QFileSystemWatcher fileChanged() signal to QML slot

texasRanger

Hello,

I have a GUI display that shows data values from an external file. I want to be able to update the display whenever the values in the file change. I'm trying to use QFileSystemWatcher fileChanged() to emit a signal to the SLOT in qml that will update the display. I can't seem to figure out the sending of the signal when the file is read.

//readJSON.h

class readJSON :public QObject
{
    Q_OBJECT

public:
    readJSON();

public slots:
    QString readingJson(const QString &name);

};


//main.cpp

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


    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    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);



    QFileSystemWatcher watcher;
    watcher.addPath("data.json");

    readJSON* myGlobal = new readJSON();

    engine.rootContext()->setContextProperty("myGlobalObject", myGlobal);


    QObject::connect(&watcher, SIGNAL(fileChanged()),
                     myGlobal, SLOT(updateDisplay()));
    
    engine.load(url);

    return app.exec();
}

//main.qml

Window {
    visible: true
    width: 800
    height: 800
    title: qsTr("Screen Display")


    DisplaysForm {

        function callCPP(text) {
            var dataOne = myGlobalObject.readingJson(text)
            dataOne = parseFloat(dataOne)

            return dataOne
        }

       function updateDisplay() {

            var value = callCPP("Engine_Spd")
            engSpd_txt.text = value

            var value1 = callCPP("Engine_Power")
            engPwr_txt.text = value1
       }

ODБOï

hi

@texasRanger said in QFileSystemWatcher fileChanged() signal to QML slot:

QObject::connect(&watcher, SIGNAL(fileChanged()),
myGlobal, SLOT(updateDisplay()));

you are trying to connect the signal fileChanged of watcher to the slot updateDisplay of myGlobal, but myGlobal has no slot
updateDisplay, that function (slot) is defined in QML

here is an example how to connect c++ signal to qml function
https://forum.qt.io/topic/116084/c-signal-to-a-qml-file/4

also you sould use "new" signal/slot syntax
https://wiki.qt.io/New_Signal_Slot_Syntax

eyllanesc

Export watcher:

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


    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    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);



    QFileSystemWatcher watcher;
    watcher.addPath("data.json");

    readJSON myGlobal;

    engine.rootContext()->setContextProperty("myGlobalObject", &myGlobal);
    engine.rootContext()->setContextProperty("watcher", &watcher);
    
    engine.load(url);

    return app.exec();
}

and use Connections:

Window {
    visible: true
    width: 800
    height: 800
    title: qsTr("Screen Display")


    DisplaysForm {

        function callCPP(text) {
            var dataOne = myGlobalObject.readingJson(text)
            dataOne = parseFloat(dataOne)

            return dataOne
        }

       function updateDisplay() {

            var value = callCPP("Engine_Spd")
            engSpd_txt.text = value

            var value1 = callCPP("Engine_Power")
            engPwr_txt.text = value1
       }
    Connections{
        target: watcher
        function onFileChanged(path){
                console.log(path)
                updateDisplay()
        }
    }
}