How to deal with "Back"?

I found this http://forums.xamarin.com/discussion/18565/override-cancel-pop-when-hardware-back-button-pressed-using-navigatepage

but it doesn't provide any clear answer as to how we are supposed to deal with the situation where a user presses back (either hardware, or via the navigation bar back button) and there is unsaved data in the current view.

Is there any standard guidance around this common scenario?

On iOS I can just set the page to modal, but what about hardware back button on Android?

Is it possible to intercept the navigation bar back button press?

Thanks for help on this.