Health Kit Data Access

HealthKit is Apple's framework for accessing health and fitness data on iOS and watchOS. Apps can read user health metrics (steps, heart rate, sleep) from HealthKit and write results (workout data, nutrition). HealthKit aggregates data from multiple sources: Apple Watch, fitness trackers, third-party health apps, and manual entry. Unlike other health APIs, HealthKit is on-device, fast, and privacy-focused. Apple doesn't see the data. Users control permissions granularly.