Is this safe? Do I have to use signals to call UI widget methods?

ideaplus

Is this safe? Do I have to use signals to call UI widget methods on the main thread?

import sys

from PySide2.QtCore import QThread
from PySide2.QtWidgets import QApplication, QWidget


class MyThread(QThread):
    def __init__(self, main_window, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.main_window = main_window

    def run(self):
        # Is this safe? Do I have to use signals to call UI widget methods?
        print(self.main_window.geometry())
        # Is this safe? Do I have to use signals to call UI widget methods?
        self.main_window.hide()
        print(self.parent())


if __name__ == '__main__':
    app = QApplication(sys.argv)
    my_widget = QWidget()
    my_widget.show()
    my_thread = MyThread(my_widget)
    my_thread.start()
    sys.exit(app.exec_())

refer link:
https://stackoverflow.com/questions/53058849/is-it-ok-to-read-qt-widgets-from-another-thread
https://stackoverflow.com/questions/42876085/is-it-possible-to-hide-qt-widget-window-from-other-thread

mrjj

Hi
No its not safe.
Say your thread reads main_window.geometry() at the exact same time
you resize the window and the main thread updates the info.

Its best to use signals and slots for this.

Else its just a crash waiting to happen.

ideaplus

Thanks. I understand. :-)

jsulm

@Denni-0 A QWidget can be placed in a QThread, but it is not supported and can lead to crashes and misbehaviour. And nothing stops you from calling methods on widgets from a non GUI thread (but of course you should not). So, @mrjj is not wrong.

SGaist

Technically: QWidget based objects should only be manipulated in the same thread where the QApplication object was created. Usually, it's in the main thread, the one created to run the "main" function but it can be another one although it's not the most used technique.

SGaist

Did you actually take the time to fully understand what I wrote ? At no point did I say that you could use a QWidget in a secondary thread beside the main thread, quite the contrary. I wrote that the thread creating the QApplication object was the only one where you are allowed to modify QWidget objects and that this precise thread might be a secondary thread even if it's not the common use case.

jsulm

This post is deleted!