https://thenextweb.com/augmented-reality/2018/10/25/pokemon-go-step-tracking-offline-adventure-sync/

Niantic today announced a new feature for Pokemon GO called “Adventure Sync” — a lofty name for a very simple feature: step tracking.

Specifically, Adventure Sync will record your walking distance in the background, even if you’re doing other things with your phone. Up to now, the only way for the app to track your steps, which was a crucial part of advancing gameplay, was with your phone awake and the app open (or use the Pokemon Go Plus tracker).