exposing C++ attributes (missing a step)

[mzimmers]

Hi all -

After an 18-month hiatus, I'm returning to QML-land...currently trying scrape the rust off my memory.

I'm trying to connect a C++ object to a QML display. I'm roughly following the instructions here with minor modifications (I have a QQmlApplicationEngine instead of a QQuickView), but I must be missing a step, because I'm getting a runtime error.

Here's a snippet from my main.cpp code:

    QQmlApplicationEngine engine;
    Clock clock;
    const QUrl url(u"qrc:/nga/main.qml"_qs);

    engine.rootContext()->setContextProperty("clock", &clock);
    engine.load(url);

Clock.h:

class Clock : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString m_timeStr READ time WRITE setTime NOTIFY timeChanged)
public:
    void setTime(const QString &time);
    QString time() {
        return m_timeStr;
    }
private:
    QString m_timeStr;
...

and my main.qml:

 import (a bunch of stuff)
 ApplicationWindow {
    Text {
        text: clock.time
    }
...

I'm getting a runtime error "TypeError: Cannot read property 'time' of null." Could someone please tell me what I left out?

Thanks...

[fcarney]

@mzimmers said in exposing C++ attributes (missing a step):

Q_PROPERTY(QString m_timeStr READ time WRITE setTime NOTIFY timeChanged)

I think this should be:
Q_PROPERTY(QString time READ time WRITE setTime NOTIFY timeChanged)

But it does not explain why "clock" is null.

[mzimmers]

@fcarney I just changed that...the only effect seems to be when the error messages are displayed...heh.

[Creaperdown]

Hey, I tried your setup in a sample project (Qt 5.15) and it works just fine:

// Clock.hpp
#pragma once
#include <QObject>

class Clock : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString m_timeStr READ time WRITE setTime CONSTANT)
    
public:
    void setTime(const QString &time)
    {
        m_timeStr = time;
    }
    QString time() {
        return m_timeStr;
    }
    
private:
    QString m_timeStr = "test";
};

// main.cpp
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    
    Clock clock;
    
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    
    engine.rootContext()->setContextProperty("clock", &clock);
    
    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
ApplicationWindow
{
    id: root
    visible: true
    width: 1200
    height: 800
    
    Component.onCompleted: console.log(clock.m_timeStr)
}

(This prints "test")

I am sure we could help you better, if you'd provide a reproducible example

[mzimmers]

@Creaperdown normally my questions are easy enough for this audience that snippets reveal my error, but evidently not so in this case. So, here's the whole thing:

main.cpp

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

#include "clock.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/nga/main.qml"_qs);
    Clock clock;

    engine.rootContext()->setContextProperty("clock", &clock);
    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();
}

clock.h

#ifndef CLOCK_H
#define CLOCK_H

#include <QObject>
#include <QTime>
#include <QTimer>

class Clock : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString time MEMBER m_timeStr READ time WRITE setTime NOTIFY timeChanged)
public:
    explicit Clock(QObject *parent = nullptr);
    void setTime(const QString &time);
    QString time() {
        return m_timeStr;
    }

signals:
    void timeChanged(QString time);
public slots:
    void update();
private:
    QString m_timeStr;
    QTime m_time;
    QTimer m_timer;
};

#endif // CLOCK_H

clock.cpp

#include <QDebug>

#include "clock.h"

Clock::Clock(QObject *parent)
    : QObject{parent}
{
    connect(&m_timer, &QTimer::timeout, this, &Clock::update);
    m_timer.start(1000);
    m_timeStr = "uninitialized";
}
void Clock::setTime(const QString &time) {
    if (time != m_timeStr) {
        m_timeStr = time;
        emit timeChanged(m_timeStr);
    }
}
void Clock::update() {
    QString l_timeStr;

    m_time = QTime::currentTime();
    l_timeStr = m_time.toString("hh:mm:ss");
    if (l_timeStr != m_timeStr) {
        m_timeStr = l_timeStr;
        qDebug() << "m_timeStr updated to" << m_timeStr;
    }
}

and main.qml:

import QtQml 2.2
import QtQuick 2.15
import QtQuick.Controls 2.15
import QtQuick.Controls.Material 2.15
import QtQuick.Layouts 1.15

 ApplicationWindow {

    visible: true
    width: 800
    height: 480

   Mytoolbar { // ignore this
        id: toolBar
    }
    Text {
        anchors {
            top: toolBar.bottom
        }
        text: clock.time
    }
}

Thanks...

[J.Hilk]

@mzimmers said in exposing C++ attributes (missing a step):

timeChanged

you never call setTime, so the changed signal is never emitted -> ui never updates.

the Cannot read property 'time' of null is because of the stack FIFO, create the Clock instance before the GuiApplication instance and that too will go away :D

[Creaperdown]

This post is deleted!

[mzimmers]

@J-Hilk that was it (sometimes the most obvious omissions are the hardest to find).

clock.cpp:

Clock::Clock(QObject *parent)
    : QObject{parent}
{
    connect(&m_timer, &QTimer::timeout, this, &Clock::update);
    m_timeStr = "00:00 AM";
}
void Clock::setTime(const QString &time) {
    if (time != m_timeStr) {
        m_timeStr = time;
        emit timeChanged(m_timeStr);
    }
}
void Clock::update() {
    QString l_timeStr;

    m_time = QTime::currentTime();
    l_timeStr = m_time.toString("hh:mm AP");
    setTime(l_timeStr);
}

I had to move the m_timer.start() out of the c'tor, because I got this runtime error:

QObject::startTimer: Timers can only be used with threads started with QThread

So I created a start() function and I call it from main.cpp. Works like a charm now. Thanks!