How to use a Qt GUI with Vala?

Question

I have found zero documentation on this subject (Have I been searching in the wrong places? It seems strange to me).

I simply need to be able to use a Qt GUI for my Vala application. An example would be appreciated.

Glassine

I don't think there are any Vala bindings for Qt. – Neckpiece

I don't think either and don't quite see the motivation for this kind of thing. In a way, Qt is to C++ what Vala is to C (or more spefically, the Gnome C API). Usually people would just go with either depending what they're more comfortable with. – Cerebrospinal

There's not no way, but it would involve so much extra work that you'd probably be wasting your time. Maybe you can say why you need to use Qt and Vala together, then someone might be able to suggest an alternate solution? – Jasmine

@Jasmine Mainly because I want to learn Vala and because Qt Designer works way easier than Glade Designer. – Glassine

I would pick either Qt or Vala and not try to learn both at the same time. Vala's object system is quite different from C++'s object system and I don't foresee that anyone will think it worthwhile to write code to unite the two. – Jasmine

Answer 1

As others have mentioned, Qt and Vala don't work hand-in-hand, but that doesn't mean it's impossible to get them to cooperate. It's mostly about understanding what's going on beneath the covers.

Vala generates C code which is then fed to gcc (or another installed compiler) to produce a binary. Note that one of the Vala designers' chief goals was for Vala to produce C-based libraries. These can then be used by other languages that accept C-based bindings -- Python, Ruby, Java, and so on.

So, you could use Vala to code a C-based library that your Qt C++ GUI application calls into. The Vala compiler produces a .h file that your Qt app merely #includes.

The problem with this is that Qt and Vala use different object systems: QObject for Qt, GObject for Vala. (Vala does allow for other backends, and there's even some thought about Vala producing Qt C++ instead of GObject-based C, but that's far in the future.) QObject and GObject are not compatible, and so for your QObjects to talk with GObjects, you need to do a lot of manual C-based work. (Writing GObject in C is quite verbose, hence the allure of Vala to hide all of it.)

But it can be done. Note that Qt will even use GLib's event loop rather than its own, allowing for the code to commingle in an event-driven application.

I can't heartily recommend the above, but in theory it's possible, mostly because C++ code can easily call C code.

Another possibility worth considering is make Vala code a DBus server and your Qt code a DBus client. DBus is a fancy IPC, and so this is not suitable for all applications, but it might be for yours. It's attractive because Vala can easily produce DBus clients and servers (they look like ordinary objects). Tools to produce Qt DBus bindings are available as well. Note that this means your Vala code runs as a separate process and is not an in-process library. See http://live.gnome.org/Vala/DBusServerSample and http://live.gnome.org/Vala/DBusClientSamples

Alfie

Answer 2

This is not quite what you want, but take a look at https://wiki.gnome.org/Projects/GObjectIntrospection/GObjectConsume That brings gobject-introspection to C++. With GObjectConsume you could use your vala library in C++ and qt.

Guerrero