librosqt

This is a simple library that merges the event loop of ROS and Qt.

Usage

In your main.cpp init the node in the following way :

int main(int _argc, char ** _argv)
{
    QCoreApplication app(_argc, _argv);
    QRosCallBackQueue::replaceGlobalQueue();
    ros::init(_argc, _argv, "librosqt_simple_example", ros::init_options::NoSigintHandler);
    
//  [...]
    
    app.exec();
    return 0;
}

Examples

• Simple example

Motivations

The Qt library can be useful for ROS nodes when developping GUI applications, or even in headless nodes using network communications (tcp/ip, modbus, canbus, bluetooth ...).

Unfortunatly both Qt and ROS need to have their own event loop spinning : ros::spin() for ROS, QCoreApplication::exec() for Qt.

Multiple methods can be used to take care of both event loops :

Alternatives

Polling using a timer

void onTimeoutFromROS(const ros::TimerEvent&)
{
    qDebug() << "Timeout from ros timer";
}

int main(int _argc, char ** _argv)
{
    QCoreApplication app(_argc, _argv);
    ros::init(_argc, _argv, "polling_example", ros::init_options::NoSigintHandler);

    QTimer pollingTimer;
    QObject::connect(&pollingTimer, &QTimer::timeout, [](){
        ros::spinOnce();
    });
    pollingTimer.start(100); // 10 Hz

    ros::NodeHandle nh;
    ros::Timer timer = nh.createTimer(ros::Duration(1.0), onTimeoutFromROS, false, true);

    return app.exec();
}

Complete source code

One way to easily merge the event loops is to periodically call spinOnce() from Qt's main thread. However this method reduces the application reactivity to the frequency of the pollingTimer (10 Hz in this case).

Spawning another thread

void onTimeoutFromROS(const ros::TimerEvent&)
{
    qDebug() << "Timeout from ros in thread : " << QThread::currentThreadId()";
}

void onTimeoutFromQt()
{
    qDebug() << "Timeout from Qt in thread : " << QThread::currentThreadId()";
}

int main(int _argc, char ** _argv)
{
    QCoreApplication app(_argc, _argv);
    ros::init(_argc, _argv, "polling_example", ros::init_options::NoSigintHandler);

    ros::NodeHandle nh;

    std::thread thread_ros([](){
        ros::spin();
    });

    ros::Timer timer = nh.createTimer(ros::Duration(1.0), onTimeoutFromROS, false, true);

    QTimer pollingTimer;
    QObject::connect(&pollingTimer, &QTimer::timeout, onTimeoutFromQt);
    pollingTimer.start(1000); // 1 Hz

    return app.exec();
}

Complete source code

Using this method the callbacks from Qt (onTimeoutFromQt()) and from ROS (onTimeoutFromROS()) are called without any latency. The ROS callbacks are called from another thread, so we have to take care to protect any shared ressources with locking mechanisms like mutexes or mailboxes.

License

MIT