Volume Power App

Is your phone's power button broken? It happens more often than you might think: after years of use, a drop, or simple wear, the Power key can stop responding. A repair at a service center often costs as much as a used phone, and in the meantime you're left with a device that's still perfectly usable but with no convenient way to lock the screen or wake it up.

Volume Power App offers a software solution: it remaps the Volume up key so it does the Power button's job:

• Screen on → locks the phone;
• Screen off → wakes the phone.

It's not an exact replacement for the hardware button (accessibility permissions, a persistent notification, and some volume trade-offs are required — see Limitations), but for anyone with a broken Power button it's often enough to get by until a repair or phone replacement.

In technical terms: an Android app that starts a background service and closes immediately; from that point on, Volume + no longer raises the volume but handles screen lock and wake.

What the app does (summary)

1. Tap the Volume Power App icon once.
2. A foreground service starts (persistent notification) and the activity closes: you don't stay "inside" the app.
3. After the initial setup, the Volume + key no longer raises the volume but:
   • screen on → locks the device (like "stand-by" / screen off with lock);
   • screen off → tries to turn the screen on (wake lock).

The Volume down button is not modified and continues to lower the volume normally.

Note: If multimedia audio is playing in the background (e.g. music from Spotify, YouTube Music, or a podcast), wake with Volume + does not work: Android routes volume keys to the active media session instead of this app.

Architecture

The app consists of four main components and one helper:

┌─────────────────┐
│   MainActivity  │  Startup: notification permission → start FGS → (if needed) accessibility settings → finish()
└────────┬────────┘
         │
         ▼
┌─────────────────────────────────┐
│ VolumePowerForegroundService    │  Ongoing notification + MediaSession + VolumeProvider + silent AudioTrack
└──┬──────────────────────────────┘
   │   ▲ routes Volume +/− when screen is OFF (via MediaSession)
   │   │
   │   └─► ScreenWaker.wakeScreen(ctx) ◄──┐
   │                                       │
   ▼                                       │
┌──────────────────────────────────┐       │
│ VolumePowerAccessibilityService  │───────┘  onKeyEvent(): intercepts Volume + when screen is ON
└──────────────────────────────────┘
   │
   ▼
┌──────────────┐
│ WakeActivity │  Transparent activity with setTurnScreenOn(true) / setShowWhenLocked(true)
└──────────────┘

Why two paths for Volume + (accessibility + MediaSession)

Android delivers key events in two different ways depending on screen state:

• Screen on/unlocked: the AccessibilityService receives onKeyEvent() and runs GLOBAL_ACTION_LOCK_SCREEN. Works everywhere.
• Screen off (lock screen): on MIUI/EMUI/OneUI the system does not call the accessibility service's onKeyEvent(): the key is handled at a lower level (PhoneWindowManager) and routed directly to the audio subsystem. To intercept it, the app registers an active MediaSessionCompat + VolumeProviderCompat REMOTE. The system delegates volume control to our onAdjustVolume(). To be "the current music session" and win routing, on ACTION_SCREEN_OFF an AudioTrack with zero PCM samples is started (real silence, volume 0, MODE_STATIC in an infinite loop = negligible CPU cost).

MainActivity

• Transparent activity and excludeFromRecents: the user doesn't see it in the task switcher after launch.
• On Android 13+ requests Notifications permission (required for the foreground service).
• Starts VolumePowerForegroundService and exits with finish().
• If the accessibility service isn't active yet, shows a message and opens Settings → Accessibility.

VolumePowerForegroundService

• Type: foregroundServiceType="mediaPlayback|specialUse" (Android 14+), with subtype declared in the manifest.
• Shows a non-dismissible notification while the service is active (tap → accessibility settings).
• START_STICKY: if the system kills the process, Android may restart the service.
• Registers an active MediaSessionCompat with PLAYING state and setPlaybackToRemote(VolumeProviderCompat): when the system needs to adjust volume it calls our onAdjustVolume(direction).
  • ADJUST_RAISE → ScreenWaker.wakeScreen() (volume + never raises the volume).
  • ADJUST_LOWER → delegates to AudioManager.adjustStreamVolume(STREAM_MUSIC, ADJUST_LOWER, FLAG_SHOW_UI) (volume − works normally).
• On ACTION_SCREEN_OFF: acquires a PARTIAL_WAKE_LOCK, reactivates the session, and starts a silent AudioTrack in MODE_STATIC infinite loop. On ACTION_SCREEN_ON releases everything.

