CODEWITHSUNDEEP
c++multithreadingqt
user3518432
user3518432

Reputation: 17

How to use threads in Qt GUI

I have an application that continuously reads an image from a camera and displays this to a user. The user can adjust different sliders such as exposure and threshold to modify the image in real-time. I also do a bunch of calculations on this image afterwards which sometimes makes the GUI unresponsive, so I decided to use threads to divide the workload.

However, I can't get it to work properly, sometimes I get segmentation faults and a bunch of "assertion ` GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT (object)' failed" warnings when the slider values are changed or if I try to save the image (save button in my GUI), and the GUI sometimes stops updating the image or freezes so you can't move the sliders or push any buttons.

What I tried to do was to use the standard std::thread in C++, and connect it to the start button through a slot. 

QObject::connect(btnStart, SIGNAL(clicked()), this, SLOT(RunStartThread()));

void MainMenu::RunStartThread(){
std::thread t1;
t1= std::thread(&MainMenu::Start,this);
t1.detach();
}

void MainMenu::Start() {
run = true;    
    window->mngr->ReadCalibration();
    window->mngr->InitializeCameras();

    while (run) {

        window->mngr->CaptureImage();
        window->mngr->ProcessImages();            
        UpdateLabels();
      }  

window->mngr->Stop();
}

When the user changes the slider values they change variables in my manager (mngr above) that captureImage and ProcessImages uses. I tried using a std::mutex lock/unlock when a variable was to be accessed, but it did not change anything. I've tried to find examples of how to do this online, but have yet to find something that has a continuous while-loop.

I'm a newbie when it comes to threads, so just tell me if I'm approaching this in the wrong way.

Upvotes: 1

Views: 1178

Answers (1)

Marek R
Marek R

Reputation: 38209

First for all for inter thread communication use singnals and slots. By default Qt connections do nice thread hopping between threads which lets to avoid complex synchronization.

Secondly you have three ways of using threads: QThread, QRunnable, QtConcurrent::Run (my favorite, since requires minimum amount of code).

In case QThread please do not subclass it! It is common design mistake.

Example:

SomeClass::~SomeClass()
{
    SignalStop();
    future.result();
}

void SomeClass::RunStartThread(){
     future = QtConcurrent::run(this, &SomeClass::DoOnThread);
}

void SomeClass::DoOnThread()
{
    while (ShouldContinueToRun()) {
        QImage im1 = CaptureImage();
        emit ImageCaptured(im1);
        QImage im2 = ProcessImages(im1);            
        emit ImageProcessed(im2);
    }
    emit JobCompleted();
}

Please note that QObject::connect has last argument which defines how invocation of slot is performed if different thread is involved. See documentation of enumeration used for this argument.
So by default Qt detects if thread hopping is needed or not. Reading carefully about QObject::moveToThread should also help to understand the problem (note you can't move object to different thread if it has a parent).

Upvotes: 1

Related Questions