engine.rootContext()->setContextProperty is failing to add property

jcdelve

Hello, I'm trying to expose some C++ objects to my QML code, and it won't work. The object appears to be unrecognized in qml (it doesn't highlight a different color), and when I try and return engine.rootObjects(), the list is empty. If this is not how it should be working, then please let me know.

Here is my main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "updatescreen.h"
int main(int argc, char *argv[])
{
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;

UpdateScreen* screen = new UpdateScreen();

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

screen->init();

engine.rootContext()->setContextProperty("UpdateScreen", screen);

if (engine.rootObjects().isEmpty()) {
    return -1;
}
engine.load(url);
return app.exec();
}

My declaration of UpdateScreen.h as a Q_Object:

class UpdateScreen : public QObject {
Q_OBJECT

Q_PROPERTY(int foo READ getFoo WRITE setFoo NOTIFY fooChanged)
public:
explicit UpdateScreen(QObject *parent = nullptr);
void init();

And my qml file trying to access the property:

Connections {
    target: UpdateScreen
    onFooChanged: {
        a.text = UpdateScreen.getFoo();
    }
}

Please let me know how I can fix this and get it working! Thank you!!

DBoosalis

Where are you emitting your signal fooChanged() . Also if using Version 6 of Qt you need to change signature for connections to :

Connections {
target: UpdateScreen
function onFooChanged() {
a.text = UpdateScreen.getFoo();
}
}

KroMignon

@jcdelve to complete @DBoosalis answer, you should also take care about QML naming convention.
I don't remember where I found this in Qt documentation, or if it is a JavaScript convention, but all type should (must?!?) be UpperCamelCase and all instances and methodes in lowerCamelCase.

I would suggest you to change your code to:

engine.rootContext()->setContextProperty("updateScreen", screen);

And

Connections {
    target: updateScreen
    function onFooChanged() {
        a.text = updateScreen.getFoo() + "";  // to converter int into string
    }
}

jcdelve

@DBoosalis
I update it in UpdateScreen.cpp:

void UpdateScreen::setFoo(int rate)
{
    foo = rate;
    emit fooChanged();
}

I made both your changes, but it still doesn't seem to recognize the QT Object and the application returns -1. Since I return -1, it would seem to imply that it's not adding the object correctly in the rootContext? But I'm not sure why...

dheerendra

Since it is not giving any object undefined error, it means that object is recognized. I don't see any issue with the object. I suspect that setting value is not right sequence. Where are you setting the value of foo variable ? When are you setting it ? May complete example will help here.

sierdzio

@jcdelve said in engine.rootContext()->setContextProperty is failing to add property:

if (engine.rootObjects().isEmpty()) {
return -1;
}

This will not work. This method returns objects instantiated by the engine, using load() method, which you call later. It does not return context properties.

engine.rootContext()->setContextProperty("UpdateScreen", screen);

Only QML components should start with upper case. Try with "updateScreen".

jcdelve

@dheerendra Right now, I'm just doing it in the init() method that I call in main.cpp. I'm just trying to establish this functionality before actually implementing it how I need it.

void UpdateScreen::init()
{
    setFoo(20);
}

main.cpp

screen->init();

engine.rootContext()->setContextProperty("updateScreen", screen);
if(engine.rootObjects().isEmpty()) {
        return -1;
    }
    engine.load(url);
    return app.exec();

@sierdzio I made that change following @KroMignon's post but it did not change anything

GrecKo

@jcdelve said in engine.rootContext()->setContextProperty is failing to add property:

if(engine.rootObjects().isEmpty()) {
return -1;
}

remove that.

jcdelve

Yes, I removed it, and the program runs, but the number I'm looking for doesn't get set to 20 like it should

sierdzio

You call init() before your QML is loaded. The fooChanged() signal is fired long before your QML slot (Connections) is ready to receive it.

Invoke init() after some delay and it will work.

jcdelve

@sierdzio
Good catch, that would definitely be part of the problem. However, the object is still not recognized in the qml file. Using QT Creator, the object I know should appear highlighted and italicized, but it's not. And when I try and call the .getFoo() function, it says it's not a function

JoeCFD

@jcdelve said in engine.rootContext()->setContextProperty is failing to add property:

Q_PROPERTY

public:
    Q_INVOKABLE int getFoo(); // This will work.

jcdelve

@JoeCFD That worked! I didn't realize I needed that for every single property.
I will say it still looks weird, because the object still looks like normal text instead of highlighted or whatever, but it works so I'm not complaining. Thank you!

JoeCFD

@jcdelve Q_INVOKABLE means the func is visible in qml. I am learning as well.

sierdzio

@KroMignon said in engine.rootContext()->setContextProperty is failing to add property:

a.text = updateScreen.getFoo() + ""; // to converter int into string

You don't need to make getFoo() invokable! Just call the property instead of a function:

a.text = updateScreen.foo

GrecKo

Do it declaratively like so:

// ...
Label {
    text: UpdateScreen.foo
}

it will automatically update when the fooChanged signal is emitted.
Doing Q_INVOKABLE int getFoo(); is ugly.

jcdelve

Interesting. By all other conventions of coding standards, it's bad practice to have a variable directly accessible like that... Is there a reason QT/QML prefers that over doing an invokable getter?

sierdzio

@jcdelve said in engine.rootContext()->setContextProperty is failing to add property:

Interesting. By all other conventions of coding standards, it's bad practice to have a variable directly accessible like that... Is there a reason QT/QML prefers that over doing an invokable getter?

Please read up on how Q_PROPERTY system works. You are not accessing a variable, you are accessing a property. QML (via Meta Object System) will use the getter / setter you provide in Q_PROPERTY declaration. It will not access the variable directly.