How to schedule a function to run on the main UI thread?

[relent95]

I want a worker thread created by the standard threading module to interact with the main UI thread, by scheduling a function on the worker thread, where that function manipulates widgets and will be run on the main UI thread.(Like Gdk.threads_add_idle() in GTK or runOnUiThread() in Android)

There are plenty of articles on QThread and they suggest defining a class with custom signals, customized for each specific problem. But I don't want that pattern. I want a reusable pattern, which separates the worker and UI implementations as much as possible.

I came to a solution like the following, that sends dummy signals to a global dummy QObject.

import sys
import time
import threading
from PySide2.QtCore import *
from PySide2.QtWidgets import *

class IdleRunner(QObject):
    run = Signal(object, tuple, float)
    def __init__(self):
        super().__init__()
        self.run.connect(self.on_run)
    def on_run(self, func, args, delay):
        if delay: QTimer.singleShot(delay * 1000, lambda: func(*args))
        else: func(*args)
_idle_runner = IdleRunner()
def run_on_idle(func, *args, delay = 0):
    _idle_runner.run.emit(func, args, delay)

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__(windowTitle = sys.argv[0])
        self.resize(400, 300)
        self.setAttribute(Qt.WA_DeleteOnClose, True)

        self.label = label = QLabel('aaa', alignment = Qt.AlignCenter)
        self.setCentralWidget(label)

        def thread_entry():
            time.sleep(1)
            run_on_idle(lambda: self.label.setText('bbb'))
            run_on_idle(self.close, delay = 1)
        self.thread = thread = threading.Thread(target = thread_entry)
        thread.start()
    def close(self):
        self.thread.join()
        super().close()

app = QApplication()
main_window = MainWindow()
main_window.show()
app.exec_()

I have two questions.

1. What is the best solution to this?(I expects a more elegant one in a Qt way.)
2. What are the possible problems of this solution? (Such as a memory leak or a runtime overhead).

[jsulm]

@relent95 said in How to schedule a function to run on the main UI thread?:

by scheduling a function manipulating widgets inside the worker thread entry

Manipulating UI from different threads than GUI thread is not supported!
Normal way in Qt to communicate between threads are signals/slots. If you want to notify main thread from another thread implement and emit a signal in that other thread. Connect this signal to a slot in main thread.

[relent95]

@jsulm Thanks for a quick reply. I didn't mean to manipulate UI directly in the worker thread, but schedule a function that does that. I'll update my question to be more clear.