CODEWITHSUNDEEP
pythonmultithreadingpyqtsignals
And0rian
And0rian

Reputation: 170

PyQt Signals across threads

I've been messing around with PyQt and signals/slots across threads. Here a situation where I can't find my mistake:

I have a class (MultipleProcessLauncher) that is able to launch multiple processes in separate threads. I catch the stdout of each processes and send those messages to a single queue that is read by another thread (OutputWorker), this last thread should send a signal onNewMessage (I think it doesn't) catch on the main class but the callback function is never called.

But: - The signal of the reading thread doesn't seems to emit anything, so the callback function of the main thread is never called...

Your help would be much appreciated, I think I'm missing something with cross threads signals...

class OutputWorker(QObject):
    onNewMessage = pyqtSignal(['QString'])

    def __init__(self, queue, parent=None):
        super(OutputWorker, self).__init__(parent)
        self.queue = queue

    def work(self):
        while True:
            item = self.queue.get()
            self.onNewMessage.emit(item)
            self.queue.task_done()

class MultipleProcessLauncher(QObject):
    commandEvent = pyqtSignal(['QString'])

    def __init__(self, parent=None):
        super(MultipleProcessLauncher, self).__init__(parent)

        self.messaging_queue = Queue()

        # Start reading message
        self.reading_thread = QThread()

        self.worker = OutputWorker(self.messaging_queue)
        self.worker.moveToThread(self.reading_thread)
        self.worker.onNewMessage.connect(self.command_event)

        self.reading_thread.started.connect(self.worker.work)
        self.reading_thread.start()

    def execute(self, command):
        p = subprocess.Popen(command, stdout=subprocess.PIPE)
        t = Thread(target=self.enqueue, args=(p.stdout, self.messaging_queue))
        t.daemon = True
        t.start()

    def enqueue(self, stdout, queue):
        for line in iter(stdout.readline, b''):
            queue.put(line.decode())
        stdout.close()

    def command_event(self, event):
        # This point is never reached
        print('message received')

if __name__ == '__main__':
    manager = MultipleProcessLauncher()
    manager.execute('ipconfig')

    time.sleep(100)

Upvotes: 4

Views: 1431

Answers (1)

Oliver
Oliver

Reputation: 29591

Qt's cross-thread signaling is based on event loop, so you need to exec a QApplication so that there is a main event loop to process signals from other threads. For example: 

if __name__ == '__main__':
    app = QApplication([])
    manager = MultipleProcessLauncher()
    manager.execute('ipconfig')
    MAX_WAIT_MSEC = 100 * 1000  # 100 seconds
    QTimer.singleShot(MAX_WAIT_MSEC, app.quit) 
    app.exec()

In your real application you will probably execute the manager based on user input so the execute would be in a slot, and there wouldn't be a need to quit, etc, but you get the idea.

Upvotes: 3

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