Qt Bluetooth - no devices discovered.

[jenya7]

Based on an example I do

#include "bluetooth/device.h"

static Device bt_device;

void BLE_Scan()
{
    bt_device.startDeviceDiscovery();

    QVariant devices_list = bt_device.getDevices();

}

I see no device in devices_list . So I check it from a command line

sudo hcitool lescan

and I see the list

sudo hcitool lescan
LE Scan ...
14:3A:B7:53:DF:27 (unknown)
58:80:3C:47:1F:DA (unknown)
58:80:3C:47:1F:DA Amazfit-765D
6B:CA:29:3A:63:03 (unknown)
20:FB:2D:0F:D6:DA (unknown)
EF:A6:D6:17:CB:8D (unknown)
7E:F1:45:DF:D3:A4 (unknown)
5A:79:C1:65:91:D6 (unknown)
5A:79:C1:65:91:D6 (unknown)

Something I miss?

I've modified the code

void BLE_Scan()
{
    bt_device.startDeviceDiscovery();

   while (bt_device.state()) ; //here we get a scan state

    QVariant devices_list = bt_device.getDevices();

}

Now it stays in a while loop.

[J.Hilk]

@jenya7 macOS (possibly Monterey) ?

possible this situation:
https://bugreports.qt.io/browse/QTCREATORBUG-25252

[jenya7]

@J-Hilk
I added qDebug() and I see

Last device added:  "14-7D-DA-B5-DA-DE"
Last device added:  "43-8F-37-BA-48-86"
Last device added:  "42-E5-CA-7D-CB-90"
Last device added:  "13-82-BE-60-4F-B8"
Last device added:  "62-7C-68-B4-D6-7F"
Last device added:  "JBL Pulse 3"
Last device added:  "67-FB-3F-F2-97-4A"
Last device added:  "64-C5-8A-03-6E-58"
Last device added:  "54-1C-9E-74-DB-3C"
Last device added:  "3F-9D-0D-CB-1C-B8"
Last device added:  "75-88-C7-2C-7B-E1"
Last device added:  "1A-AF-D3-12-C3-57"
Last device added:  "25-30-77-C8-E9-43"
Last device added:  "Amazfit-765D"

Obviously I miss the end of the scan. I see

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);

but discoveryAgent is a private member. How can get ScanFinished event?

[jsulm]

@jenya7 said in Qt Bluetooth - no devices discovered.:

but discoveryAgent is a private member

Private member of what?

[jenya7]

@jsulm said in Qt Bluetooth - no devices discovered.:

@jenya7 said in Qt Bluetooth - no devices discovered.:

but discoveryAgent is a private member

Private member of what?

I use this class from the example (lowenergyscanner).
class Device: public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVariant devicesList READ getDevices NOTIFY devicesUpdated)
    Q_PROPERTY(QVariant servicesList READ getServices NOTIFY servicesUpdated)
    Q_PROPERTY(QVariant characteristicList READ getCharacteristics NOTIFY characteristicsUpdated)
    Q_PROPERTY(QString update READ getUpdate WRITE setUpdate NOTIFY updateChanged)
    Q_PROPERTY(bool useRandomAddress READ isRandomAddress WRITE setRandomAddress NOTIFY randomAddressChanged)
    Q_PROPERTY(bool state READ state NOTIFY stateChanged)
    Q_PROPERTY(bool controllerError READ hasControllerError)
public:
    Device();
    ~Device();
    QVariant getDevices();
    QVariant getServices();
    QVariant getCharacteristics();
    QString getUpdate();
    bool state();
    bool hasControllerError() const;

    bool isRandomAddress() const;
    void setRandomAddress(bool newValue);

public slots:
    void startDeviceDiscovery();
    void scanServices(const QString &address);

    void connectToService(const QString &uuid);
    void disconnectFromDevice();

private slots:
    // QBluetoothDeviceDiscoveryAgent related
    void addDevice(const QBluetoothDeviceInfo&);
    void deviceScanFinished();
    void deviceScanError(QBluetoothDeviceDiscoveryAgent::Error);

    // QLowEnergyController realted
    void addLowEnergyService(const QBluetoothUuid &uuid);
    void deviceConnected();
    void errorReceived(QLowEnergyController::Error);
    void serviceScanDone();
    void deviceDisconnected();

    // QLowEnergyService related
    void serviceDetailsDiscovered(QLowEnergyService::ServiceState newState);

Q_SIGNALS:
    void devicesUpdated();
    void servicesUpdated();
    void characteristicsUpdated();
    void updateChanged();
    void stateChanged();
    void disconnected();
    void randomAddressChanged();

private:
    void setUpdate(const QString &message);
    QBluetoothDeviceDiscoveryAgent *discoveryAgent;
    DeviceInfo currentDevice;
    QList<QObject *> devices;
    QList<QObject *> m_services;
    QList<QObject *> m_characteristics;
    QString m_previousAddress;
    QString m_message;
    bool connected = false;
    QLowEnergyController *controller = nullptr;
    bool m_deviceScanState = false;
    bool randomAddress = false;
};

[jsulm]

@jenya7 So, if this is your own class simply add a signal to it and connect it to &QBluetoothDeviceDiscoveryAgent::finished

[jenya7]

@jsulm said in Qt Bluetooth - no devices discovered.:

@jenya7 So, if this is your own class simply add a signal to it and connect it to &QBluetoothDeviceDiscoveryAgent::finished

I use this class from the example (lowenergyscanner).
I get an error - finished is a private member.

