A simple foreground MP3 player for Android 8.0+ (API 26). Kotlin, Jetpack Compose, Media3 ExoPlayer, Room.
- Songs: every MP3 on the device (read via
MediaStore,audio/mpeg+IS_MUSIC), sorted by title. Tap to play the whole list from that song. - Playlists: create, rename, delete; add songs from the Songs tab; remove and drag-to-reorder inside a playlist. A song appears at most once per playlist.
- Controls: play/pause, previous (restarts the track if past ~3s), next, seek bar with elapsed/total time, shuffle, repeat (off → all → one).
- Pauses when the app leaves the foreground, when headphones are unplugged, and on audio-focus loss (e.g. a phone call).
| Area | Decision |
|---|---|
| Track identity | Playlists store the MediaStore _ID. Each scan prunes entries for files that no longer exist; an empty scan never prunes (protects against an unmounted SD card wiping playlists). Missing tracks are also hidden at display time. |
| Ordering | playlist_tracks(playlist_id, track_id, position) with PK (playlist_id, track_id). Positions are kept dense; every mutation (add/remove/move/prune) is a single Room transaction. |
| Reorder | Drag updates a local list for smooth feedback; on drop one id-based move (track → slot of target track) is persisted, so a stale UI list can't move the wrong row. |
| Playback queue | A snapshot of the list you played from. Editing a playlist doesn't disturb what's currently playing. |
| Threading | MediaStore scan on Dispatchers.IO; Room suspend APIs; prune deletes in chunks of 500 to stay under SQLite's 999-variable limit. Progress polls every 500 ms only while playing. |
| Lifecycle | PlayerViewModel owns ExoPlayer so playback survives rotation; released in onCleared. Paused in Activity.onStop. |
data/ Track, TrackRepository (MediaStore), PlaylistRepository, db/ (Room entities + DAO)
player/ AudioPlayer interface, ExoAudioPlayer, PlayerState, RepeatMode
ui/ LibraryViewModel, PlayerViewModel, Compose screens, PlayerBar
Requires JDK 17+ and the Android SDK (platform 35).
./gradlew assembleDebug # app/build/outputs/apk/debug/app-debug.apk
./gradlew testDebugUnitTest # JVM: unit + Robolectric (Room, ViewModels, Compose UI, E2E)
./gradlew connectedDebugAndroidTest # E2E on a device/emulatorsrc/sharedTest holds the end-to-end flow test (launches MainActivity, creates/renames/opens/deletes a playlist). It runs both under Robolectric and as an instrumented test.