calling ros::spin or ros::spinOnce from other function in QT

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Hello,

I am very new to ROS and I have just manged to integrate the ROS libraries into my QT IDE.

I am building a fairly complex application, however the ROS subscriber example I am attempting to implement fairly simple.

If I implement my code this way (per the examples), I get subscriber callbacks as expected:

void MainWindow::rtsp1Callback(const sensor_msgs::Image::ConstPtr& msg)
{
    qDebug() << "rtsp1 Image has data";
}

With this initializer:

void MainWindow::on_ROSSubscribeButton_clicked()
{
    ros::init(m_argc, m_argv, "camera_check");
    ros::NodeHandle n1;
    ros::Subscriber sub1 = n1.subscribe("/rtsp_1/image_raw", 100, &MainWindow::rtsp1Callback, this);
    ros::spin();
}

But of course, this is an undesirable design, as ros::spin(); then blocks the rest of my program.

So I execute a timer to call ros::spinOnce(); on demand:

Callback unchanged:

void MainWindow::rtsp1Callback(const sensor_msgs::Image::ConstPtr& msg)
{
    qDebug() << "rtsp1 Image has data";
}

Initializer with timer:

void MainWindow::on_ROSSubscribeButton_clicked()
{
    ros::init(m_argc, m_argv, "camera_check");
    ros::NodeHandle n1;
    ros::Subscriber sub1 = n1.subscribe("/rtsp_1/image_raw", 100, &MainWindow::rtsp1Callback, this);
    CameraCheckTimer.start(100);  //calls ros::spinOnce() every 1000ms;
}

And this spin call (on timer execution):

void MainWindow::checkCameras()
{
    ros::spinOnce();
    qDebug() << "Spin!";
}

However I do not get any callbacks.

I also tested calling ros::spin() - which was working in my initial example, by calling a singleShot timer. When I do this, my program blocks as expected, however I do not get any callbacks.

This makes me suspect that ros::spin() and ros::spinOnce() are somehow getting an implicit pointer to the GlobalCallbackQueue from the ros::init() function. Is this the case?

If so, what is the data type of ros::GlobalCallbackQueue (so that I can pass a global pointer to it into my timer loop)?

Or, if you have an example of how to simply fix my problem, I will accept that as well!