[jsulm]

@jenya7 https://doc.qt.io/qt-5/qbluetoothdevicediscoveryagent.html#finished - does not look like private to me.
Please post whole error message and code.

[jenya7]

@jsulm said in Qt Bluetooth - no devices discovered.:

@jenya7 https://doc.qt.io/qt-5/qbluetoothdevicediscoveryagent.html#finished - does not look like private to me.
Please post whole error message and code.

QBluetoothDeviceDiscoveryAgent - it is global.

but in the class

private:
    QBluetoothDeviceDiscoveryAgent *discoveryAgent;

and in constructor

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);

Actually it's a pointer ( *discoveryAgent). How do I pass my object (QBluetoothDeviceDiscoveryAgent ) to the class?

[jsulm]

@jenya7 said in Qt Bluetooth - no devices discovered.:

Actually it's a pointer ( *discoveryAgent). How do I pass my object (QBluetoothDeviceDiscoveryAgent ) to the class?

You don't.
What I suggested is:

• Add a signal to Device class
• Connect discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished to this new signal
• Other classes can now connect to this new signal

No need to pass anything to anybody...

[jenya7]

@jsulm said in Qt Bluetooth - no devices discovered.:

@jenya7 said in Qt Bluetooth - no devices discovered.:

Actually it's a pointer ( *discoveryAgent). How do I pass my object (QBluetoothDeviceDiscoveryAgent ) to the class?

You don't.
What I suggested is:

• Add a signal to Device class
• Connect discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished to this new signal
• Other classes can now connect to this new signal

No need to pass anything to anybody...

OK.

Device::Device()
{
    //! [les-devicediscovery-1]
    discoveryAgent = new QBluetoothDeviceDiscoveryAgent();
    discoveryAgent->setLowEnergyDiscoveryTimeout(5000);
    connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::deviceDiscovered,
            this, &Device::addDevice);
    connect(discoveryAgent, QOverload<QBluetoothDeviceDiscoveryAgent::Error>::of(&QBluetoothDeviceDiscoveryAgent::error),
            this, &Device::deviceScanError);
    connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);
    //! [les-devicediscovery-1]

    setUpdate("Search");
}

a private  finished  connected to a private  deviceScanFinished. What's  the point in it?

[jsulm]

This post is deleted!

[jsulm]

@jenya7 Did you actually read what I wrote?!
For the third time: you add a new SIGNAL to Device class...

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::scanFinished); // scanFinished is the new signal

[jenya7]

@jsulm said in Qt Bluetooth - no devices discovered.:

@jenya7 And why is deviceScanFinished private?
It is actually apparent that it should be public...

it's an official example in the examples folder (C:\Qt\Examples\Qt-5.12.0\bluetooth\lowenergyscanner).
I wouldn't dare to change it, only if you say so.

[jsulm]

@jenya7 Please read my post above your last post...

[jenya7]

@jsulm
What if I do deviceScanFinished public?

OK. I see what's going on. Just a strong encapsulation.

The signal already set

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);

and it sets m_deviceScanState

void Device::deviceScanFinished()
{
    emit devicesUpdated();
    m_deviceScanState = false;
    emit stateChanged();
    if (devices.isEmpty())
        setUpdate("No Low Energy devices found...");
    else
        setUpdate("Done! Scan Again!");
}

so my

 while (bt_device.state()) ;

actually tests the memeber

bool Device::state()
{
    return m_deviceScanState;
}

so deviceScanFinished never occurs.

[jsulm]

@jenya7 said in Qt Bluetooth - no devices discovered.:

What if I do deviceScanFinished public?

To achieve what exactly? deviceScanFinished is a slot not signal in Device - how will making it public help other classes?

"The signal already set" - the signal from private member is "set", this does not help other classes as you already discovered.

Why don't you try my approach? Signals/slots are one of the most important features in Qt and it is really not much work:

class Device...
{
signals:
    void scanFinished();
}

// In Device constructor:
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::scanFinished);

// Now in some other class you simply do:
connect(&bt_device, &Device::scanFinished, this, &SomeOtherClass::someSlot);

Isn't this easy and clear? And you do not expose any private members from Device to outer world (which would be bad design).

[jenya7]

@jsulm
It seems OK. But here in the constructor

BT_DEVICE::BT_DEVICE(QObject *parent) : QObject(parent)
{
    connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
}

I get - "undefined reference to `vtable for BT_DEVICE' ". And I don't have any virtual function.

[jsulm]

@jenya7 Please read https://doc.qt.io/qt-5/signalsandslots.html
If you want to have signals and/or slots in a class you have to derive this class from QObject and add Q_OBJECT macro right at the beginning of your class.

#include <QObject>

class BT_DEVICE : public QObject
{
Q_OBJECT

signals:
    void scanFinished();
...

Do a complete rebuild of your project after these changes.

[jenya7]

@jsulm
this is h file

class BT_DEVICE : public QObject
{
    Q_OBJECT

    public:
    BT_DEVICE(QObject *parent = nullptr);
    void BLE_Scan();

signals:
    void ScanFinished();
};

and cpp file

BT_DEVICE::BT_DEVICE(QObject *parent) : QObject(parent)
{
    connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
}

void BT_DEVICE::BLE_Scan()
{
   scanning = 1;

   bt_device.startDeviceDiscovery();

   while (scanning) ;

   QVariant devices = bt_device.getDevices();
}

void BT_DEVICE::ScanFinished()
{
    scanning = 0;
}