Bluetooth Example with Kotlin

Here’s a simple Kotlin program to toggle Bluetooth on/off, search for nearby Bluetooth devices, and list the available devices. This is done in an Android environment using Kotlin

AndroidManifest.xml

<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp">

    <Button
        android:id="@+id/btn_turn_on"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Turn Bluetooth On" />

    <Button
        android:id="@+id/btn_turn_off"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Turn Bluetooth Off" />

    <ListView
        android:id="@+id/device_list"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

MainActivity.kt

package com.androindian.bluetoothex

import android.Manifest
import android.app.Activity
import android.bluetooth.BluetoothAdapter
import android.bluetooth.BluetoothDevice
import android.content.Intent
import android.content.pm.PackageManager
import android.os.Bundle
import android.widget.ArrayAdapter
import android.widget.Button
import android.widget.ListView
import android.widget.Toast
import androidx.core.app.ActivityCompat
import androidx.core.content.ContextCompat

class MainActivity : Activity() {

    private val bluetoothAdapter: BluetoothAdapter? by lazy {
        BluetoothAdapter.getDefaultAdapter()
    }
    private val REQUEST_ENABLE_BT = 1
    private val REQUEST_LOCATION_PERMISSION = 2
    private lateinit var deviceListAdapter: ArrayAdapter<String>
    private val discoveredDevices = mutableListOf<String>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val listView: ListView = findViewById(R.id.device_list)
        val btnTurnOn: Button = findViewById(R.id.btn_turn_on)
        val btnTurnOff: Button = findViewById(R.id.btn_turn_off)

        deviceListAdapter = ArrayAdapter(this, android.R.layout.simple_list_item_1, discoveredDevices)
        listView.adapter = deviceListAdapter

        checkBluetoothSupport()

        // Request location permission (required for device discovery)
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION)
            != PackageManager.PERMISSION_GRANTED
        ) {
            ActivityCompat.requestPermissions(
                this,
                arrayOf(Manifest.permission.ACCESS_FINE_LOCATION),
                REQUEST_LOCATION_PERMISSION
            )
        }

        // Enable Bluetooth
        btnTurnOn.setOnClickListener {
            if (bluetoothAdapter?.isEnabled == false) {
                val enableBtIntent = Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE)
                startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT)
            } else {
                Toast.makeText(this, "Bluetooth is already ON", Toast.LENGTH_SHORT).show()
            }
        }

        // Disable Bluetooth
        btnTurnOff.setOnClickListener {
            if (bluetoothAdapter?.isEnabled == true) {
                bluetoothAdapter?.disable()
                Toast.makeText(this, "Bluetooth turned OFF", Toast.LENGTH_SHORT).show()
            } else {
                Toast.makeText(this, "Bluetooth is already OFF", Toast.LENGTH_SHORT).show()
            }
        }

        // Start device discovery
        startDiscovery()
    }

    private fun checkBluetoothSupport() {
        if (bluetoothAdapter == null) {
            Toast.makeText(this, "Bluetooth not supported on this device", Toast.LENGTH_SHORT).show()
            finish()
        }
    }

    private fun startDiscovery() {
        bluetoothAdapter?.let {
            if (it.isDiscovering) {
                it.cancelDiscovery()
            }
            if (it.startDiscovery()) {
                Toast.makeText(this, "Starting discovery...", Toast.LENGTH_SHORT).show()
                registerReceiver(
                    BluetoothBroadcastReceiver { device ->
                        runOnUiThread {
                            discoveredDevices.add("${device.name} (${device.address})")
                            deviceListAdapter.notifyDataSetChanged()
                        }
                    },
                    BluetoothBroadcastReceiver.getIntentFilter()
                )
            } else {
                Toast.makeText(this, "Failed to start discovery", Toast.LENGTH_SHORT).show()
            }
        }
    }

    override fun onDestroy() {
        super.onDestroy()
        bluetoothAdapter?.cancelDiscovery()
    }
}

BluetoothBroadcastReceiver.kt

package com.androindian.bluetoothex

import android.bluetooth.BluetoothDevice
import android.content.BroadcastReceiver
import android.content.Context
import android.content.Intent
import android.content.IntentFilter

class BluetoothBroadcastReceiver(private val onDeviceDiscovered: (BluetoothDevice) -> Unit) : BroadcastReceiver() {
    override fun onReceive(context: Context?, intent: Intent?) {
        when (intent?.action) {
            BluetoothDevice.ACTION_FOUND -> {
                val device: BluetoothDevice? = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE)
                device?.let { onDeviceDiscovered(it) }
            }
        }
    }

    companion object {
        fun getIntentFilter(): IntentFilter {
            val filter = IntentFilter()
            filter.addAction(BluetoothDevice.ACTION_FOUND)
            return filter
        }
    }
}