit.
This mechanism works on Linux and Windows but not on macOS (I'm using 10.13.4)
So basically I have no way to implement a "Save before exit" step.
[EDIT] after comments (and adding event->ignore()), the issue is now only related to Cmd+Q or "Quit" from menu. I haven't found a way on macOS to prevent the application to quit