[PyQt] How to change the value which is shown on GUI

[Johoski, Timothy R]

hi Harry,

At the end of the updateLabel() function, it calls QTimer.singleShot to ensure that Qt will call updateLabel again in another 5 seconds.  So, the sequence of operations is something like this:
  - at the bottom of the file, the main function is called
  - main calls initUI
  - initUI constructs the widgets and calls QTimer.singleShot to schedule updateLabel to be called in 5 seconds
  - initUI returns back to main
  - main calls QApplication.exec_ to bring up the gui
  - QApplication.exec_ enters a loop to process mouse, button, key, and timer events
  - after 5 seconds, the QTimer expires and Qapplication's event loop calls updateLabel
  - updateLabel checks the voltage, current temperature, etc and updates the gui as appropriate
  - updateLabel calls QTimer.singleShot to schedule updateLabel to be called in another 5 seconds
  - control returns back to Qapplication's event loop and it processes mouse, button, key, and timer events again...  and after 5 seconds, the timer expires and calls updateLabel again.  and so on...

So, in essence, the updateLabel function is equivalent to your while True loop, it ensures that updateLabel is called every 5 seconds.  If you have other values like current temperature, etc, I recommend updating those values in the updateLabel function.  Hope this answers your questions..  Good luck.

- Tim



From: 吉文 [mailto:jiwen0304 at gmail.com] 
Sent: Friday, July 26, 2013 2:54 PM
To: Johoski, Timothy R
Cc: pyqt at riverbankcomputing.com
Subject: Re: [PyQt] How to change the value which is shown on GUI

Hi, Tim, Thank you very much for helping me, I will try it tomorrow. If there is other value such as current, temperature etc. , this value like the voltage, is also the real changing data, how to realize this function? Calling updateLabel() many times?
 
One more question, the voltage will change in every 5 seconds, i.e. raspberrypi will receive the value of voltage every 5 seconds, can I write the code like the following. 
 
while True:
          QtCore.QTimer.singleShot(5000, lambda: self.updateLabel(voltageEdit))
 
Thanks in advance
Best regards
Harry

2013/7/26 Johoski, Timothy R <timothy.r.johoski at intel.com>
> From: pyqt-bounces at riverbankcomputing.com [mailto:pyqt-bounces at riverbankcomputing.com] On Behalf Of ??
> Sent: Friday, July 26, 2013 9:45 AM
> To: pyqt at riverbankcomputing.com
> Subject: [PyQt] How to change the value which is shown on GUI
>
> Hello list, I want to design a battery monitoring system via raspberrypi. The voltage of battery is real changing data, I try to write some code, but the voltage can not change when I change the value of voltage.
>
> here is part of my code:
>
> import sys
> from PyQt4 import QtGui,QtCore
> import time
>
> class Example(QtGui.QWidget):
>
>         def __init__(self):
>               super(Example,self).__init__()
>
>               self.initUI()
>
>          def initUI(self):
>                voltage=12 #e.g. I get the value of voltage is 12v
>
>                voltageEdit=QtGui.QLabel(str(voltage),self)
>                voltageEdit.resize(voltageEdit.sizeHint())
>                voltageEdit.move(160,100)
>
>                time.sleep(5) # I want the GUI shows another value of voltage 5 seconds later
>
>                voltage=22
>                voltageEdit=QtGui.QLabel(str(voltage),self)
>                voltageEdit.resize(voltageEdit.sizeHint())
>                voltageEdit.move(160,100)
>
>                grid=QtGui.QGridLayout()
>                self.setLayout(grid)
>
>                self.setGeometry(300,200,600,400)
>                self.setWindowTitle("battery status")
>                self.show()
>
> def main():
>
>          app=QtGui.QApplication(sys.argv)
>          ex=Example()
>          sys.exit(app.exec())
>
> if __name__='__main__':
>          main()
>
>
> When I executed this code, GUI shows the voltage is 12, and it will not change, please tell me how to fix it.
>
> thanks in advance,
>
> Harry
hi Harry, the following illustrates how to update the label every 5 seconds using a QTimer.  In the original code, time.sleep(5) will actually put the application to sleep and Qt will not process any events (mouse motion, button presses, repaint events, etc).  But in the code below, QTimer.singleShot "schedules" the updateLabel function to be called in 5 seconds.  QTimer.singleShot returns immediately, allowing the next lines of code to run and show the widget on the screen.  5 seconds later, Qt calls updateLabel which changes the voltage text (and then schedules another call to itself 5 more seconds from now).
Hope this helps.
- Tim

import sys
from PyQt4 import QtGui,QtCore
import time

class Example(QtGui.QWidget):

        def __init__(self):
              super(Example,self).__init__()

              self.initUI()

        def initUI(self):
              voltage=12 #e.g. I get the value of voltage is 12v

              voltageEdit=QtGui.QLabel(str(voltage),self)
              voltageEdit.resize(voltageEdit.sizeHint())
              voltageEdit.move(160,100)
              # after 5 seconds (5000 milliseconds), call self.updateLabel
              QtCore.QTimer.singleShot(5000, lambda: self.updateLabel(voltageEdit))

              grid=QtGui.QGridLayout()
              self.setLayout(grid)

              self.setGeometry(300,200,600,400)
              self.setWindowTitle("battery status")
              self.show()
        def updateLabel(self, voltageEdit):
              # change the following line to retrieve the new voltage from the device
              newvoltage=int(voltageEdit.text()) + 1
              voltageEdit.setText(str(newvoltage))
              QtCore.QTimer.singleShot(5000, lambda: self.updateLabel(voltageEdit))

def main():

         app=QtGui.QApplication(sys.argv)
         ex=Example()
         sys.exit(app.exec_())

if __name__=='__main__':
         main()