Nav BT Display

Shows turn-by-turn navigation hints on car stereos that only support Bluetooth audio (A2DP/AVRCP) and no Android Auto.

Such stereos can display the "now playing" metadata (title / artist / album) of the phone. This app reads the ongoing guidance notification of a navigation app and publishes its text as "now playing" metadata, so the car shows e.g.

Title:  300 m · Turn right          (distance + maneuver)
Artist: Main St                     (street / direction)
Album:  12 min · 5.3 km · ETA 18:34 (trip summary)

If a part is missing, the next one moves up (e.g. no street → summary as artist), so the title always carries the most important information. The display is refreshed every time the navigation app updates its notification.

Status: experimental. Reading other apps' notifications is unofficial. Navigation apps can change their notification at any time, and Google Maps does not officially share guidance outside Android Auto.

How it works

1. NavListenerService (a NotificationListenerService) receives the notifications of the supported navigation apps. Only ongoing (or CATEGORY_NAVIGATION) notifications that are not group summaries are considered guidance.
2. NotificationDump/NavText take EXTRA_TITLE, EXTRA_TEXT (or the first line of EXTRA_BIG_TEXT), EXTRA_SUB_TEXT and EXTRA_INFO_TEXT. If the extras carry no text, the custom RemoteViews layout is inflated off-screen and its TextViews are read. Heuristics (English, German, Czech) then split the text into distance, maneuver, street/direction and trip summary.
3. NavPublisher puts the text into a framework MediaSession. When the guidance notification disappears, the session is released.

The hard part: the car only shows one player

Verified in AOSP source (frameworks/base MediaSessionStack and AudioPlayerStateMonitor, packages/modules/Bluetooth MediaPlayerList):

• The Bluetooth AVRCP stack sends the car the metadata of the media button session (onMediaKeyEventSessionChanged → setActivePlayer).
• Android picks the media button session from the app that most recently started an audio player (any usage; only isActive() counts). An app that has a media session but plays no audio never wins against a music app that is actually playing.
• When the head of that list stops playing, the most recent app that is still playing moves back to the front.

So the app plays digital silence (SilentAudio, USAGE_MEDIA, no audio focus request, so the music keeps playing unchanged) while it wants the display, and stops it and releases its session to hand the display back.

Display modes

Mode	What the car shows	Downsides
Takeover (default)	Navigation for the whole trip, distance counting down	Music title is not visible. Car buttons reach this app (see below).
Flash	Each new maneuver for 8 s, then the music title again	You only see a hint when the maneuver changes, not the countdown to it. Each switch is a "player change" for the car; some head units show a source popup or reset the display.
Passive	Session only, no silent audio	Practically invisible while music plays; only useful for cars that let you pick the player in a Bluetooth browse menu.

Car buttons (play / pause / next / previous)

While this app holds the display, the car's buttons are delivered to its session. They are forwarded to the music app's session if this app can see it (MediaSessionManager.getActiveSessions, same Android profile only). Otherwise (e.g. the music app runs in the work profile) the app hands the display back and re-sends the key via AudioManager.dispatchMediaKeyEvent, so it reaches the music app; in Takeover mode the display is then left to the music app until the next maneuver.

Some head units send "play" by themselves when the active player changes; that would be forwarded to the music app and may start paused music.

Profiles

A notification listener only sees notifications of its own Android profile. The navigation app and this app must be installed in the same profile (personal or work). The music app may be in another profile.

Supported navigation apps

Whether an app's notification contains usable text has to be checked on a real device (Settings → Debug log, or adb logcat -s NavBtDisplay).

App	Package(s)	Notification text
Google Maps	com.google.android.apps.maps	not yet verified
Mapy.com	cz.seznam.mapy	not yet verified

Other apps (Waze, OsmAnd, Organic Maps, CoMaps, HERE WeGo) are left out of the first version. OsmAnd would be best supported through its official AIDL navigation API rather than its notification.

Testing

1. Install, open the app and grant notification access.
2. Connect the phone to the car, start music, press Send test hint – the car should show the test text for 10 s and then the music title again.
3. Start navigation in a supported app; the debug log shows what the app put into its notification and what was sent to the car.

Useful on the phone (via adb):

adb shell dumpsys media_session        # "Media button session" = what AVRCP shows
adb shell dumpsys bluetooth_manager    # AvrcpTargetService section: active player
adb logcat -s NavBtDisplay

Build

./gradlew assembleDebug

Requires JDK 17+ and the Android SDK (compileSdk 37). No third-party libraries.

License

GPL-3.0, see LICENSE.