@bolloxed what do you mean exactly when you say "open and running"? Do you mean the app is in the foreground and visible on the screen?

It should work even when you suspend the app, i.e. when you navigate to another app or turn off the screen. It will however not work if you terminate the app by force quitting it ("swipe kill" in the recent apps list, https://daringfireball.net/2017/07/you_should_not_force_quit_apps).