Android Ble Hardware

Lineage: Built upon and expanded from the official Android group skills repository at github.com/android/skills. That repo covers areas where evaluations show LLMs underperform on standard Android patterns. This skill extends that foundation into BLE hardware integration — an area the official set does not cover.

Overview

Use the Nordic Semiconductor Android BLE library (no.nordicsemi.android:ble + no.nordicsemi.android:ble-ktx) as the BLE abstraction layer. Prefer this over raw BluetoothGatt callbacks — it handles connection retries, queued operations, and MTU negotiation.

Dependencies

// libs.versions.toml
[versions]
nordic-ble = "2.7.0"

[libraries]
nordic-ble     = { group = "no.nordicsemi.android", name = "ble",     version.ref = "nordic-ble" }
nordic-ble-ktx = { group = "no.nordicsemi.android", name = "ble-ktx", version.ref = "nordic-ble" }

BLE Permissions (Android 12+)

Declare in AndroidManifest.xml and request at runtime before any BLE operation:

<uses-permission android:name="android.permission.BLUETOOTH_SCAN"
    android:usesPermissionFlags="neverForLocation" />
<uses-permission android:name="android.permission.BLUETOOTH_CONNECT" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-feature android:name="android.hardware.bluetooth_le" android:required="false" />

Required permissions by API level:

• API < 31: BLUETOOTH, BLUETOOTH_ADMIN, ACCESS_FINE_LOCATION
• API 31+: BLUETOOTH_SCAN, BLUETOOTH_CONNECT (+ ACCESS_FINE_LOCATION if deriving location)

Suppress @SuppressLint("MissingPermission") only after confirming permissions are held at the call site. Never apply it at the class level.

Device Type Detection

Define device types with UUID and name-pattern matching. Never scatter raw UUIDs across files — keep them in the companion object of the manager that owns that GATT profile.

enum class DeviceType {
    GPS_LOGGER,  // e.g. Nordic UART service: 6E400001-B5A3-F393-E0A9-E50E24DCCA9E
    OBD_ELM327,  // name often starts with "OBDII" or "ELM327"
    HEART_RATE,  // standard BLE Heart Rate Service: 0x180D
    UNKNOWN;

    companion object {
        fun detect(device: BluetoothDevice, serviceUuids: List<ParcelUuid>?): DeviceType {
            if (hasMatchingService(serviceUuids, UART_SERVICE_UUID)) return GPS_LOGGER
            if (matchesDeviceName(device.name, listOf("OBDII", "ELM327", "OBD"))) return OBD_ELM327
            if (hasMatchingService(serviceUuids, UUID.fromString("0000180D-0000-1000-8000-00805F9B34FB"))) return HEART_RATE
            return UNKNOWN
        }

        private fun hasMatchingService(uuids: List<ParcelUuid>?, target: UUID) =
            uuids?.any { it.uuid == target } == true

        private fun matchesDeviceName(name: String?, patterns: List<String>) =
            name != null && patterns.any { name.uppercase().contains(it) }
    }
}

Scanning (callbackFlow pattern)

Expose scanning as a Flow<ScannedDevice> so the ViewModel can collect it. Never hold a scan callback reference in a ViewModel — keep it in a repository or manager that outlives configuration changes.

@SuppressLint("MissingPermission")
fun scanForDevices(deviceTypes: Set<DeviceType> = emptySet()): Flow<ScannedDevice> =
    callbackFlow {
        val scanner = bluetoothAdapter?.bluetoothLeScanner
            ?: run { close(IllegalStateException("BLE not available")); return@callbackFlow }

        val settings = ScanSettings.Builder()
            .setScanMode(ScanSettings.SCAN_MODE_LOW_LATENCY)
            .build()

        val callback = object : ScanCallback() {
            override fun onScanResult(callbackType: Int, result: ScanResult) {
                val detected = DeviceType.detect(result.device, result.scanRecord?.serviceUuids)
                if (deviceTypes.isEmpty() || detected in deviceTypes) {
                    trySend(ScannedDevice(result.device, detected, result.rssi))
                }
            }
            override fun onScanFailed(errorCode: Int) {
                close(RuntimeException("BLE scan failed: $errorCode"))
            }
        }

        scanner.startScan(buildFilters(deviceTypes), settings, callback)
        awaitClose { scanner.stopScan(callback) }
    }

Rules:

• Use SCAN_MODE_LOW_LATENCY during active device selection; switch to SCAN_MODE_LOW_POWER in background.
• Always call stopScan in awaitClose — an unclosed scanner drains battery continuously.
• Use trySend (not send) in scan callbacks — the callback fires on a non-coroutine thread.

