Connect and read data automatically from BLE weight scale in Qt application - qt

Question

I'm developing a Qt app which need to read data from weight scale model and can't quite understand how exactly the Bluetooth Low Energy works and how to implement it in Qt.
I have a UC-352BLE weight scale which uses BLE to send data. What I want to achieve is this:
After initial pairing the scale with my Raspberry Pi on which my app is running, when the scale sends data (it does it automatically when you take a measurement), my app receive it. For example I have a blood pressure monitor which uses normal Bluetooth and here it's easy. In my app, I create a QBluetoothServer and call its listen() method. Then when the (already paired) device sends a measurement, it connects with my app automatically, then I create QBluetoothSocket and read the data. But with the scale and it's BLE it seems that you can't do it that way. I tried to follow the Qt documentation on this and right now I just have to manually press the button which connects to the scale when it's sending the data. And every time it connects to the device it discovers it's characteristics and services etc. Don't know if I can do it so the app automatically receives a connection and reads a data when the scale sends a measurement. And even when I try to connects like that sometimes it connects and sometimes don't (I get Unknown error form QLowEnergyController::Error when connecting). Here is what I already have:
Bletest::Bletest(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::Bletest)
{
ui->setupUi(this);
if (localDevice.isValid()) {
localDevice.powerOn();
localDevice.setHostMode(QBluetoothLocalDevice::HostDiscoverable);
connect(&localDevice, &QBluetoothLocalDevice::deviceConnected, this, &Bletest::deviceConnected);
connect(&localDevice, &QBluetoothLocalDevice::deviceDisconnected, this, &Bletest::deviceDisconnected);
connect(&localDevice, &QBluetoothLocalDevice::pairingFinished, this, &Bletest::pairingFinished);
}
discoveryAgent = new QBluetoothDeviceDiscoveryAgent(this);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::deviceDiscovered, this, &Bletest::addDevice);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Bletest::scanFinished);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::canceled, this, &Bletest::scanFinished);
discoveryAgent->start(QBluetoothDeviceDiscoveryAgent::LowEnergyMethod);
}
Bletest::~Bletest()
{
delete ui;
}
// Local device slots
void Bletest::deviceConnected(const QBluetoothAddress &address)
{
qDebug() << address.toString() << " connected";
}
void Bletest::deviceDisconnected(const QBluetoothAddress &address)
{
qDebug() << address.toString() << " disconnected";
}
void Bletest::pairingFinished(const QBluetoothAddress &address, QBluetoothLocalDevice::Pairing pairing)
{
}
// Agent slots
void Bletest::addDevice(const QBluetoothDeviceInfo &device)
{
if (device.coreConfigurations() & QBluetoothDeviceInfo::LowEnergyCoreConfiguration) {
if (device.name().contains("352")) {
bleDevice = device;
qDebug() << "Found: " + device.name() + "\t" + device.address().toString();
}
}
}
void Bletest::scanFinished()
{
qDebug() << "Devices scan finished";
}
///////////
void Bletest::on_connectButton_clicked()
{
controller = QLowEnergyController::createCentral(bleDevice, this);
connect(controller, &QLowEnergyController::serviceDiscovered, this, &Bletest::serviceDiscovered);
connect(controller, &QLowEnergyController::discoveryFinished, this, &Bletest::serviceScanFinished);
connect(controller, static_cast<void (QLowEnergyController::*)(QLowEnergyController::Error)>(&QLowEnergyController::error),
this, [this](QLowEnergyController::Error error) {
qDebug() << "Cannot connect to device: " + QString::number(error);
});
connect(controller, &QLowEnergyController::connected, this, [this]() {
qDebug() << "Connected to device";
controller->discoverServices();
});
connect(controller, &QLowEnergyController::disconnected, this, [this]() {
qDebug() << "Disconnected";
});
controller->connectToDevice();
}
// Controller slots
void Bletest::serviceDiscovered(const QBluetoothUuid &gatt)
{
qDebug() << "Service discovered: " << gatt.toString();
if (gatt.toString().contains("0000181d-0000-1000-8000-00805f9b34fb")) {
service = controller->createServiceObject(QBluetoothUuid(QBluetoothUuid::WeightScale));
if (service) {
qDebug() << "Found weight scale service";
connect(service, &QLowEnergyService::stateChanged, this, &Bletest::serviceStateChanged);
connect(service, &QLowEnergyService::characteristicChanged, this, &Bletest::updateWeight);
connect(service, &QLowEnergyService::characteristicRead, this, &Bletest::updateWeight);
service->discoverDetails();
}
}
}
void Bletest::serviceScanFinished()
{
qDebug() << "Service scan finished";
}
////////////////////////////
// Service slots
void Bletest::serviceStateChanged(QLowEnergyService::ServiceState newState)
{
if (controller->state() == QLowEnergyController::UnconnectedState)
return;
if (newState == QLowEnergyService::DiscoveringServices) {
qDebug() << "Discovering services state";
} else if (QLowEnergyService::ServiceDiscovered) {
qDebug() << "Service discovered.";
const QLowEnergyCharacteristic weightChar = service->characteristic(QBluetoothUuid(QBluetoothUuid::WeightMeasurement));
if (!weightChar.isValid()) {
qDebug() << "Weight data not found";
return;
}
qDebug() << "Weight data found";
//service->readCharacteristic(weightChar);
desc = weightChar.descriptor(QBluetoothUuid::ClientCharacteristicConfiguration);
if (desc.isValid()) {
qDebug() << "Writing descriptor";
service->writeDescriptor(desc, QByteArray::fromHex("0100"));
}
}
}
void Bletest::updateWeight(const QLowEnergyCharacteristic &c, const QByteArray &value)
{
qDebug() << "Updating weight";
if (c.uuid() != QBluetoothUuid(QBluetoothUuid::WeightMeasurement))
return;
double weight = qFromLittleEndian<qint16>(value.mid(1, 2).data()) * RESOLUTION;
qDebug() << "New weight: " << value.mid(1, 2);
qDebug() << "New weight: " + QString::number(weight, 'f', 2);
}
///////////////////////////
Can someone point me in the right direction with this? Is is even possible with BLE to automatically connects to my app and receive data?
Thanks.