GUI Freezes using QProcess during data acquisition

Question

PROBLEM DEFINITION: I have an external application called runSensor that communicates with a sensor. The sensor sends its data over UDP. Using the terminal, runSensor has two arguments to communicate with the sensor for data acquisition:start and stop.

Once at the terminal I call: $ runSensor start, a sample output is as follows:

[Time 07:20:11:000]: Device PoweredOn.
[Time 07:20:11:010]: x=1.231, y=-0.022, z=0.001
[Time 07:20:11:015]: x=1.235, y=-0.024, z=0.001
[Time 07:20:11:020]: x=1.241, y=-0.024, z=0.002
[Time 07:20:11:025]: x=1.258, y=-0.027, z=0.002

I need to call start and stop using a QT-UI. For that, I have a QDialog as follows:

Dialog::Dialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
    ui->setupUi(this);
    this->socketHandler         = std::make_shared<udpHandler>();
    this->runSensorStartProcess = std::make_shared<QProcess>();
    this->runSensorStopProcess  = std::make_shared<QProcess> ();

    //--------------------------
    // SIGNAL - SLOT
    //--------------------------
    connect(ui->startButton, SIGNAL(clicked()), this,  SLOT(onStartButtonClicked()));
    connect(ui->stopButton, SIGNAL(clicked()), this, SLOT(onStopButtonClicked()));
    connect(this->socketHandler.get(), SIGNAL(sendUdpContent(QString)), this, SLOT(updateMessageBrowser(QString)));
    connect(this->runSensorStartProcess.get(), SIGNAL(readyReadStandardError()), this, SLOT(printError()));
}

I use QProcess to call the start and stop of runSensor application. When I push the GUI's start button, data acquisition starts, but the the GUI freezes and I cann't click on the stop button.

CODE EXTRACTS: Here is how I implemented the start button click:

void Dialog::onStartButtonClicked()
{ 
    this->runSensorStartProcess->start("start");
    //this->runSensorStartProcess->waitForFinished(-1);

    //--------------------------
    // udp socket handler starts picking data and write it into a file
    //--------------------------

    if (!this->runSensorStartProcess->waitForStarted())
    {
        qWarning() << "Warning: Cannot start Cygwin process!";
    }
}

stop button click implementation is similar:

void Dialog::onStopButtonClicked()
{
    if(this->runSensorStartProcess.get() != NULL)
    {
        this->runSensorStartProcess->close();
    } 
    this->runSensorStopProcess->start("stop");

    if (!this->runSensorStopProcess->waitForStarted())
    {
        qWarning() << "Warning: Cannot stop Cygwin!";
    }
}

Question:

1. How can I keep th GUI responsive after runSensorStartProcess starts?
2. How can I stop that process on demand (in fact by starting runSensorStartProcess)?
3. Do I need a separate thread for runSensorStartProcess?

Answer 1

1. Don't use any of the waitForXxx methods. That's all. Yes, it's that simple.

   Note that for every waitForXxx method there's a signal you can attach to and thus react to the event you're looking for.

2. Aren't QProcess::kill and QProcess::terminate what you need?

3. Never. See also answer 1 and answer 2.