Passing a Javascript callback to a C++ Invoked method in Qml

Question

In C++ I have a class with an invokable function, what I would like to do is call that method from QML/Javascript (this I've gotten to work) and pass it a Javascript callback.

In code, I define my class like:

class MyObject: public QObject
{

Q_OBJECT

public:
    Q_INVOKABLE void doSomething(quint64 x, /* what goes here? */ jsCallback)
    {
        x += 1;

        // I suspect this will require a invocation mechanism but 
        // this shows what I'd like to do
        jsCallback(x);
    }
};

And in my QML, I would like to do something like:

Rectangle {

    function myCallback(x){
        console.log("x=" + x);
    }

    MouseArea{
        anchors.fill: parent
        onClicked:{
            myObject.doSomething(2, myCallback);
        }
    }
}

So that when I click on the Rectangle, I would see x=3 in the console. How would I define the parameter in C++ and invoke the callback in order to accomplish this?

Thank you!

BaCaRoZzo · Accepted Answer · 2015-07-30 20:25:58Z

18

I think I've figured this out. What I ended up doing was implementing this in my C++ class like such:

class MyObject: public QObject
{

Q_OBJECT

public:
    Q_INVOKABLE void doSomething(quint64 x, QJSValue jsCallback)
    {
        x += 1;

        QJSValue val = jsCallback.engine()->newObject();
        val.setProperty("x", x);

        jsCallback.call(QJSValueList { val });
    }
};

And then I can access the value in my callback like:

function myCallback(x){
    console.log("x=" + x.x);
}

edited Jul 30, 2015 at 20:25

BaCaRoZzo

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answered Jul 24, 2015 at 20:47

Addy

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4 Comments

Alex Over a year ago

Also if you use in lambda function inside function (for example, after network reply), pass QJSValue as mutable \ by value.

tomalbrc Over a year ago

Can anyone clarify if this is considered good practice? I'm facing a similar problem where I want to push an Item onto a StackView but only after an asynchronous operation (called from QML, implemented in C++) is finished

user2580621 Over a year ago

As of now QJSValue::engine() is deprecated but there's no suggested replacement in the Qt documentation. I'd be great to have an updated answer accounting for this.

Maxim Paperno Over a year ago

Since Qt5.5, assuming MyObject is being invoked from a script running in a QJSEngine (or QQmlEngine), the current engine instance can be obtained with qjsEngine(this) (or qmlEngine(this)). doc.qt.io/qt-6/qjsengine.html#qjsEnginex

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