Following a friend’s run live should not require spectators to sign up or the runner to publish a location history.
The constraint
The spectator side is private and accountless: an unlisted link opens a waiting page with no location, and nothing is sent until the runner explicitly confirms the start.
The approach
SwiftUI on iPhone and watchOS, Sign in with Apple owning each session, ordered GPS telemetry from the phone, a minimal Live Activity and optional heart rate from the Watch, served to a spectator web app on Cloudflare Workers and D1.
The outcome
The MVP runs end to end; production has not shipped.
A native iOS Formula 1 companion: standings, the season calendar with session weather, driver statistics, trends and widgets, from public OpenF1 and Open-Meteo data with no account.