[Development] Builtin QML property type for functions possible?

[Sebastian Lehmann]

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