This HRV biofeedback device uses an Empatica E4 to detect individual heartbeats. It shows a breathing pacing signal, instant heart rate, and a frequency analysis of instant heart rate (breathing rate when breathing with a slow, even pattern). Even without an E4, the app will display the pacing signal.

The heartbeat detection algorithm is explained in this paper.

Information on the Empatica E4 API can be found here.