QQmlEngine and QJSEngine evaluate differently

phlipper

Hello folks,

I'm trying to evaluate some script code in a Qt GUI, where I can enter some code in a QTextEdit and then let it be evaluated by one of the plenty engines available in Qt.

First I started with QScriptEngine, which worked fine, but I moved over to QJSEngine because the former is already deprecated. QJSEngine does evaluate scripts just as well.
But it does lack many functionality, like loading plugins, etc.
So I thought maybe just use QQmlEngine, it inherits from QJSEngine, and provides much more functionality, i.e. settings include paths, importing plugins, etc.
So basically this is exactly what I need, BUT it cannot evaluate complex scripts.

A simple example:

oEngine->evaluate("var i = 0; i + 2;", "name");

If oEngine is a QJSEngine the script will be evaluated, but a QQmlEngine throws following exception:

Error in line 1: ReferenceError: i is not defined

So my question is, is it possible to evaluate code with QQmlEngine just like with QJSEngine/QScriptEngine, because I would really like to add some plugins, or do I have to stick with QScriptEngine for now?

Thanks

p3c0

@phlipper I'm not sure how you are using it but following works for me:

QQmlEngine engine;
QJSValue result =  engine.evaluate("var i = 0; i + 2;", "name");
qDebug() << result.toString(); //prints 2

phlipper

First of all I am using Ubuntu 16.04 with Qt 5.5.1 and QtCreator 4.1.

I isolated the code to a very simple example:

#include <QCoreApplication>
#include <QDebug>
#include <QJSEngine>
#include <QQmlEngine>

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

  // switch between engines with comment
  QQmlEngine engine;
  //QJSEngine engine;
  QJSValue val = engine.evaluate("var i = 0; i + 2;", "name");

  if (val.isError())
  {
    qCritical() << "Error in line " << val.property("lineNumber").toString() << ": " << val.toString();
  }
  else
  {
    qInfo() << "Execution successful";
  }

  return 0;
}

If I use QQmlEngine the output is:

QML debugging is enabled. Only use this in a safe environment.
"QML Debugger: Invalid argument 'services:DebugMessages' detected. Ignoring the same."
"QML Debugger: Invalid argument 'QmlDebugger' detected. Ignoring the same."
"QML Debugger: Invalid argument 'V8Debugger' detected. Ignoring the same."
"QML Debugger: Invalid argument 'QmlInspector' detected. Ignoring the same."
QML Debugger: Waiting for connection on port 34039...
Error in line "1" : "ReferenceError: i is not defined"
Press <RETURN> to close this window...

If I use QJSENgine the output is:

QML debugging is enabled. Only use this in a safe environment.
Execution successful
Press <RETURN> to close this window...

Do I need to use a newer Qt version to get the correct behaviour?

Thank you.

p3c0

@phlipper Your code works perfectly for me. The control goes into the else and prints 2 as expected.
Apart from that you should use exec for the event loop to work properly.

Do I need to use a newer Qt version to get the correct behaviour?

Which version did you try ?

phlipper

I am using Ubuntu 16.04 with Qt 5.5.1.

p3c0

@phlipper Not sure if the version could be the problem but try with latest version. I tested using Qt 5.6

phlipper

Downloaded latest Qt 5.7 and it now works.
Thanks.