QtDBus, Not receiving signals from dbus-send when using QT DBus xml interface

Question

System information:

QT5.10
Ubuntu 16.04

With QtDBus, and using the following XML, I generated the xyz_interface.hpp

<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node>
<interface name="com.blah.XYZ">
    <signal name="Event">
        <arg name="event_json" type="s" direction="out"/>
    </signal>
    <method name="AuthenticateUser">
        <arg name="auth_req_json" type="s" direction="in"/>
        <arg name="result" type="s" direction="out"/>
    </method>
</interface>
</node>

After creating the interface, and connecting to the interface's signal;

com::blah::Xyz interface = new com::blah::Xyz("com.blah.XYZ", "/com/blah/XYZ", QDBusConnection::systemBus(), this);
connect(interface , &com::blah::Xyz::Event, this, &SomeClass::HandleEvents)

I'm unable to trigger the breakpoint inside the function SomeClass::HandleEvents when sending a signal from Xyz by through command line with dbus-send;

dbus-send --system --type=signal /com/blah/Xyz com.blah.XYZ.Event string:"hello"

using dbus-monitor --system, I can see the signal is being correctly emitted.

Then, in the unit tests, I created a test signal as;

QDBusMessage xyz_signal = QDBusMessage::createSignal(
    "/com/blah/XYZ", "com.blah.XYZ", "Event");
xyz_signal << "hello";
ASSERT_TRUE(connection.send(xyz_signal ));

That also failed to trigger the breakpoint.

After hours of searching for a solution, i gave up and created some fake python dbus services that emitted the Event signal on command (replaced dbus-send) and voila.. the breakpoint was triggered and signal was caught! (not a single change to the QT dbus code)

The fact that i'm able to receive a signal emitted from python code, leads me to think that my dbus-send --system --type=signal is wrongly formatted, but can't seem to fix it.

here's the python code;

import gobject

import dbus
import dbus.service
import dbus.mainloop.glib


class XyzService(dbus.service.Object):

    @dbus.service.method("com.blah.Xyz",
                        in_signature='s', out_signature='s')
    def AuthenticateUser(self, auth_req_json):
        print (str("hello_message"))
        return '''
                {
                    "AuthenticationResponce" : "SUCCESS"
                }
                '''

    @dbus.service.method("com.blah.Xyz",
                        in_signature='s')
    def EmitEvent(self, str):
        self.Event(str)

    @dbus.service.signal('com.blah.Xyz', signature='s')
    def Event(self, event_json):
        # The signal is emitted when this method exits
        # You can have code here if you wish
        pass


if __name__ == '__main__':
    dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)

    system_bus = dbus.SystemBus()
    name = dbus.service.BusName("com.blah.Xyz", system_bus)
    object = XyzService(system_bus, '/com/blah/Xyz')

    mainloop = gobject.MainLoop()
    print "Running example service."
    mainloop.run()

Update

I just found out that if I create the interface with empty strings for path and name, then I am able to receive the signals sent by command line dbus-send

com::blah::Xyz interface = new com::blah::Xyz("", "", QDBusConnection::systemBus(), this);

Also the unit test above is passing.

but now i'm unable to call the method AuthenticateUser! This is very confusing

Update 2

I was able to reproduce the issue with the RemoteControlledCar dbus example provided by QT found here; https://code.qt.io/cgit/qt/qtbase.git/tree/examples/dbus/remotecontrolledcar?h=5.10

In controller.cpp, in the constructor, i added the following;

Controller::Controller(QWidget *parent)
    : QWidget(parent)
{
    ui.setupUi(this);
    car = new org::example::Examples::CarInterface("org.example.CarExample", "/Car",
                           QDBusConnection::sessionBus(), this);

    // added signal listener
    connect(car, &org::example::Examples::CarInterface::crashed,this,[](){qDebug("hello signal");});
    startTimer(1000);
}

when triggering the signal with dbus-send, the message hello signal is not printed. But when I change the service path and name to empty string, then I successfully receive the signal;

car = new org::example::Examples::CarInterface("", "",
                       QDBusConnection::sessionBus(), this);

from command line

dbus-send --session --type=signal /Car org.example.Examples.CarInterface.crashed

Cheers, Simon

QtDBus, Not receiving signals from dbus-send when using QT DBus xml interface

0 Answers0