Long button press to trigger an event

Hello Nordic team,

I am trying to develop a long button press handler, but I am unable to find the proper documentation or example code to help with that. I am not sure if I should use bsp.c or app_button.c. 

Reading through devzone has me more inclined to using app_button instead of bsp.c but I am unsure as to what the differences really are. Is there example code that can support my development??

Thanks for your time and support,