Why does the slot emit it's connected signal ?

Quentin91

*Hi !

I'm using QOpenGL to print a video on a texture for an QML HMI. I'm using a code that I don't understand and that does not work yet. And i'm completely new to OpenGL

So today's question is :
Why would a slot emit the signal that calls this same slot ?

The signal I'm talking about, void newImageAvailable(); is emitted when a new image is ready to be printed. It's emitted periodically by a QThread. i got this. But the slot is only emitting the signal.
here are the specific samples of code.

	void ImageSourceShm::onNewImageAvailable(){
		// Attention : peut-être exécuté dans le thread du monitor
		Q_EMIT newImageAvailable();
	}

i'm trying to understand how the code is supposed to print the image get in the shared memory on a texture...

fcarney

@Quentin91
Are you 100% sure that the newImageAvailable signal is connected to onNewImageAvailable?

JKSH

@quentin91 said in Why does the slot emit it's connected signal ?:

The signal I'm talking about, void newImageAvailable(); is emitted when a new image is ready to be printed. It's emitted periodically by a QThread.

Which class contains that signal?

here are the specific samples of code.

  void ImageSourceShm::onNewImageAvailable(){
  	// Attention : peut-être exécuté dans le thread du monitor
  	Q_EMIT newImageAvailable();
  }

If that signal belongs to ImageSourceShm, then your program will enter an infinite recursion and crash when ImageSourceShm::newImageAvailable() is emitted.

Quentin91

@fcarney

@Quentin91
Are you 100% sure that the newImageAvailable signal is connected to onNewImageAvailable?

yes, i am

connect(m_pMonitorThread, SIGNAL(firstImageAvailable(int, int)), this, SLOT(onFirstImageAvailable(int, int)), Qt::DirectConnection);
connect(m_pMonitorThread, SIGNAL(newImageAvailable()), this, SLOT(onNewImageAvailable()), Qt::DirectConnection);

If that signal belongs to ImageSourceShm, then your program will enter an infinite recursion and crash when ImageSourceShm::newImageAvailable() is emitted.

That's weird as this program is the source code of a pyd lib well spread in my companny... The programs using it don't crash.
Ok, they are sometimes but mostly works.

Maybe it's because a thread is emitting the signal periodically too ? i still don't get how this program works to print images...

	void ImageSourceShmMonitorThreadByPeriod::run()
	{
		m_pPrecisionTimer->start();//mesure du temps d'attente
		while (m_pImageSourceShm->isAttached() == false)
		{
			QThread::msleep(1);
		}m_pData = m_pShm->data();
		QThread::msleep(1);
		Q_EMIT firstImageAvailable(m_pImageSourceShm->shm()->getWidth(), m_pImageSourceShm->shm()->getHeight());

		m_pPrecisionTimer->start();
		while (1)
		{
			if (m_pPrecisionTimer->elapsed() >= m_dPeriod)
			{
				m_pPrecisionTimer->start();
				Q_EMIT newImageAvailable();//THERE 
			}
			QThread::msleep(1);
		}
	}

JKSH

@quentin91 said in Why does the slot emit it's connected signal ?:

connect(m_pMonitorThread, SIGNAL(newImageAvailable()), this, SLOT(onNewImageAvailable()), Qt::DirectConnection);

The sender and receiver are different objects. The sequence is:

The signal, m_pMonitorThread->newImageAvailable(), is emitted
The slot, this->onNewImageAvailable(), is invoked by the previous signal
The signal, this->newImageAvailable(), is emitted in the previous slot
Nothing else happens, because nothing is connected to this->newImageAvailable()

Notice that #1 and #3 are different signals. I can't figure out what the author is trying to do here, however...

i'm trying to understand how the code is supposed to print the image get in the shared memory on a texture...

I don't see any image printing code in your posts.

P.S. FYI, ImageSourceShm subclassses QThread, implements slots in the subclass, implements an infinite while() loop inside run(), and then forces a Qt::DirectConnection. This is an anti-pattern; please don't do this in new code!

Quentin91

Thank you very much for this explanation. It's not surprising if I didn't understand then... I'll change this code

@jksh

I don't see any image printing code in your posts.

There aren't any image printing there. For now, a two classes are defined as QQuickItems. One is creating opengl textures, vao and stuff and one other is getting pictures in the sharedMemory. I still don't know how the data are supposed to be sent from the shm classes to the textures classes to be printed but I suppose I'll make another topic about it in a few days...