Feel Every Beat: AnyMusic Brings Haptic-Synchronized Music to iOS & Android

Audio players expose position at irregular intervals, and native haptic APIs have their own latency. Naive trigger approaches caused vibrations that felt noticeably off from the music — ruining the core experience.

Built a custom HapticEngine that runs a 16ms polling loop against the just_audio position stream. It maintains a sorted event queue and fires any events within a ±20ms window of the current playback position. Seek detection (>500ms position jumps) triggers an automatic queue resync. Events missed by more than 30ms are silently skipped to avoid late jarring vibrations.

Limiting free users to 30 cumulative seconds — across all songs and across app sessions — required a reliable counter that couldn't be gamed by force-quitting the app.

PlaytimeService maintains a local delta counter during playback and flushes to Firestore (atomic increment on users/{uid}.totalPlayedSeconds) every 10 seconds, on pause, and on dispose. On app launch, the remaining free seconds are loaded from Firestore before any audio starts. If quota reaches zero, the player pauses immediately and the RevenueCat paywall is presented.

Different audio types — classical music, cat purring, chants, lullabies — require completely different haptic strategies. A one-size-fits-all approach produced unsatisfying results across the content library.

The Python Cloud Function computes four acoustic features per uploaded track: onsets-per-second (attack density), spectral centroid (frequency balance), low-frequency energy ratio (bass heaviness), and zero-crossing rate (texture). Decision tree logic maps these to four haptic modes, each with its own event generation algorithm — onset-triggered for music, energy-envelope for continuous sounds.