Updating qml child label element text after completing a function in C++

[qtdevana]

I have an application where when after finish executing a function in C++, i need to update the a label in QML with C++ function status.

Below is the C++ code where it calls qml function to let know about it's status.

@ void UiHandler::updateStatus(QVariant msg)
{
QQmlEngine engine;
QQmlComponent component(&engine, "qml/Window.qml");
QObject *object = component.create();

    QMetaObject::invokeMethod(object, "**StatusUpdate**",
            Q_ARG(QVariant, msg));
    delete object;
}@

And my QML looks like below

@ import QtQuick 2.2
import QtQuick.Controls 1.1
import QtQuick.Layouts 1.1

Rectangle {
    id: container
    property alias StatusLabel: ui_label_status

    Component.onCompleted: {
        console.debug("Window>> onCompleted"); 
    }

    onVisibleChanged: {
        console.debug("Window>> onVisibleChanged");
    }
    function **StatusUpdate**(msg) {
        **ui_label_status**.text = "Click the button to start"
        ui_label_status.color = "blue"
        console.log("Got message:", msg)
    }

    color: "lightgreen"

    ColumnLayout {
        id: settings_layout
        anchors.fill: parent

        GroupBox {
            id: group
            anchors {
                top: parent.top
                topMargin: 25
                left: parent.left
                leftMargin: 10
            }
            height: children.height
            width: children.width
            title: "Test"

            GridLayout {
                id: layout
                anchors.fill: parent

                Label {
                    id: ui_label_status
                }
            }
        }
    }
}@

Now, when i tried to change the text of my label (ui_label_status) inside the function StatusUpdate it has no effect. Could someone point me how I can update the label text for my scenario above ? And also i am on QT 5.2.

[melghawi]

I have to ask...

Have you actually tried running this code?

A number of issues here.

Firstly you are creating an instance of "qml/Window.qml" and then deleting it at the end of your method call so any changes you make will obviously be lost.

Secondly you create a QQmlEngine on stack which would be invalid to use to create new components.

Thirdly your Qml does not compile.

[melghawi]

The following works for me:

main.qml

@import QtQuick 2.2
import QtQuick.Controls 1.1
import QtQuick.Layouts 1.1

Rectangle {
id: container
property alias statusLabel: ui_label_status

Component.onCompleted: {
console.debug("Window>> onCompleted");
}

onVisibleChanged: {
console.debug("Window>> onVisibleChanged");
}

function statusUpdate(msg) {
ui_label_status.text = "Click the button to start"
ui_label_status.color = "blue"
console.log("Got message:", msg)
}

color: "lightgreen"

ColumnLayout {
id: settings_layout
anchors.fill: parent

  GroupBox {
      id: group
      anchors {
          top: parent.top
          topMargin: 25
          left: parent.left
          leftMargin: 10
      }
      height: children.height
      width: children.width
      title: "Test"

      GridLayout {
          id: layout
          anchors.fill: parent

          Label {
              id: ui_label_status
          }
      }
  }

}
}@

main.cpp

@#include <QtGui/QGuiApplication>
#include "qtquick2applicationviewer.h"

int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);

QtQuick2ApplicationViewer viewer;
viewer.setMainQmlFile(QStringLiteral("qml/Testing/main.qml"));
viewer.showExpanded();

QMetaObject::invokeMethod((QObject*)viewer.rootObject(), "statusUpdate", Q_ARG(QVariant, "TESTING"));

return app.exec();

}@

[jeremy_k]

Another approach is to declare a property in C++ object, register the class or an instance with the QML engine, and emit the property's update signal when the value should be re-read from QML.