MasterDetailPage - page data is lost when navigated to another detail page

I have an MD page with 3 detail pages (of which each of them are of type ContentPage)

The problem I have now is that when the user navigates to another detail page, the data on the current page are lost (which is not fine) and when the user goes back to that page, it's like it's initialised all over again (which is fine)

Now, I am looking to find a way to raise an event on the detail page to realise when the page is left so that I can save data accordingly.

I know how to do it on the MasterDetailPage by accessing an event:

MasterPage.ListView.ItemSelected += ListView_ItemSelected;

but I need to raise this event on the detail page itself so I can save data first.

Thanks