PyQt5 add second function to thread but not work - multithreading

Question

Previously I have tried to use Flask for doing the followings simultaneously:
Display live video streaming
Display real-time data streaming
Control the robot car
As the above is just for demonstration, with the video streaming performance not good enough, I decided to change the whole application to PyQt5 for further development and production. Now I can create the GUI for displaying live video streaming well, while the real-time data streaming cannot be done well. The error is
QObject::startTimer: Timers can only be used with threads started with QThread
The following is the whole program. Please help to see what's wrong in the adding thread issue. Thanks!
import sys
from PyQt5 import QtWidgets, QtCore, QtGui
import cv2
from vidgear.gears import CamGear
from random import random
data_list=[]
fps=60
options_cam={"CAP_PROP_FRAME_WIDTH":640,"CAP_PROP_FRAME_HEIGHT":480,"CAP_PROP_FPS":fps}
stream=CamGear(source=0,logging=False,**options_cam).start()
class MainWindow(QtWidgets.QWidget):
def __init__(self,*args):
super(MainWindow, self).__init__()
self.setWindowTitle('Vehicle control')
self.grid_layout=QtWidgets.QGridLayout()
self.video_label = QtWidgets.QLabel('Video streaming',self)
self.video_frame = QtWidgets.QLabel(self)
self.grid_layout.addWidget(self.video_label,0,0)
self.grid_layout.addWidget(self.video_frame,1,0)
self.data_label = QtWidgets.QLabel('Data streaming',self)
self.data_frame = QtWidgets.QListWidget(self)
self.grid_layout.addWidget(self.data_label,0,1)
self.grid_layout.addWidget(self.data_frame,1,1)
self.setLayout(self.grid_layout)
#self.thread=QtCore.QThread()
#self.thread.started.connect(self.nextFrameSlot)
#self.thread.start()
self.timer=QtCore.QTimer()
self.timer.timeout.connect(self.video_stream)
self.timer.start(0)
self.thread=QtCore.QThread()
self.thread.start()
self.timer2=QtCore.QTimer()
self.timer2.moveToThread(self.thread)
self.timer2.timeout.connect(self.data_stream)
self.timer2.start(0)
def video_stream(self):
frame = stream.read()
# My webcam yields frames in BGR format
frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
img = QtGui.QImage(frame, frame.shape[1], frame.shape[0], QtGui.QImage.Format_RGB888)
pix = QtGui.QPixmap.fromImage(img)
self.video_frame.setPixmap(pix)
QtCore.QThread.sleep(0)
def data_stream(self):
print("data stream")
stream_data=round(random()*10,3)
data_list.insert(0,str(stream_data)+'\n')
if len(data_list)>10:
del data_list[-1]
for i in range(len(data_list)):
self.data_frame.addItem(data_list[i])
self.data_frame.show()
QtCore.QThread.sleep(1000)
if __name__ == '__main__':
import sys
app = QtWidgets.QApplication(sys.argv)
window = MainWindow()
window.show()
sys.exit(app.exec_())
Edit:
Thanks #musicamante's answer. I have updated the code as follows but still have the error "segmentation fault" for the video streaming, while if I run for data stream only, the updated list can be shown. So what's wrong with the setPixmap function? Thanks again!
import sys
from PyQt5 import QtWidgets, QtCore, QtGui
import cv2
from vidgear.gears import CamGear
from random import random
fps=60
options_cam={"CAP_PROP_FRAME_WIDTH":480,"CAP_PROP_FRAME_HEIGHT":480,"CAP_PROP_FPS":fps}
stream=CamGear(source=0,logging=False,**options_cam).start()
class CamGrabber(QtCore.QThread):
frame = QtCore.pyqtSignal(QtGui.QImage)
def run(self):
while True:
new_frame = stream.read()
new_frame = cv2.cvtColor(new_frame, cv2.COLOR_BGR2RGB)
img = QtGui.QImage(new_frame, new_frame.shape[1], new_frame.shape[0], QtGui.QImage.Format_RGB888)
self.frame.emit(img)
class DataProvider(QtCore.QThread):
data = QtCore.pyqtSignal(object)
def run(self):
while True:
newData = round(random()*10,3)
self.data.emit(newData)
QtCore.QThread.sleep(1)
class MainWindow(QtWidgets.QWidget):
def __init__(self,*args):
super(MainWindow, self).__init__()
self.setWindowTitle('Vehicle control')
self.grid_layout=QtWidgets.QGridLayout()
self.video_label = QtWidgets.QLabel('Video streaming',self)
self.video_frame = QtWidgets.QLabel(self)
self.grid_layout.addWidget(self.video_label,0,0)
self.grid_layout.addWidget(self.video_frame,1,0)
self.data_label = QtWidgets.QLabel('Data streaming',self)
self.data_frame = QtWidgets.QListWidget(self)
self.grid_layout.addWidget(self.data_label,0,1)
self.grid_layout.addWidget(self.data_frame,1,1)
self.setLayout(self.grid_layout)
self.camObject = CamGrabber()
self.camObject.frame.connect(self.newFrame)
self.camObject.start()
self.dataProvider = DataProvider()
self.dataProvider.data.connect(self.newData)
self.dataProvider.start()
def newFrame(self, img):
self.video_frame.setPixmap(QtGui.QPixmap.fromImage(img))
def newData(self, data):
self.data_frame.insertItem(0,str(data))
if self.data_frame.count() > 10:
self.data_frame.takeItem(9)
if __name__ == '__main__':
import sys
app = QtWidgets.QApplication(sys.argv)
window = MainWindow()
window.show()
sys.exit(app.exec_())```

