How to transfer QPolygonF via DBus

skylendar

Hi there and thx for reading and answering if you can.

I'm on a linux box x86_64 with Qt5.4 downloaded from the qt site, and I need to transfer a QPolygonF to another process via DBus.

In order to do that, I wrote:

Q_DECLARE_METATYPE(QPolygonF)

then in a function:

qDBusRegisterMetaType<QPolygonF>();

also, 2 adequate operators:

const QDBusArgument &operator>>(const QDBusArgument &argument, QPolygonF& pol)
{
    argument.beginArray();
    pol.clear();

    while ( !argument.atEnd() ) {
        QPointF qp;
        argument >> qp;
        pol.append(qp);
    }

    argument.endArray();
    return argument;
}

const QDBusArgument &operator<<(QDBusArgument &argument, const QPolygonF& pol)
{
	argument.beginArray();	for(QPolygonF::const_iterator it = pol.begin(); it != pol.end(); ++it)
			argument << static_cast<QPointF>(*it));
    argument.endArray();
 return argument;
}

So far, gcc accepts these declarations, but when I run my app, it crashes.

What's wrong ?

skylendar

Ah, and I forgot to say that I got this output from the crashed app:

QDBusArgument: read from a write-only object
process 9243: arguments to dbus_message_unref() were incorrect, assertion "message->generation == _dbus_current_generation" failed in file dbus-message.c line 1695.
This is normally a bug in some application using the D-Bus library.
  D-Bus not built with -rdynamic so unable to print a backtrace

skylendar

Do I also need to declare all the shebang for QPointF ?

skylendar

Noticed the following behavior:

QPolygonF pol;
QDBusVariant dbv;
QVariant v, v1;
v.setValue<QPolygonF>(pol);
dbv.setVariant(v);
QVariant v1 = QVariant::fromValue(dbv);
qDebug() << "typename" << v.typeName() << v1.typeName();

QPolygonF QDBusVariant

As you can see, there is a discrepancy between both QVariants. Is this a bug ?

SGaist

Hi,

AFAIK, no, you are effectively putting a QDBusVariant containing a QPolygonF in a QVariant. So at first level, what you see it correct.

skylendar

Thx for answering.

But I still need to transfer a QPolygonF via QDBusVariant. If I do as above, dbv contains a DBusVariant, not the corresponding QVariant.

How to initialize dbv so that QVariant::fromValue() returns a QVariant containing a QPolygonF, and not its QDBusVariant ?

SGaist

You can't, why not juste:

     QDBusVariant dbusVariant = qvariant_cast<QDBusVariant>(v1);
     QPolygonF result = qvariant_cast<QPolygonF>(dbusVariant.variant());

?

skylendar

Thanks again for proposing a solution, but it doesn't work. I even got:

QDBusMarshaller: cannot add a null QDBusVariant

as a log message when running my stuff.

when qdbusxml2cpp produces the corresponding dbus glue method, the body of this method relies on QVariant::fromValue(myQDBusVariant) for creating its argumentList, but instead of returning the QVariant containing a QPolygonF, fromValue returns a QVariant containing a QDbusVariant containing the QpolygonF.

Arghhh :-(

skylendar

OK, found the error !!! My method used QDBusvariant instead of const QPolygonF. But the solution of one problem yields the opening of a new one:

in my header parsed by qdbuscpp2xml in my CMakeList, my method is completely ignored; the corresponding xml file doesn't include any entry relative to it. Example:

foo.h

#include <QPolygonF>
Q_DECLARE_METATYPE(QPolygonF)

class Foo
{
 Q_OBJECT
 Q_CLASSINFO("D-Bus Interface", "mysoft.foo")
public:

 void MyMethod(int i, const QPolgonF& p);
};

MyMethod is ignored. qdbuscpp2xml returns: Type not registered with QtDBus in parameter list: QPolygonF