HRV real time and passively

the specifics are pretty straight forward we just need to the code to get HRV in realtime and pasively like the zendo app in the app store https://github.com/zendotools/zendo but that github doesnt compile. on the watch i can get the Heart rate to update but not hear rate variability. I need it in real time while the user is looking at his watch and passively throught out the day,.