Question 185673 in Json Question posted in
Our archive of expertly curated questions and answers provides insights and solutions to common problems related to this popular data interchange format. From parsing and manipulating JSON data to integrating it with various programming languages and web services, our archive has got you covered. Start exploring today and take your JSON skills to the next level

Json – Issues with C++ and Qt/QML data exchange

Why do I get this error in the QML code: ReferenceError: m_jsonDocStr is not defined,
even if m_jsonDocStr is specified here in the C++ code:

Q_PROPERTY(QString m_jsonDocStr READ jsonDocToStr WRITE setJsonDocStr NOTIFY jsonDocStrChanged)

And why the QmlLink.open() C++ function is not opened from QML? (neither as a normal member function nor as a slot)

TypeError: Property 'open' of object [object Object] is not a function

Next, with the "<–" comment I point to the critical lines of the code, with the error messages I get.

This is the QML file:

/** Main.qml */

import QtCore
import QtQuick
import QtQuick.Window
import QtQuick.Controls
import QtQuick.Dialogs
import QtQuick.Layouts
import main

ApplicationWindow {
    id: window
    width: 640
    height: 480
    visible: true
    title: qsTr("Data input widget")
    
    property string dataFilename: ""
    property string dataFile: ""
    
    menuBar: MenuBar {
        Menu {
            title: qsTr("&File")
            Action {
                text: qsTr("&Open...")
                onTriggered: fileDialog.open()
            }
        }
    }

    ScrollView {
        id: fileContents
        anchors.fill: parent
        property string dataFile: m_jsonDocStr  // <-- ReferenceError: m_jsonDocStr is not defined
        TextArea {
            text: "File: " + dataFilename + "n" + dataFile + "nLength: " + dataFile.length
        }
    }

    FileDialog {
        id: fileDialog
        title: "Open data file"
        nameFilters: ["JSON files (*.json)"]
        onAccepted: {
            dataFilename = selectedFile
            console.log("Open file: " + dataFilename)
            QmlLink.open(dataFilename)      // <-- TypeError: Property 'open' of object [object Object] is not a function
            console.log("File contents: " + m_jsonDocStr)
        }
        onRejected: {
            console.log("No file selected.")
        }
    }
}

Here is the header file:

/** QmlLink.hpp */

#ifndef QMLLINK_HPP
#define QMLLINK_HPP

#include <QObject>
#include <QJsonDocument>
#include <QtQml/qqmlregistration.h>

class QmlLink : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QJsonDocument m_jsonDoc READ jsonDoc WRITE setJsonDoc NOTIFY jsonDocChanged)
    Q_PROPERTY(QString m_jsonDocStr READ jsonDocToStr WRITE setJsonDocStr NOTIFY jsonDocStrChanged)
    QML_ELEMENT

public:
    explicit QmlLink(QObject *parent = nullptr);

public slots:
    QJsonDocument jsonDoc() const { return m_jsonDoc; }
    void          setJsonDoc(const QJsonDocument &jsonDoc) { m_jsonDoc = jsonDoc; }
    QString       jsonDocToStr() const { return m_jsonDoc.toJson(); }
    void          setJsonDocStr(const QString &jsonDoc) { m_jsonDoc = QJsonDocument::fromJson(jsonDoc.toUtf8()); }

signals:
    void jsonDocChanged();
    void jsonDocStrChanged();

public slots:
    void menuOpen();
    int  open(QString filePath);

private:
    QJsonDocument m_jsonDoc;
    QString m_jsonDocStr;

    QString m_jsonFileName = "default.json";
};

#endif // QMLLINK_HPP

This is the implementation file:

/** QmlLink.cpp */

#include "QmlLink.hpp"
#include <iostream> 
#include <QDebug>
#include <QFile>
#include <QQmlContext>
#include <QQmlEngine>
#include <QQuickView>

QmlLink::QmlLink(QObject *parent) : QObject{parent} {}

void QmlLink::menuOpen() {
    open(m_jsonFileName);
}

int QmlLink::open(QString filePath)
{
    m_jsonFileName = filePath;

