PyQt: How to deal with QThread? - multithreading

Question

In the following code I try to deal with QThread. In this executable example there are three buttons: first for start, second for stop and third for close. Well, when I start the task its runs like a charm. BUT when I want the while-loop to stop I click on the stop-button. And now, there is a problem: the while-loop doesn't stop.
You see, the stop-button emits a signal to call the stop() method on TestTask().
What is wrong?
from sys import argv
from PyQt4.QtCore import QObject, pyqtSignal, QThread, Qt, QMutex
from PyQt4.QtGui import QDialog, QApplication, QPushButton, \
QLineEdit, QFormLayout, QTextEdit
class TestTask(QObject):
def __init__(self, parent=None):
QObject.__init__(self, parent)
self._mutex = QMutex()
self._end_loop = True
def init_object(self):
while self._end_loop:
print "Sratus", self._end_loop
def stop(self):
self._mutex.lock()
self._end_loop = False
self._mutex.unlock()
class Form(QDialog):
stop_loop = pyqtSignal()
def __init__(self, parent=None):
QDialog.__init__(self, parent)
self.init_ui()
def init_ui(self):
self.pushButton_start_loop = QPushButton()
self.pushButton_start_loop.setText("Start Loop")
self.pushButton_stop_loop = QPushButton()
self.pushButton_stop_loop.setText("Stop Loop")
self.pushButton_close = QPushButton()
self.pushButton_close.setText("Close")
layout = QFormLayout()
layout.addWidget(self.pushButton_start_loop)
layout.addWidget(self.pushButton_stop_loop)
layout.addWidget(self.pushButton_close)
self.setLayout(layout)
self.setWindowTitle("Tes Window")
self.init_signal_slot_pushButton()
def start_task(self):
self.task_thread = QThread(self)
self.task_thread.work = TestTask()
self.task_thread.work.moveToThread(self.task_thread)
self.task_thread.started.connect(self.task_thread.work.init_object)
self.stop_loop.connect(self.task_thread.work.stop)
self.task_thread.start()
def stop_looping(self):
self.stop_loop.emit()
def init_signal_slot_pushButton(self):
self.pushButton_start_loop.clicked.connect(self.start_task)
self.pushButton_stop_loop.clicked.connect(self.stop_looping)
self.pushButton_close.clicked.connect(self.close)
app = QApplication(argv)
form = Form()
form.show()
app.exec_()

Answer 1

The stop_loop signal is converted to an event and sent to the thread of the signal receiver. But your worker object is running a blocking while-loop, and this prevents the thread processing any pending events in its event-queue. So the slot connected to the stop_loop signal will never be called.
To work around this, you can call processEvents in the while-loop to allow the thread to process its pending events:
def init_object(self):
while self._end_loop:
QThread.sleep(1)
QApplication.processEvents()
print "Status", self._end_loop
Alternatively, you could call the worker's stop() method directly instead of emitting a signal. Strictly speaking, this is not thread-safe, but it would only be a problem if multiple threads could call stop() at the same time. So a more correct way to do that is to use a mutex to protect the value being changed:
class TestTask(QObject):
def __init__(self, parent=None):
QObject.__init__(self, parent)
self._mutex = QtCore.QMutex()
self._end_loop = True
def stop(self):
self._mutex.lock()
self._end_loop = False
self._mutex.unlock()
And now calling stop() directly from the main thread is thread-safe.