ChatServer.kt

private fun connectToChatDevice(device: BluetoothDevice) {
        gattClientCallback = GattClientCallback()
        gattClient = device.connectGatt(app, false, gattClientCallback)
}

private class GattClientCallback : BluetoothGattCallback() {
        override fun onConnectionStateChange(gatt: BluetoothGatt, status: Int, newState: Int) {
            super.onConnectionStateChange(gatt, status, newState)
            val isSuccess = status == BluetoothGatt.GATT_SUCCESS
            val isConnected = newState == BluetoothProfile.STATE_CONNECTED
            if (isSuccess && isConnected) {
                gatt.discoverServices()
            }
        }

        override fun onServicesDiscovered(discoveredGatt: BluetoothGatt, status: Int) {
            super.onServicesDiscovered(discoveredGatt, status)
            if (status == BluetoothGatt.GATT_SUCCESS) {
                gatt = discoveredGatt
                val service = discoveredGatt.getService(SERVICE_UUID)
                messageCharacteristic = service.getCharacteristic(MESSAGE_UUID)
            }
        }
}