    QUrl url(filePath);
    QFile jsonFile;
    jsonFile.setFileName(url.toLocalFile());

    if(jsonFile.open(QIODevice::ReadOnly) == false) {
        return -1;
    }

    const QByteArray data = jsonFile.readAll();
    m_jsonDoc = QJsonDocument::fromJson(data);

    const QString strDoc(m_jsonDoc.toJson(QJsonDocument::Compact));

    std::cout << "Data file contents: " << strDoc.toStdString() << std::endl;

    return 0;
}

This is a classic main() function:

/** main.cpp */

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "QmlLink.hpp"

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

    QmlLink qmlLink;

    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/main/Main.qml"_qs);
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
        &app, []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);
    engine.load(url);

    QQmlContext *rootContext = engine.rootContext();
    rootContext->setContextProperty("QmlLink", &qmlLink);

    return app.exec();
}

This is the trivial JSON file:

{
    "Array": [
        true,
        999,
        "string"
    ],
    "Key": "Value",
    "null": null
}

Tools versions:

  • Qt 6.5
  • OS: Linux
  • gcc 13.1
  • clang 15
Tags:

2

Answers


  1. 0

    You should set the context property before loading the url :

    QGuiApplication app(argc, argv);

QmlLink qmlLink;

QQmlApplicationEngine engine;
// set the context property here
QQmlContext *rootContext = engine.rootContext();
rootContext->setContextProperty("QmlLink", &qmlLink)

const QUrl url(u"qrc:/main/Main.qml"_qs);
QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
    &app, []() { QCoreApplication::exit(-1); },
    Qt::QueuedConnection);
engine.load(url);

    and fix the Main.qml :

     // in ScrollView
 property string dataFile: QmlLink.m_jsonDocStr
 // ...
 // in FileDialog
 console.log("File contents: " + QmlLink.m_jsonDocStr)
    Login or Signup to reply.
  2. 0

    I think you can’t pass QJsonDocument from C++ to QML.
    As you mentioned you will get an "Unable to assign [undefined] to QString" error.
    But My Idea is to pass QByteArray.

    Here is my example:

    In main.cpp:

    #include <QApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include <QFileDialog>
#include <QJsonDocument>
#include <QJsonObject>
#include <QDebug>

class FileLoader : public QObject
{
    Q_OBJECT

public:
    explicit FileLoader(QObject *parent = nullptr) : QObject(parent) {}

public slots:
    void openFile()
    {
        QString filePath = QFileDialog::getOpenFileName(nullptr, "Open JSON File", "", "JSON Files (*.json)");

        if (!filePath.isEmpty()) {
            QFile file(filePath);
            if (file.open(QIODevice::ReadOnly | QIODevice::Text)) {
                QByteArray data = file.readAll();
                file.close();

                QJsonDocument jsonDoc = QJsonDocument::fromJson(data);
                qDebug() << "JSON File Loaded:" << filePath;
                emit fileLoaded(jsonDoc.toJson(QJsonDocument::Indented));
            }
        }
    }

signals:
    void fileLoaded(const QByteArray &jsonData);
};

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

    QQmlApplicationEngine engine;

    // Create an instance of the FileLoader
    FileLoader fileLoader;

    // Register the FileLoader as a context property
    engine.rootContext()->setContextProperty("fileLoader", &fileLoader);

    // Load the QML file
    const QUrl url(QStringLiteral("qrc:/QmlJson/Main.qml"));
    engine.load(url);

    return app.exec();
}

#include "main.moc"

    in Main.qml

    import QtQuick 2.15
import QtQuick.Controls 2.15

ApplicationWindow {
    visible: true
    width: 400
    height: 300
    title: "JSON Viewer"

    Text {
        id: jsonDataText
        anchors.centerIn: parent
    }

    Button {
        text: "Open JSON"
        onClicked: fileLoader.openFile()
    }

    Connections {
        target: fileLoader
        function onFileLoaded(jsonData) {
            jsonDataText.text = jsonData
        }
    }
}

    This is my result:

    enter image description here

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search