[Development] Builtin QML property type for functions possible?

[Hausmann Simon]

This is already possible :) just use var properties, they map to QJSValue in C++.

Simon
Fra: Sebastian Lehmann
Sendt: 19:31 onsdag 8. januar 2014
Til: development at qt-project.org
Emne: [Development] Builtin QML property type for functions possible?


Hi guys,

We can write JS functions in QML, but they can't be passed around via
properties. The type "variant" can't be assigned a JS function.

Can either the possibility of such an assignment (to "variant"
properties) or a "function" property type be added to the QML core?
I'm not asking for someone to do it, but I'd like to know if it is
possible (with reasonable effort) to do so, as I think this might be a
nice extension.

In which scenarios would this feature help? Can't we use signals and
slots for C++/QML interoperability? Well, there are some scenarios in
which we want to pass "behavior" from C++ to QML. This could always be
wrapped in a QObject with a slot to be called, but such a wrapper
can't be created with little effort / nice syntax from within QML to
pass a behavior back to C++, afaik.

The following would then be possible: Assign something like a callback
from C++ to a QML property (maybe using some QQmlFunctionProperty type
as a wrapper, like we have one for list properties; this could wrap
around any "callable" thing like std::function does). Then a call this
function from QML's JS like it were a JS function; also assign a
function from QML to such a property and call it in C++ (the
QQmlFunctionProperty then wraps around a function of the underlying JS
engine and has `operator()()` or `invoke()`, or similar). Signature is
of course a problem, but only supporting `void()` would be a start.
`QVariant(QVariantList)` could make it flexible yet ugly and slow.

It would also be cool to first try to find the nicest way to do both
things with the existing tools.

Thanks for any feedback on this topic!

Cheers,

Sebastian Lehmann
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