QT: Updates can only be scheduled from GUI thread or from QQuickItem::updatePaintNode()

Myself.Shivu · wrote on 10 Jan 2020, 12:17

I'm trying to change Image with Pushbutton (GPIO PINs) stored inside a folder using QML with PyQt5

Python Code:

from gpiozero import Button
from signal import pause

from PyQt5.QtCore import *
from PyQt5.QtGui import *
from PyQt5.QtQml import *
import os, time, sys


def btn_pressed():
    global r
    return lambda: r.setProperty("source", "/home/pi/Desktop/example/sample/img/img1.jpg")

button1 = Button(20)        
myApp = QGuiApplication([])
myEngine = QQmlApplicationEngine()
directory = os.path.dirname(os.path.abspath(__file__))
myEngine.load(QUrl.fromLocalFile(os.path.join(directory, 'simple1.qml')))
if not myEngine.rootObjects():
    print("root object not found")

r = myEngine.rootObjects()[0].findChild(QObject, "DisplayImage")
dir(r)
print("Main Thead id ",myApp.thread())
updateUI = UpdateUI()

button1.when_pressed = btn_pressed()   
myEngine.quit.connect(myApp.quit)
sys.exit(myApp.exec_())

QML:

import QtQuick 2.10
import QtQuick.Controls 1.6
import QtQuick.Window 2.2

ApplicationWindow {
    id : main
    title: qsTr("Test")
    width: 640
    height: 480
    visible: true

    Rectangle{
        width: parent.width
        height: parent.height

        Image {
            id: img
            source: ""
            width : main.width;
            fillMode : Image.PreserveAspectFit
            objectName: "DisplayImage"
        }
    }
}

When I Pressed the Push Button Connected to GPIO 20 in raspberry Pi 4 , I'm Getting below error message



list itemQObject: Cannot create children for a parent that is in a different thread
(Parent is QQmlApplicationEngine(0x10f5ba0),
parents thread is QThread(0xf6f7c0)
current thread is QThread(0xa98007c8)
Updates can only be scheduled from GUI thread or from QQuickItem::updatePaintNode()


I also tried creating class with a method changing Image Source Property and then calling the same method(via Class Object) upon PushButton press but it shows same error message

Is there a way to set Image Source Property in QML - from parent thread in Python.
In Winforms we can Avoid "Cross-Thread Violation Error" by using Delegates.
Can we Use `Signal and Slot` to Solve this Problem in PyQt5