terminate While loop inside a QThread

[Igor86]

Hi all!

I have a situation that is beyond my knowledge and cant figure out how to solve this.

I have a class, that runs on its own Thread. This class calls a method that has a while loop in it, that keeps checking a camera buffer to see if the image has been aquired.

Problem is, i need in certain situations to be able to abort this while loop from the main thread (UI), but since it is in the same thread as my class I cannot figure out how to Stop it from outside, as teh whole worker is locked.

Any help is appreciated...

class UiMainWindow(QMainWindow):
    def __init__(self):
        self.thread = QThread()
        self.worker = Worker()       
        self.thread.started.connect(self.worker.load_camera)
        self.worker.finished.connect(self.thread.quit)
        self.thread.start()


class Worker(QObject):
    def get_color(self, idin):
        try:
            color_img = crateColor.ColorImage(ia=self.ia, pool=self.pool, 
            machine_type=self.machine_type)
        except BaseException as be:
            print("Color recognition error:", be)

cratecolor:

class ColorImage:
    def __init__(self, ia=None, pool=str, machine_type=str):    
            self.abort = False

    def get_image(self):
        while not self.abort:
            buffer = self.ia.try_fetch(timeout=0)
            if buffer:
                 do stuff...
                 break

is there a way to abort this while loop from the user Interface?
Thanks to anybody willing to help me out..

[JonB]

@Igor86

• I would use QThread::requestInterruption() from the UI and QThread::isInterruptionRequested() in the thread's while not ... test.

• Or you could have your thread run() the Qt event loop and then send a signal from the UI where the worker has placed a slot.

• Or I presume Python would allow you to go from the UI

self.worker.color_img_member_variable.abort = True

But I wouldn't recommend it :)

As an aside, your

        while not self.abort:
            buffer = self.ia.try_fetch(timeout=0)

Depending on how that try_fetch(timeout=0) behaves, you may have a thread here which is very "busy" and uses up to 100% of its CPU time, you should check.

[SGaist]

Hi,

Beside the good point of @JonB and based on the code you show, your camera acquisition still happens in the main thread.

[Igor86]

@JonB thank you very much! I't have some questions tough...

for point (1) is there a way to revert the request once it has been set?
for point (2), sorry, i didnt understand this point at all.. could you please elaborate a little bit?

I set the timeout of the camera to 0.001 now and it is not locking up anymore. thanks for the tips!

@SGaist , thank you for your answer. I noticed i removed the "move to thread" line while posting. I do have it in the code tough..

Best regards

[JonB]

@Igor86 said in terminate While loop inside a QThread:

for point (1) is there a way to revert the request once it has been set?

There is no clearInterruption() method, so no. If you look at https://codebrowser.dev/qt5/qtbase/src/corelib/thread/qthread.cpp.html#_ZN7QThread19requestInterruptionEv you can see it's just a boolean flag wrapped in a QMutexLocker so you can do the same yourself on, say, your abort flag and allow clear if you wish. I have to say it's getting a bit weird that you want request an interruption and then change your mind.

for point (2), sorry, i didnt understand this point at all.. could you please elaborate a little bit?

See https://doc.qt.io/qt-6/qthread.html#details

QThreads begin executing in run(). By default, run() starts the event loop by calling exec() and runs a Qt event loop inside the thread.

and read the run() and particularly the exec(). If you are running the Qt event loop you can communicate from the main thread to the worker thread via signals/slots. You would need to change while not self.abort: to something like a QTimer(1) so that the event loop is re-entered regularly, or maybe call QCoreApplication::processEvents() if you keep your while loop.

As I wrote earlier, personally I would start by looking at whether your thread currently uses anywhere close to 100% CPU utilization. If it does then you need some kind of non-busy delay between every call to self.ia.try_fetch(timeout=0).

[Igor86]

@JonB Thank you very much!

[Igor86]

@JonB

as per why I change my mind on the abort, I try to explain my situation:

a external system sends a comand with a "job id" via TCP/IP to start the camera. at this point the software jumps into that loop and keeps checking the buffer waiting for the image. some time later, the external device triggers the camera with a digital contact and the image is in the buffer.
What can happen tough, is that we get a new comand with a new job id, and have to abort the previous one and start the get_color again with the new id. (the worker is startet at startup and keeps running, its just the get_color that needs to be restarted).
the idea was to request an abort on every new job id, so IF there is something running it will be aborted. But this means that the new get_color gets killed too. So I would need a way to reset that flag before I enter the loop, so i can be sure that on ENTERING the loop, its always false.

But maybe there are better ways to do this. All I would need to do is to exit the get_color whenever the TCP gets a new jobId (tcp running on another Qtask) and start it over again with the new ID.

hope the weird question makes a bit more sense now..