How to create an exit dialog box that pops up when back button is clicked

Hi!
Usually, you have a screen stack, where you can check whether the current view is the last in the stack or not. With this, you can achieve this functionality. You can use some flag for the initial screen e.g: "firstView" or smth like that, and then observe the screen stack for change. If the current view has this flag (or a similar one), and back btn has been pressed - show dialog. 

I hope you get the idea.