ChatServer.kt

private fun setupGattServer(app: Application) {
        gattServerCallback = GattServerCallback()

        gattServer = bluetoothManager.openGattServer(
            app,
            gattServerCallback
        ).apply {
            addService(setupGattService())
        }
    }


private fun setupGattService(): BluetoothGattService {
        val service = BluetoothGattService(SERVICE_UUID, BluetoothGattService.SERVICE_TYPE_PRIMARY)
       
        val messageCharacteristic = BluetoothGattCharacteristic(
            MESSAGE_UUID,
            BluetoothGattCharacteristic.PROPERTY_WRITE,
            BluetoothGattCharacteristic.PERMISSION_WRITE
        )
        service.addCharacteristic(messageCharacteristic)
        return service
}

private class GattServerCallback : BluetoothGattServerCallback() {
        override fun onConnectionStateChange(device: BluetoothDevice, status: Int, newState: Int) {
            super.onConnectionStateChange(device, status, newState)
            val isSuccess = status == BluetoothGatt.GATT_SUCCESS
            val isConnected = newState == BluetoothProfile.STATE_CONNECTED
            if (isSuccess && isConnected) {
                setCurrentChatConnection(device)
            } else {
                _deviceConnection.postValue(DeviceConnectionState.Disconnected)
            }
        }

        override fun onCharacteristicWriteRequest(
            device: BluetoothDevice,
            requestId: Int,
            characteristic: BluetoothGattCharacteristic,
            preparedWrite: Boolean,
            responseNeeded: Boolean,
            offset: Int,
            value: ByteArray?
        ) {
            super.onCharacteristicWriteRequest(
                device,
                requestId,
                characteristic,
                preparedWrite,
                responseNeeded,
                offset,
                value
            )
            if (characteristic.uuid == MESSAGE_UUID) {
                gattServer?.sendResponse(device, requestId, BluetoothGatt.GATT_SUCCESS, 0, null)
                val message = value?.toString(Charsets.UTF_8)

                message?.let {
                    _messages.postValue(Message.RemoteMessage(it))
                }
            }
        }
}