Forter — Opening an SPP Connection to the Adapter

This post was compiled by Claude.

Goal

v0.1.3: tap the adapter in the bonded list, open an RFCOMM socket, send ATZ, and show the ELM327 response on screen. One connect, one command, one close. No service, no reconnect, no polling yet.

Reading MainActivity first

Before adding code, I stopped to understand when each line actually runs. The file reads top to bottom, but the lines do not run at the same time.

• setContent { } runs inside onCreate, but it only stores the lambda. The lambda runs later, when the view is attached to the window (after onResume).
• remember { } runs on the first composition only, then returns the stored value.
• LaunchedEffect(Unit) { } is scheduled during composition and runs once after the first frame.
• Event lambdas (onRefresh, onSelect) run when the user does something.
• Changing a mutableStateOf value re-runs the parts that read it.

@Composable marks a function the Compose compiler rewrites so the runtime can track where it was called, remember values, and re-run it. ForterTheme, Column, and Text are all functions, not classes. The capital letter is a Compose naming convention.

That was enough. The goal of Forter is collecting car data, not mastering Android, so I decided to learn Compose only as far as the app needs and spend the depth on the connection, the service, and the data.

SppConnection

class SppConnection(
    private val context: Context,
    private val address: String,
) {
    private var socket: BluetoothSocket? = null

    @SuppressLint("MissingPermission")
    suspend fun connect() = withContext(Dispatchers.IO) {
        val adapter = context.getSystemService(BluetoothManager::class.java).adapter
        val s = adapter.getRemoteDevice(address).createRfcommSocketToServiceRecord(SPP_UUID)
        s.connect()
        socket = s
    }

    suspend fun send(command: String, timeoutMs: Long = 3000): String = withContext(Dispatchers.IO) {
        val s = socket ?: error("Not connected")
        s.outputStream.write("$command\r".toByteArray())
        s.outputStream.flush()

        val input = s.inputStream
        val buffer = StringBuilder()
        withTimeout(timeoutMs) {
            while (true) {
                if (input.available() > 0) {
                    val c = input.read().toChar()
                    if (c == '>') break
                    buffer.append(c)
                } else {
                    delay(10)
                }
            }
        }
        buffer.toString().replace("\r", "\n").trim()
    }

    fun close() {
        runCatching { socket?.close() }
        socket = null
    }

    companion object {
        private val SPP_UUID: UUID = UUID.fromString("00001101-0000-1000-8000-00805F9B34FB")
    }
}

• Socket connect and read block, so both run on Dispatchers.IO. On the main thread the UI would freeze.
• 00001101-... is the standard Serial Port Profile UUID.
• ELM327 ends every response with >. Read until it arrives, or give up after 3 seconds.

Wiring it to the screen

suspend functions cannot be called from a plain click lambda, so MainActivity gets a coroutine scope and a status state.

val scope = rememberCoroutineScope()
var status by remember { mutableStateOf("기기를 선택하세요") }

onSelect = { device ->
    scope.launch {
        val conn = SppConnection(context, device.address)
        try {
            status = "연결 중: ${device.name}"
            conn.connect()
            val response = conn.send("ATZ")
            status = "응답:\n$response"
        } catch (e: Exception) {
            status = "실패: ${e.message}"
        } finally {
            conn.close()
        }
    }
}

Mistakes along the way

• A missing BluetoothManager import failed the build. The editor showed no error in the file itself; the build output did.
• I typed "/r" instead of "\r". It compiles, but the response would never split into lines.
• A mismatched brace put companion object inside onCreate.

Result at home

With the adapter powered off, tapping it shows:

실패: read failed, socket might closed or timeout, read ret: -1

That is the expected failure. The exception is caught, the app stays responsive, and the reason is on screen. The version reads v0.1.3-e45c6ea.

Next

• Test in the car with ACC on and other OBD apps closed. Expect ELM327 v1.5.
• Tag v0.1.3 once the car test passes.
• Move the connection into a Foreground Service.