Updating QML GUI from C++

TomHoe

Hi everyone,

I am trying to change a parameter in my qml file from c++ using the following code:

    QQmlEngine ccbengine;
    QQmlComponent ccbcomponent(&ccbengine, QUrl(QLatin1String("qrc:/pages/ccbframe.qml")));
    QObject *object = ccbcomponent.create();

    QObject *rect = object->findChild<QObject*>("ccb_latchValue");
    if (rect)
        rect->setProperty("text", "1");
    qDebug() << "Property value:" << QQmlProperty::read(rect, "text").toInt();

The debugger gives the output I want.. but the gui isn't updating and stays at the last value. Is there a way to update the gui?

raven-worx

@TomHoe
whats the type of the item with ccb_latchValue as object name?
Is this a custom created type?

TomHoe

@raven-worx Its a label,

                Label {
                    objectName: "ccb_latchValue"
                    width: parent.width * 0.5 / 7
                    wrapMode: Label.Wrap
                    horizontalAlignment: Qt.AlignHCenter
                    text: "?"
                }

raven-worx

@TomHoe
event loop is running?
you are not halting on a breakpoint in the debugger while you check the GUI?
Does the GUI update after a resize?
QtQuickControls 1 or 2?
What Qt version / OS?

TomHoe

@raven-worx

I'm compiling for Android now,
-there is no break-point.
-The gui doesn't update after a resize.
-QtQuickControls 2.

So I guess there is no loop running, here is the main c++ code:

int main(int argc, char *argv[])
{
    QGuiApplication::setApplicationName("Reader");
    QGuiApplication::setOrganizationName("Tom");
    QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QSettings settings;
    QString style = QQuickStyle::name();
    if (!style.isEmpty())
        settings.setValue("style", style);
    else
        QQuickStyle::setStyle(settings.value("style").toString());

    QQmlApplicationEngine engine;
    engine.load(QUrl("qrc:/main.qml"));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

Hi! The following works for me:

main.qml

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Rectangle {
        objectName: "MainRect"
        anchors.fill: parent
        color: "plum"
    }
}

CcbFrame.qml

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

Text {
    anchors.centerIn: parent
    objectName: "MyObject"
    text: "Default text"
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QDebug>
#include <QQmlProperty>
#include <QQmlComponent>
#include <QQuickItem>
#include <QQmlContext>

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    QQmlComponent ccbcomponent(&engine, QUrl(QLatin1String("qrc:/CcbFrame.qml")));
    QObject *object = ccbcomponent.create();
    QQuickItem *mainRect = engine.rootObjects().first()->findChild<QQuickItem*>("MainRect");
    static_cast<QQuickItem*>(object)->setParentItem( mainRect );
    object->setProperty("text", "New text");

    return app.exec();
}

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Updating QML GUI from C++