Say hello to QtScript! 
Posted by Kent on Friday, January 05, 2007 @ 09:25

QtScript is a new module that will be introduced in the next minor release of Qt (4.3). It is a scripting environment that you embed in your C++ Qt application. While the release of Qt 4.3 is still a few months away, fortunately QtScript has just recently been integrated into the daily Qt snapshots, so you can start playing with it now. All you need to do is add

QT += script

to your .pro file, and

#include

in your source file(s), and you’re ready to rock.

QtScript conforms to the ECMA-262 ECMAScript scripting language standard, the core language that’s the basis of JavaScript and Flash ActionScript (to name two well-known languages). The key ECMAScript extension provided by QtScript is the QObject extension; QtScript leverages the introspection capabilities of the Qt Meta-Object System to provide dynamic bindings of the signals and slots and properties of Qt objects (QObject and subclasses thereof).

To give you an idea of what it looks like, here’s a small example showing how to register a widget in the scripting environment, and then manipulate it in script code:

#include
#include
#include 
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QPushButton button;

QScriptEngine engine;
QScriptValue scriptButton = engine.scriptValueFromQObject(&button);
engine.globalObject().setProperty(”button”, scriptButton);
engine.evaluate(”button.text = ‘Hello World!’”);
engine.evaluate(”button.styleSheet = ‘font-style: italic’”);
engine.evaluate(”button.show()”);

return app.exec();
}

In script code, you can connect to signals of a Qt object. The target of the connection can either be a Qt slot, or a script function. Example of the latter:

function onButtonClicked(checked) {
print ("button was checked:", checked);
print ("button text:", this.text);
}
button.clicked.connect(onButtonClicked);

As you can see, the sender() of the signal is available as the this object in the script function. You can also use array-style member access:

button['toggled(bool)'].connect(app.quit);

You can disconnect from the signal again like so:

button.clicked.disconnect(onButtonClicked);

If you’re never going to explicitly disconnect from the signal again, the target function can be anonymous:

button.clicked.connect(
function (checked) {
print ("button was checked:", checked);
print ("button text:", this.text);
}
}

Child objects are accessible as members too, so long as the child has its objectName set. A natural application of this feature is when building user interfaces at runtime: Use QUiLoader to build the form, then manipulate the form from script code:

form.buttonBox.accepted.connect(/* some function */)
form.buttonBox.rejected.connect(/* some function */)
form.userNameEdit.text = "Ned Flanders";
form.exec();

There’s more interesting stuff you can do, such as registering your own functions for value conversion (marshalling), binding C++ functions (not just slots), and making slots “script-aware” (using a class called QScriptable). See the QtScript docs in the snapshots for more information.

And finally, some quick questions and answers:

Q: What about QSA?
A: For those of you familiar with QSA (Qt Script for Applications): The new QtScript module bears no relation to QSA. The API is new and the implementation is new. QSA will reach the end of its lifetime in 2008. We’re not touting QtScript as a “QSA replacement” per se, but QtScript clearly offers a lot of the same functionality as QSA, and will naturally offer even more in the future. Just don’t expect to be able to port your existing QSA projects to QtScript in under an hour; as already stated, QSA and QtScript are similar in concept, but very different in practice.

Q: What about performance?
A: QtScript offers far better performance than, say, QSA (by a couple of orders of magnitude). The performance is comparable to SpiderMonkey in most areas.

Q: What about bindings for the rest of Qt?
A: The emphasis is not on bindings for the 4.3 release, but rather on the core scripting environment; e.g. interpreter, ECMAScript compliance, QObject bindings. We offer an API you can use for writing your own bindings, in case the QObject bindings are not sufficient. Don’t expect to be able to write entire applications in only QtScript, at least not with the first release.

Well, have fun and let us know what you think. Remember, there’s still time for us to take your thoughts into account before the first official QtScript release.