can't get dbus signal listener to work in C with gnome multimedia keys

问题

I'm trying to make my own application that will listen to the media-keys in gnome using dbus. I have found a script that works in python and I'm trying to make it work by converting it to C. My c-code builds and runs correctly but it doesn't respond to the dbus signal.

this python code works:

#!/usr/bin/env python
"""Printing out gnome multi media keys via dbus-python.
"""
import gobject
import dbus
import dbus.service
import dbus.mainloop.glib


def on_mediakey(comes_from, what):
    """ gets called when multimedia keys are pressed down.
    """
    print ('comes from:%s  what:%s') % (comes_from, what)
    if what in ['Stop','Play','Next','Previous']:
        print ('Got a multimedia key!')
    else:
        print ('Got a multimedia key...')

# set up the glib main loop.
dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)
bus = dbus.Bus(dbus.Bus.TYPE_SESSION)
bus_object = bus.get_object('org.gnome.SettingsDaemon', 
                            '/org/gnome/SettingsDaemon/MediaKeys')

# this is what gives us the multi media keys.
dbus_interface='org.gnome.SettingsDaemon.MediaKeys'
bus_object.GrabMediaPlayerKeys("MyMultimediaThingy", 0, 
                               dbus_interface=dbus_interface)

# connect_to_signal registers our callback function.
bus_object.connect_to_signal('MediaPlayerKeyPressed', 
                             on_mediakey)

# and we start the main loop.
mainloop = gobject.MainLoop()
mainloop.run()

my c-code does not work, what is missing?

#include <glib.h>
#include <dbus/dbus-glib.h>

static void media_key_pressed(DBusGProxy *proxy, const char *value_name, gpointer user_data) {
    g_print("mediakey: %s",value_name);
}

int
main (int argc, char **argv)
{
  DBusGConnection *bus;
  DBusGProxy *proxy;
  GError *error = NULL;
  GMainLoop *loop;

  g_type_init();

  loop = g_main_loop_new (NULL, FALSE);

  bus = dbus_g_bus_get (DBUS_BUS_SESSION, &error);
  if (!bus) {
    g_printerr("Failed to connect to the D-BUS daemon: %s", error->message);
    return 1;
  }

  proxy = dbus_g_proxy_new_for_name(bus,
                                    "org.gnome.SettingsDaemon",
                                    "/org/gnome/SettingsDaemon/MediaKeys",
                                    "org.gnome.SettingsDaemon.MediaKeys");

  if(!proxy) {
      g_printerr("Could not create proxy object");
  }

  dbus_g_proxy_add_signal(proxy,
                          "MediaPlayerKeyPressed",
                          G_TYPE_STRING,
                          G_TYPE_INVALID);

  dbus_g_proxy_connect_signal(proxy,
                              "MediaPlayerKeyPressed",
                              G_CALLBACK(media_key_pressed),
                              NULL,
                              NULL);

  g_print("Starting media key listener");
  g_main_loop_run (loop);
  return 0;
}

回答1:

I solved the problem! Must call dbus_g_proxy_call with the name "GrabMediaPlayerKeys" (which you also can see in the python code)

Must also register a marshaller which is done by first running this command:

glib-genmarshal --header --body mediaplayerkeypressed.list > marshal.h

and then use it in the dbus_g_object_register_marshaller function.

The complete C-code:

#include <glib.h>
#include <dbus/dbus-glib.h>
#include "marshal.h"

static void media_key_pressed(DBusGProxy *proxy, const char *value1, const char *value2, gpointer user_data) {
    g_print("mediakey: %s\n",value2);
}

int
main (int argc, char **argv)
{
    DBusGConnection *conn;
    DBusGProxy *proxy;
    GError *error;
    GMainLoop *loop;

    g_type_init();
    error = NULL;

    conn = dbus_g_bus_get (DBUS_BUS_SESSION, &error);
    if (!conn) {
        g_printerr("Failed to connect to the D-BUS daemon: %s\n", error->message);
        g_error_free(error);

        return 1;
    }

    loop = g_main_loop_new (NULL, FALSE);
    if(!loop) {
        g_printerr("Could not create mainloop\n");

        return 1;
    }

    proxy = dbus_g_proxy_new_for_name(conn,
                                    "org.gnome.SettingsDaemon",
                                    "/org/gnome/SettingsDaemon/MediaKeys",
                                    "org.gnome.SettingsDaemon.MediaKeys");

    if(!proxy) {
        g_printerr("Could not create proxy object\n");
    }

    error = NULL;
    if(!dbus_g_proxy_call(proxy,
                      "GrabMediaPlayerKeys", &error,
                      G_TYPE_STRING, "WebMediaKeys",
                      G_TYPE_UINT, 0,
                      G_TYPE_INVALID,
                      G_TYPE_INVALID)) {
        g_printerr("Could not grab media player keys: %s\n", error->message);
    }

    dbus_g_object_register_marshaller (
            g_cclosure_user_marshal_VOID__STRING_STRING,
            G_TYPE_NONE,
            G_TYPE_STRING,
            G_TYPE_STRING,
            G_TYPE_INVALID);

    dbus_g_proxy_add_signal(proxy,
                          "MediaPlayerKeyPressed",
                          G_TYPE_STRING,
                          G_TYPE_STRING,
                          G_TYPE_INVALID);

    dbus_g_proxy_connect_signal(proxy,
                              "MediaPlayerKeyPressed",
                              G_CALLBACK(media_key_pressed),
                              NULL,
                              NULL);

    g_print("Starting media key listener\n");
    g_main_loop_run (loop);

    return 0;
}

来源：https://stackoverflow.com/questions/5744041/cant-get-dbus-signal-listener-to-work-in-c-with-gnome-multimedia-keys