Answer 1

The QTimer error basically means that the a QTimer can only be started from the thread it exists.
Besides that, GUI element should always be directly accessed or modified from the main thread, not from another one.
In order to accomplish that, you'll need to create a separate "worker" thread, and communicate with the main one by taking advantage of the signal/slot mechanism.
class CamGrabber(QtCore.QThread):
frame = QtCore.pyqtSignal(QtGui.QImage)
def run(self):
while True:
frame = stream.read()
frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
img = QtGui.QImage(frame, frame.shape[1], frame.shape[0], QtGui.QImage.Format_RGB888)
self.frame.emit(img)
class DataProvider(QtCore.QThread):
data = QtCore.pyqtSignal(object)
def run(self):
while True:
newData = round(random()*10,3)
self.data.emit(newData)
# note that QThread.sleep unit is seconds, not milliseconds
QtCore.QThread.sleep(1)
class MainWindow(QtWidgets.QWidget):
def __init__(self,*args):
super(MainWindow, self).__init__()
self.setWindowTitle('Vehicle control')
self.grid_layout=QtWidgets.QGridLayout()
self.video_label = QtWidgets.QLabel('Video streaming',self)
self.video_frame = QtWidgets.QLabel(self)
self.grid_layout.addWidget(self.video_label,0,0)
self.grid_layout.addWidget(self.video_frame,1,0)
self.data_label = QtWidgets.QLabel('Data streaming',self)
self.data_frame = QtWidgets.QListWidget(self)
self.grid_layout.addWidget(self.data_label,0,1)
self.grid_layout.addWidget(self.data_frame,1,1)
self.setLayout(self.grid_layout)
self.camObject = CamGrabber()
self.camObject.frame.connect(self.newFrame)
self.camObject.start()
self.dataProvider = DataProvider()
self.dataProvider.data.connect(self.newData)
self.dataProvider.start()
def newFrame(self, img):
self.video_frame.setPixmap(QtGui.QPixmap.fromImage(img))
def newData(self, data):
self.data_frame.addItem(str(data))
if self.data_frame.count() > 10:
self.data_frame.takeItem(0)
If, for any reason, you want to control the data fetching from the main thread via a QTimer, you could use a Queue:
from queue import Queue
class DataProvider(QtCore.QObject):
data = QtCore.pyqtSignal(object)
def __init__(self):
super().__init__()
self.queue = Queue()
def run(self):
while True:
multi = self.queue.get()
# simulate a time consuming process
QtCore.QThread.sleep(5)
newData = round(multi * 10, 3)
self.data.emit(newData)
def pushData(self, data):
self.queue.put(data)
class MainWindow(QtWidgets.QWidget):
def __init__(self,*args):
# ...
self.requestTimer = QtCore.QTimer()
self.requestTimer.setInterval(1000)
self.requestTimer.timeout.connect(self.requestData)
# this will cause the timer to be executed only once after each time
# start() is called, so no new requests will overlap
self.requestTimer.setSingleShot(True)
self.requestTimer.start()
def requestData(self):
value = random()
print('requesting data with value {}'.format(value))
self.dataProvider.pushData(value)
print('waiting for result')
def newFrame(self, img):
self.video_frame.setPixmap(QtGui.QPixmap.fromImage(img))
def newData(self, data):
print('data received')
self.data_frame.addItem(str(data))
if self.data_frame.count() > 10:
self.data_frame.takeItem(0)
# restart the timer
self.requestTimer.start()