BleManager Subclass (Nordic library)

Extend BleManager for each device profile. One manager instance per connected device.

class UartBleManager(context: Context) : BleManager(context) {

    companion object {
        val UART_SERVICE_UUID = UUID.fromString("6E400001-B5A3-F393-E0A9-E50E24DCCA9E")
        val UART_TX_UUID      = UUID.fromString("6E400002-B5A3-F393-E0A9-E50E24DCCA9E")
        val UART_RX_UUID      = UUID.fromString("6E400003-B5A3-F393-E0A9-E50E24DCCA9E")
    }

    private var txChar: BluetoothGattCharacteristic? = null
    private var rxChar: BluetoothGattCharacteristic? = null
    var dataListener: ((ByteArray) -> Unit)? = null

    override fun getGattCallback(): BleManagerGattCallback = Callback()

    private inner class Callback : BleManagerGattCallback() {
        override fun isRequiredServiceSupported(gatt: BluetoothGatt): Boolean {
            val svc = gatt.getService(UART_SERVICE_UUID) ?: return false
            txChar = svc.getCharacteristic(UART_TX_UUID)
            rxChar = svc.getCharacteristic(UART_RX_UUID)
            return txChar != null && rxChar != null
        }

        override fun initialize() {
            // Device → phone notifications on RX characteristic
            setNotificationCallback(rxChar).with { _, data ->
                data.value?.let { dataListener?.invoke(it) }
            }
            enableNotifications(rxChar).enqueue()
        }

        override fun onServicesInvalidated() {
            txChar = null; rxChar = null
        }
    }

    fun sendCommand(bytes: ByteArray) {
        writeCharacteristic(txChar, bytes, BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE)
            .enqueue()
    }
}

Rules:

• isRequiredServiceSupported returning false causes a clean disconnect — always return false when any required characteristic is missing rather than crashing later.
• Call .enqueue() on every operation — never fire-and-forget.
• Use WRITE_TYPE_NO_RESPONSE for high-throughput streams; WRITE_TYPE_DEFAULT for acknowledged commands.

Connection Lifecycle

// Connect — run from a scope that survives configuration changes (Service or Repository)
bleManager.connect(device)
    .useAutoConnect(false)   // true only for background reconnect after initial bond
    .timeout(10_000)
    .retry(3, 500)
    .done  { /* connected */ }
    .fail  { _, status -> /* handle */ }
    .enqueue()

// Disconnect
bleManager.disconnect().enqueue()

Automatic reconnection after unexpected disconnect:

override fun onDeviceDisconnected(device: BluetoothDevice, reason: Int) {
    super.onDeviceDisconnected(device, reason)
    if (shouldReconnect && reason != ConnectionObserver.REASON_SUCCESS) {
        coroutineScope.launch {
            delay(1_000)
            bleManager.connect(device).useAutoConnect(true).retry(5, 2_000).enqueue()
        }
    }
}

Packet Framing

For devices that stream raw bytes over UART, use a stateful PacketAssembler that buffers incoming chunks and emits complete frames:

class PacketAssembler {
    private val buffer = mutableListOf<Byte>()

    /** Returns a complete packet if one is ready, null otherwise. */
    fun feed(bytes: ByteArray): ByteArray? {
        buffer.addAll(bytes.toList())
        return tryExtract()
    }

    private fun tryExtract(): ByteArray? {
        // Example: 2-byte big-endian length prefix
        if (buffer.size < 2) return null
        val length = ((buffer[0].toInt() and 0xFF) shl 8) or (buffer[1].toInt() and 0xFF)
        if (buffer.size < length + 2) return null
        val packet = buffer.subList(2, length + 2).toByteArray()
        repeat(length + 2) { buffer.removeAt(0) }
        return packet
    }
}

Testing Without Hardware

Provide a DemoDeviceGenerator that emits synthetic ScannedDevice objects and a DemoDataGenerator that emits synthetic frames at the expected rate. Gate with BuildConfig.USE_DEMO_MODE so the flag never ships in release builds.

Anti-Patterns