Qt C++ application connecting signals to JavaScript?

[raven-worx]

@SPlatten said in Qt C++ application connecting signals to JavaScript?:

What I would like to do is be able to connect the javascript function with the signal in C++

Something like this?

QJSEngine* engine = ...;
connect( button, &QPushButton::clicked, this, [engine] () {
     QString script = ...
     QJSValue returnValue = engine->evaluate(script);
}

See the docs for how the script should look like

[SPlatten]

Thank you, I'll take a look.

[SPlatten]

What sort of thing can QJSEngine do and not do?

In my simple test code I tried:

    QObject::connect(pobjPushButton
                    ,&QPushButton::clicked
                    ,[pobjScriptEng]() {
                         pobjScriptEng->evaluate("alert('hello');");
                                       });

I put a break point on the line inside the callback and when I clicked on the pushbutton I could see the debugger stop on the evaluate line, but having executed the line, I didn't the alert displayed and no error was thrown.

Can anyone help?

I've got a little bit further by including a call:

    mspobjJSeng->installExtensions(QJSEngine::AllExtensions);

I don't think 'alert()' is supported.

[mrjj]

Hi
http://doc.qt.io/qt-5/qjsengine.html#details
Try
console.info()
instead.

[SPlatten]

Thank you, info does work and has the advantage of adding a new line for each entry...

I've just put in:

    console.info("QJSEngine");
    console.info(this);

Is there anyway to debug the script or to see what members are visible in 'this' ?

[mrjj]

Hi
As far as i know, you cant.
However, if you see in
https://doc.qt.io/qt-5.11/whatsnew57.html
section Qt QML Module
says
"Enabled all debug services to work with QJSEngine (instead of QQmlEngine), which allows non-QML JavaScript debugging or profiling."

so i assume if you have a js file, you can actualy debug this and see members.
However, Not inside a mere string in a c++ file.

[SPlatten]

I've hit another problem now, I'm trying to pass in a QString containing a script that was read from file, however when I try to assign this:

    QObject::connect(pobjPushButton
                    ,&QPushButton::clicked
                    ,[pobjScriptEng]() {
                pobjScriptEng->evaluate(static_cast<const QString>(strScript));
                            });

I get:

   variable 'strScript' cannot be implicitly captured in a lambda with no capture-default specified

What does this mean?

[Gojir4]

Hi @SPlatten,

This means you need to "capture" the variable you want to use in the lambda. This tells to the compiler that have to keep the variable, usually a local one, in the memory to be accessible when the lambda is called.

Capturing the variable is done by adding its name inside the brackets []. You can also add & suffix for passing variable by reference, in.

    QObject::connect(pobjPushButton
                    ,&QPushButton::clicked
                    ,[pobjScriptEng, &strScript]() { //or [&] to use the "by-reference capture default" mode
                pobjScriptEng->evaluate(static_cast<const QString>(strScript));
                            });

More informations here: https://en.cppreference.com/w/cpp/language/lambda#Lambda_capture

[SPlatten]

Thank you, I will take a look now.

[SPlatten]

Is there any way to link the signal with a specific function in the script?

[VRonin]

It's all in the docs http://doc.qt.io/qt-5/qjsengine.html

QJSvalue module = myEngine.importModule("./math.mjs");
QJSValue sumFunction = module.property("sum");
QJSValue result = sumFunction.call(args);

This will call the function sum from the module math.mjs passing args as arguments

[SPlatten]

Thank you, I will have a good read of the documentation....