QLowEnergyService::characteristicChanged is not emitted

NhungPham

I am using Bluetooth API of QT 5.13.0 on Windows 10 to connect and get data from my BLE device. But QLowEnergyService::characteristicChanged is not emitted.

Below is my steps:
step 1: Use QBluetoothDeviceDiscoveryAgent to discovery devices.
step 2: Use QLowEnergyController to connect to my device and get specific service of this device that is discovered by ID (My service is serviceA has ID is serviceID)
step 3: Use QLowEnergyService to connect to serviceA of my device
serviceA has 2 characteristics: characteristic1(Write) & characteristic2(Notify).
I have to write value1 to characteristic1 and enable notification of characteristic2, after that I can get data value from characteristic2 through characteristicChanged event of serviceA.

With the same source code, It works well on Mac OS but on Windows 10 QLowEnergyService::characteristicChanged is not emitted. Is there difference of QT Bluetooth API from Win to Mac? Is this a bug of QT Bluetooth API on Windows?
Please have a look my source and show me if you know how to fix it. Thanks!

Below is my code to connect to service and get data from device:

void Device::connectToService(QLowEnergyService &service)
{
       if (!service)
            return;
    if (service->state() == QLowEnergyService::DiscoveryRequired) {
        connect(service, &QLowEnergyService::stateChanged, this, &Device::serviceDetailsDiscovered);
        service->discoverDetails();
        return;
   }
}

void Device::serviceDetailsDiscovered(QLowEnergyService::ServiceState newState)
{
    QLowEnergyService *service = qobject_cast<QLowEnergyService *>(QObject::sender());
    if (!service){
        qWarning() << "BTLEQtInputDevice::onServiceDetailsDiscovered service is null";
        return;
    }

    if (newState != QLowEnergyService::ServiceDiscovered) {
        return;
    }
    QList<QLowEnergyCharacteristic> chars = service->characteristics();
    foreach (const QLowEnergyCharacteristic &currChar, chars) {
        auto currUUID = currChar.uuid();// cInfo->getUuid();
        if (currUUID == characteristic1) {
            service->writeCharacteristic(currChar, value1);
            continue;
        }
        if (currUUID == characteristic2){
            const QLowEnergyDescriptor descriptor = currChar.descriptor(QBluetoothUuid(QBluetoothUuid::ClientCharacteristicConfiguration));
            if (!descriptor.isValid()) {
                qInfo() << "Descriptor is invalid!";
            }
            service->writeDescriptor(descriptor, QByteArray::fromHex("0100"));
//Device::onCharacteristicChanged is invoked on Mac but doesn't run on Window10
            connect(service, &QLowEnergyService::characteristicChanged, this, &Device::onCharacteristicChanged);
            continue;
        }
    }
}

NhungPham

I see the following message:
qt.bluetooth.winrt: QWinRTLowEnergyServiceHandlerNew::obtainCharList
qt.bluetooth.winrt: Could not await descriptor read result

What does that mean? When does it occur?

pman22

@NhungPham

I had the same problem on Windows.

As a workaround I used QLowEnergyService::readCharacteristic(QLowEnergyChar&) to just read the characteristic and signal QLowEnergyService::characteristicRead() to perform the operation that QLowEnergyService::characteristicChanged() signal would do.

jon2014

@pman22,I had the same problem on Windows. but i can't read value by QLowEnergyService::readCharacteristic(QLowEnergyChar&), do you finaly fix this question.