VolumePowerAccessibilityService

This is the core of the remapping. Android does not allow normal apps to read or intercept hardware volume keys without an AccessibilityService with a key filter:

• flagRequestFilterKeyEvents + canRequestFilterKeyEvents="true" in res/xml/accessibility_service_config.xml.
• In onKeyEvent():
  • if the key is Volume up, returns true (event consumed → volume doesn't change);
  • on ACTION_DOWN, after a debounce of 400 ms:
    • PowerManager.isInteractive == true → performGlobalAction(GLOBAL_ACTION_LOCK_SCREEN);
    • otherwise → WakeLock with ACQUIRE_CAUSES_WAKEUP to turn the screen on.

Debounce (400 ms)

The debounce prevents rapid Volume up presses from triggering multiple actions in a row (lock + wake, or double lock) due to key bounce or duplicate events.

• Where it's declared: constant DEBOUNCE_MS = 400L in the companion object of VolumePowerAccessibilityService.kt.
• Where it's used: in handleVolumeUpPress(): if less than 400 ms have passed since the last valid action, the press is ignored (return); otherwise lastToggleAtMs is updated and lock or wake is performed.

// VolumePowerAccessibilityService.kt
private const val DEBOUNCE_MS = 400L

if (now - lastToggleAtMs < DEBOUNCE_MS) return
lastToggleAtMs = now

To change sensitivity, modify DEBOUNCE_MS (higher values = less responsive but more stable).

Permissions

Permission	Reason
FOREGROUND_SERVICE / FOREGROUND_SERVICE_SPECIAL_USE / FOREGROUND_SERVICE_MEDIA_PLAYBACK	Background service visible to the user; the mediaPlayback type is required by Android 14 when the service plays audio (even silent)
POST_NOTIFICATIONS	Service notification (Android 13+)
WAKE_LOCK	Turn screen on when it's off
TURN_SCREEN_ON	Permission to turn the screen on from Android 14
MODIFY_AUDIO_SETTINGS	Required to lower the system stream volume from VolumeProvider.onAdjustVolume(ADJUST_LOWER)

Root and signed system permissions are not required.

Important limitations (Android)

This is not a perfect copy of the Power button at the hardware/firmware level:

1. Accessibility service required
   The user must manually enable "Volume Power App – volume key" in Accessibility. Without this step, remapping won't work.

2. Screen lock ≠ full shutdown
   GLOBAL_ACTION_LOCK_SCREEN locks the device (PIN/biometrics if configured), like a lock screen. It doesn't power off the CPU like a real power-off!

3. Wake with screen off — not while media is playing
   On many ROMs (MIUI, EMUI, OneUI) key events with the screen off don't reach the AccessibilityService. The app works around this with a MediaSessionCompat + VolumeProviderCompat REMOTE + silent AudioTrack that ensures the system routes onAdjustVolume() to our app even from the lock screen. However, if another app is actively playing multimedia audio (Spotify, YouTube Music, a podcast, a video in the background, etc.), Android gives that app priority for volume keys: Volume + will not wake the screen and will only affect the other app's playback.

4. Volume up no longer raises volume
   While the accessibility service is active, Volume + doesn't change the volume. To raise it, use the on-screen volume menu, headphones, or temporarily disable the service.

5. Battery and optimizations
   Some manufacturers (Xiaomi, Huawei, Samsung, etc.) kill background services. You may need to exclude the app from battery optimizations (see GUIDA-UTENTE.md).

Getting the source code

Repository: https://github.com/depaolaluigi123/Volume-Power-App

git clone https://github.com/depaolaluigi123/Volume-Power-App.git
cd Volume-Power-App

For build and installation instructions, see README-USER-GUIDE.md.

Requirements

• Android 8.0 (API 28) or higher (GLOBAL_ACTION_LOCK_SCREEN).
• Recommended: Android 12+ for predictable foreground service behavior.

Project structure

VolumePowerApp/
├── app/src/main/
│   ├── AndroidManifest.xml
│   ├── java/com/volumepower/app/
│   │   ├── MainActivity.kt
│   │   ├── VolumePowerForegroundService.kt   ← MediaSession + VolumeProvider + silent AudioTrack
│   │   ├── VolumePowerAccessibilityService.kt ← onKeyEvent when screen is on
│   │   ├── WakeActivity.kt                    ← transparent activity that turns the panel on
│   │   ├── ScreenWaker.kt                     ← shared helper (WakeLock + WakeActivity)
│   │   ├── AccessibilityUtils.kt
│   │   └── MiuiUtils.kt
│   └── res/xml/accessibility_service_config.xml
├── README.md          ← this file (how it works)
└── GUIDA-UTENTE.md    ← installation and testing

Disabling the app

1. Settings → Accessibility → disable "Volume Power App".
2. Swipe away the notification and stop the service, or Settings → Apps → Volume Power App → Force stop.
3. Optional: uninstall the app.

After disabling the accessibility service, Volume + returns to standard behavior.

---

License

Copyright (C) 2026 Luigi De Paola

Volume Power App is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License v3.0 (GPL-3.0).