How to update the Qt GUI from a Boost signal that is raised in another thread?

Question

I have a plain C++ object that runs a data acquisition routine in a separate thread and notify process with a Boost signal named acquisitionStageChangedEvent with the following signature: boost::signal2::signal<void(const std::string &)>. How can I start the acquisition in a new thread and update the UI with this information without having a cross-thread exception?

Possible duplicate of http://stackoverflow.com/questions/5050588/how-in-boost-send-a-signal-in-a-thread-and-have-the-corresponding-slot-executed — Frank Osterfeld, May 11 '15 at 05:36

score 0 · Answer 1 · answered May 11 '15 at 05:16

0

Set an std::atomic<bool> to true in your signal handler and check that flag from a QTimer.

answered May 11 '15 at 05:16

isanae

3,253
1
22
47

score -1 · Answer 2 · answered May 11 '15 at 00:33

Here is a working example on how to launch a background thread and update the UI during progress updates and at the end of the task:

namespace Ui
{
    class DtacqAcquisitionWidget;
}

class DtacqAcquisitionWidget : public QWidget
{
    Q_OBJECT

public:
    explicit AcquisitionWidget(QWidget *parent = 0);

   ~DtacqAcquisitionWidget();

private slots:

    void on_pushButtonStart_clicked();

    void onDtacqChangeState(const QString &stage);

    /*... Other slots here */
private:
    Ui::AcquisitionWidget *ui;

    QFutureWatcher<void>  *m_future_watcher; // This is to be able to run UI code at the end of the background thread

    anlytcs::Dt100Acq m_dtacq; // The plain C++ object that raises the boost signal 'acquisitionStageChangedEvent'
};

In the .cpp file:

DtacqAcquisitionWidget::DtacqAcquisitionWidget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::DtacqAcquisitionWidget)
{
    ui->setupUi(this);

    // Run the 'onAcquisitionFinished' slot at the end of the thread
    m_future_watcher = new QFutureWatcher<void>(this);
    connect(m_future_watcher, SIGNAL(finished()), this, SLOT(onAcquisitionFinished()));

    // Acquisition stages update
    m_dtacq.acquisitionStageChangedEvent.connect([this](const std::string &stage)
    {
        this->onDtacqChangeState(QString::fromStdString(stage));
    });
}

void DtacqAcquisitionWidget::on_pushButtonStart_clicked()  // Starting the acquisition
{
    ui->pushButtonStop->setEnabled(true);
    ui->pushButtonStart->setEnabled(false);
    ui->progressBar->setValue(0);

    // Start the acquisition in a new thread
    QFuture<void> f = QtConcurrent::run(this, &DtacqAcquisitionWidget::acquireChannelData);    
    m_future_watcher->setFuture(f);
}

void DtacqAcquisitionWidget::onDtacqChangeState(const QString &stage)
{
    if (thread() != QThread::currentThread())
    {      
        QMetaObject::invokeMethod(this, "onDtacqChangeState", 
            Qt::BlockingQueuedConnection, Q_ARG(QString, stage));
    }
    else
    {
        ui->labelDtacqState->setText(stage);
        ui->progressBar->setValue(ui->progressBar->value() + 40);
    }    
}

void DtacqAcquisitionWidget::onAcquisitionFinished()
{
    // Set what to update here in the GUI here when the acquisition thread finishes     
    ui->labelDtacqState->setText("DONE!");
}

void DtacqAcquisitionWidget::acquireChannelData()
{
    // This is running on a background thread (Non UI thread)
    double time_sec = ui->spinBoxAcqTimeSec->value();
    uint32_t channel_mask = getChannelMask();

    std::string data_path = "C:/Users/pardinad/Desktop/DtacqData/";
    m_dtacq.startAcquisition(time_sec, channel_mask, 250);
    ui->labelDtacqState->setText("Acquisition Started!");

    if(m_dtacq.completeAcquisition())   
    {
        for (auto & dch : m_dtacq.dataChannels())
        {
            std::stringstream ss;
            ss << data_path << std::setw(2) << std::setfill('0') << dch.channelNumber() << ".DAT";
            std::string ch_file = ss.str();
            dch.saveToFile(ch_file);
        }
    }
}

This is a pretty dirty solution. It's much simpler to just make a data acquisition worker object and let it alone in its own thread. Mixing boost signal/slot/thread with Qt ones is just the wrong starting point. — user3528438, May 11 '15 at 03:57
@user3528438, you are aware that one does not always has a choice how things are done? Ever worked with legacy code in a paid project? — Greenflow, May 11 '15 at 06:57
@user3528438 The point is not mixing boost::signals with Qt signals/slots, rather to create a thin Qt UI wrapper around a cross-platform library that doesn't have Qt as a dependency. I know well the phrase that goes "when in Rome do as Romans do", but this is not the case. — Darien Pardinas, May 11 '15 at 12:55
@DarienPardinas Then is it, your solution, a "thin wrapper"? — user3528438, May 11 '15 at 16:45
Yes it is! The core of the logic is inside the `DtacqAcquisition` class. The UI only provide button and updates of the operations happening in a Qt-less library. I didn't say the UI is trivial though. — Darien Pardinas, May 11 '15 at 21:36

How to update the Qt GUI from a Boost signal that is raised in another thread?

2 Answers2