PyQt: Connecting a signal to a slot to start a background operation

user_482819 0
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I have the following code that performs a background operation (scan_value) while updating a progress bar in the ui (progress). scan_value iterates over some value in obj, emitting a signal (value_changed) each time that the value is changed. For reasons which are not relevant here, I have to wrap this in an object (Scanner) in another thread. The Scanner is called when the a button scan is clicked. And here comes my question ... the following code works fine (i.e. the progress bar gets updated on time).

# I am copying only the relevant code here.

def update_progress_bar(new, old):
    fraction = (new - start) / (stop - start)
    progress.setValue(fraction * 100)

obj.value_changed.connect(update_progress_bar)

class Scanner(QObject):

    def scan(self):
        scan_value(start, stop, step)
        progress.setValue(100)

thread = QThread()
scanner = Scanner()
scanner.moveToThread(thread)
thread.start()

scan.clicked.connect(scanner.scan)

But if I change the last part to this:

thread = QThread()
scanner = Scanner()
scan.clicked.connect(scanner.scan) # This was at the end!
scanner.moveToThread(thread)
thread.start()

The progress bar gets updated only at the end (my guess is that everything is running on the same thread). Should it be irrelevant if I connect the signal to a slot before of after moving the object receiving object to the Thread.

qthread pyqt signals-slots multithreading python
3 Answers

My problem was solved by movinf the connection to the spot where the worker thread is initialized, in my case because I am accessing an object which only exists after instantiation of my Worker Object class which is in another thread.

Simply connect the signal after the self.createWorkerThread()

Regards

user_88456530
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