findChild() does not find objectName() of LineSeries from c++ side

robven

Hi,
I created this qml component:

MyChartView.qml

import QtQuick 2.12
import QtCharts 2.13


Item{
    id: root
    objectName:"root"
    width: 600
    height: 440

    ChartView{
        id:chartView
        objectName: "chartView"
        anchors.fill: parent

        ValuesAxis{
            id:xAxis
            min: 0
            max: 10
        }

        ValuesAxis{
            id:yAxis
            min: 0
            max: 10
        }

        LineSeries{
            id:lineSeries
            objectName: "lineSeries"
            name: "Linea 1"
            axisX: xAxis
            axisY: yAxis
        }
    }
}

in main.cpp I want to access lineSeries through lineSeriesQobject pointer, but it doesn't find objectName() of lineSeries. Below the code of main.cpp.

main.cpp

#include <QApplication>
#include <QObject>
#include <QQuickView>
#include <QQmlProperty>
#include <QQuickItem>

    int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QApplication app(argc, argv);

    QQuickView view;
    view.setSource(QUrl::fromLocalFile(":/MyChartView.qml"));

    QQuickItem *rootObject=view.rootObject();
    if(rootObject){
        qDebug()<<rootObject->objectName();
        QObject *chartViewObject=rootObject->findChild<QObject*>("chartView");
        if(chartViewObject){
            qDebug()<<chartViewObject->objectName();
            QObject* lineSeriesObject=chartViewObject->findChild<QObject*>("lineSeries");
            if(lineSeriesObject){
                qDebug()<<lineSeriesObject->objectName();
            }
        }
    }
    view.show();

    return app.exec();
}

lineSeriesObject pointer return nullptr, what am I doing wrong?

sierdzio

@robven findChild() searched recursively so you can just search for lineSeries from rootObject, no need for nested if statements.

robven

@sierdzio Thanks for the quick reply.
Below I update the code as you suggested, but the problem remained the same findChild() still can't find objectName() of lineSeries.

main.cpp

#include <QApplication>
#include <QObject>
#include <QQuickView>
#include <QQmlProperty>
#include <QQuickItem>

    int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QApplication app(argc, argv);

    QQuickView view;
    view.setSource(QUrl::fromLocalFile(":/MyChartView.qml"));

    QQuickItem *rootObject=view.rootObject();
    if(rootObject){
        qDebug()<<rootObject->objectName();
        QObject *lineSeriesObject=rootObject->findChild<QObject*>("lineSeries",Qt::FindChildOption::FindChildrenRecursively);
        if(lineSeriesObject){
            qDebug()<<lineSeriesObject->objectName();
        }
    }
    view.show();

    return app.exec();
}

robven

Hello, I found the solution to be able to manipulate the LineSeries from the C++ side in QML. I needed this approach because I want to populate the LineSeries from C++ rather than from QML. I have to work with a large number of points, and populating the LineSeries from QML was proving to be too slow. The chart will display 100 randomly generated points as well. Below you will find the main.cpp, main.qml, and MyChartView.qml files.

main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlComponent>
#include<QQmlEngine>
#include <QAbstractSeries>
#include <QLineSeries>
#include <QRandomGenerator>


int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QApplication app(argc, argv);

    QQmlApplicationEngine engine;

    const QUrl url(QStringLiteral("qrc:/main.qml"));

    QQmlComponent component(&engine, url);
    QObject *rootObject = component.create();
    if (rootObject) {
        QObject *chartViewObject = rootObject->findChild<QObject *>("chartView");
        if (chartViewObject) {
            QAbstractSeries *series = nullptr;
            QMetaObject::invokeMethod(chartViewObject,
                                      "series",
                                      Q_RETURN_ARG(QAbstractSeries *, series),
                                      Q_ARG(int, 0));
            if (series) {
                QLineSeries *qSeries = qobject_cast<QLineSeries *>(series);
                QList<QPointF> pointFList;
                QPointF pointF;
                series->setProperty("name", "Line 1");
                series->setProperty("color", "#FF8000");
                for (int i = 0; i < 100; i++) {
                    pointF.setX(i);
                    qint32 randomNumber = QRandomGenerator::global()->bounded(101);
                    pointF.setY(randomNumber);
                    pointFList.push_back(pointF);
                }
                qSeries->replace(pointFList);
            }
        }
    }


    return app.exec();
}

main.qml

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    id: window
    width: 640
    height: 480
    visible: true
    title: qsTr("Chart")


    MyChartView{
        id:myChartView
        anchors.centerIn: parent
    }
}

MyChartView.qml

import QtQuick 2.12
import QtCharts 2.13

Item {
    id:root
    objectName: "root"
    width: 600
    height: 440

    ChartView{
        id:chartView
        objectName: "chartView"
        anchors.fill: parent

        ValuesAxis{
            id:xAxis
            min: 0
            max: 100
        }

        ValuesAxis{
            id:yAxis
            min: 0
            max: 100
        }

        LineSeries{
            id:lineSeries
            objectName: "lineSeries"
            axisX: xAxis
            axisY:yAxis
        }
    }
}

GrecKo

@robven A better solution would be to use VXYModelMapper so the C++ code can stay unaware of the QML code.

robven

@GrecKo Thanks, I'll try to look into your advice.

robven

I rewrote the code using VXYModelMapper. It actually results in a more elegant solution and as the Qt documentation points out we keep the C++ side separate from the QML side.

main.cpp

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QStandardItemModel>
#include <QStandardItem>
#include <QRandomGenerator>
#include <QQmlContext>


int main(int argc, char *argv[])
{
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
    QApplication app(argc, argv);

    //we create a QStandardItemModel object composed of 100 rows and 3 columns
    QStandardItemModel lineModel(100, 3);

    for (int row = 0; row < lineModel.rowCount(); row++) {
        QStandardItem *item1 = new QStandardItem(QString::number(row));
        QStandardItem *item2 = new QStandardItem(QString::number(row));
        qint32 randomNumber = QRandomGenerator::global()->bounded(101);
        QStandardItem *item3 = new QStandardItem(QString::number(randomNumber));

        //now I assign the items to lineModel
        lineModel.setItem(row,0,item1);
        lineModel.setItem(row,1,item2);
        lineModel.setItem(row,2,item3);
    }

    QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("lineModel",&lineModel);
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
        &app, [url](QObject *obj, const QUrl &objUrl) {
            if (!obj && url == objUrl)
                QCoreApplication::exit(-1);
        }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

main.qml

import QtQuick 2.12
import QtQuick.Window 2.12
import QtCharts 2.3

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

    ChartView {
        anchors.fill: parent
        id: chart
        antialiasing: true

        ValuesAxis{
            id: axisX
            min: 0
            max: 100
        }

        ValuesAxis{
            id: axisY
            min: 0
            max: 100
        }

        LineSeries{
            id: line
            name: "Line 1"
            axisX: axisX
            axisY: axisY
        }

        VXYModelMapper {
            id: modelMapper
            model: lineModel
            series: line
            firstRow: 0
            xColumn: 1
            yColumn: 2
        }
    }

}