Able to detect signals.

dbusVLC.cpp

/*
g++ cppdbus.cpp -I/usr/include/glib-2.0 \
                -I/usr/lib/x86_64-linux-gnu/glib-2.0/include/ \
                -lglib-2.0 -lgio-2.0 -lgobject-2.0

This small program can pause/play media in vlc. Then, if the
tracklist is modified, the program will exit.

This is the dbus-monitor output corresponding to adding a track to a playlist:

signal time=1524079919.626002 sender=:1.97 -> destination=(null destination) \
     serial=37 path=/org/mpris/MediaPlayer2; \
     interface=org.freedesktop.DBus.Properties; \
     member=PropertiesChanged
   string "org.mpris.MediaPlayer2.TrackList"
   array [
   ]
   array [
      string "Tracks"
   ]

But seemingly the same output is shown in dbus-monitor if you
remove a track from the playlist:

signal time=1524079988.735333 sender=:1.97 -> destination=(null destination) \
    serial=38 path=/org/mpris/MediaPlayer2; \
    interface=org.freedesktop.DBus.Properties; \
    member=PropertiesChanged
   string "org.mpris.MediaPlayer2.TrackList"
   array [
   ]
   array [
      string "Tracks"
   ]

Is there a way to distinguish?
*/

#include <iostream>
#include <gio/gio.h>

static GMainLoop* loop = NULL;
    
static void handle_proxy_call_sync_result( GVariant* result )
{
    const gchar* str;
    g_assert (result != NULL);
    g_assert_cmpstr (g_variant_get_type_string (result), ==, "(s)");
    g_variant_get (result, "(&s)", &str);
    std::cout << str << std::endl;
    g_variant_unref (result);
}

static void
myCallback (GDBusConnection* connection,
            const gchar* sender_name,
            const gchar* object_path,
            const gchar* interface_name,
            const gchar* signal_name,
            GVariant* parameters,
            gpointer user_data) {
  g_message ("Works for me.");
  g_main_loop_quit (loop);
}

static void
neverCalled (GDBusConnection* connection,
            const gchar* sender_name,
            const gchar* object_path,
            const gchar* interface_name,
            const gchar* signal_name,
            GVariant* parameters,
            gpointer user_data) {
  g_message ("U Can't C Me.");
  g_main_loop_quit (loop);
}


static void
handleIntrospection( GObject* source_object,
                      GAsyncResult* result,
                      gpointer user_data )
{
    const char* data = (char*) user_data;
    std::cout << "Printing data in the GAsyncResult function:" << data << std::endl;
}

int main()
{
    g_autoptr( GDBusConnection ) conn = NULL;
    g_autoptr( GError ) error = NULL;
    conn = g_bus_get_sync( G_BUS_TYPE_SESSION, NULL, &error );
    g_assert_no_error( error );
    if (conn == NULL)
        std::cout << "Error!" <<std::endl; //assert(), cerr, w/e.
    g_autoptr( GError ) error2 = NULL;
    g_autoptr( GDBusProxy ) proxy = NULL;
    proxy = g_dbus_proxy_new_sync( conn,
                                   G_DBUS_PROXY_FLAGS_NONE, 
                                   NULL,
                                   "org.mpris.MediaPlayer2.vlc",
                                   "/org/mpris/MediaPlayer2",
                                   "org.mpris.MediaPlayer2.Player",
                                   NULL,
                                   &error2 );
    g_assert_no_error( error2 );
    if( proxy == NULL )
        std::cout << "Error!!" << std::endl;

    g_dbus_proxy_call( proxy,
                       "Play", // "Pause" 
                       NULL,
                       G_DBUS_CALL_FLAGS_NONE,
                       -1,
                       NULL,
                       NULL,
                       NULL );


    g_autoptr( GError ) error3 = NULL;
    g_autoptr( GDBusProxy ) proxy2 = NULL;
    proxy2 = g_dbus_proxy_new_sync( conn,
                                   G_DBUS_PROXY_FLAGS_NONE, 
                                   NULL,
                                   "org.mpris.MediaPlayer2.vlc",
                                   "/org/mpris/MediaPlayer2",
                                   "org.freedesktop.DBus.Introspectable",
                                   NULL,
                                   &error3 );
    g_assert_no_error( error3 );
    if( proxy2 == NULL )
        std::cout << "Error!!" << std::endl;

    GVariant* ret = NULL;
    g_autoptr( GError ) error4 = NULL;
    ret =  g_dbus_proxy_call_sync( proxy2, 
                            "Introspect",
                            NULL,
                            G_DBUS_CALL_FLAGS_NONE,
                            -1,
                            NULL, 
                            &error4 );
    g_assert_no_error( error4 );

    // <sorcery>
    const gchar* str;
    g_assert (ret != NULL);
    g_assert_cmpstr (g_variant_get_type_string (ret), ==, "(s)");
    g_variant_get (ret, "(&s)", &str);
    std::cout << str << std::endl;
    g_variant_unref (ret);
    //</sorcery>

    char cData[4] = {'a', 'b', 'c', '\0'};
    gpointer data = (void*) cData; 
    std::cout << (char*) data << std::endl;

    g_dbus_proxy_call( proxy2,
                       "Introspect", 
                       NULL,
                       G_DBUS_CALL_FLAGS_NONE,
                       -1,
                       NULL,
                       (GAsyncReadyCallback) handleIntrospection,
                       data );



    //handle_proxy_call_sync_result( ret );

    guint doesNothing = g_dbus_connection_signal_subscribe( conn, 
                                        "org.mpris.MediaPlayer2.vlc",
                                        "org.mpris.MediaPlayer2.Tracklist",
                                        "TrackAdded",
                                        "/org/mpris/MediaPlayer2",
                                        NULL,
                                        G_DBUS_SIGNAL_FLAGS_NONE,
                                        (GDBusSignalCallback) neverCalled,
                                        NULL,
                                        NULL );

 
    guint sigId = g_dbus_connection_signal_subscribe( conn, 
                                        "org.mpris.MediaPlayer2.vlc",
                                        "org.freedesktop.DBus.Properties",
                                        "PropertiesChanged",
                                        "/org/mpris/MediaPlayer2",
                                        "org.mpris.MediaPlayer2.TrackList", //Filter out the signals by string
                                        G_DBUS_SIGNAL_FLAGS_NONE,
                                        (GDBusSignalCallback) myCallback,
                                        NULL,
                                        NULL );
    
    loop = g_main_loop_new (NULL, FALSE);
    g_main_loop_run (loop);
    g_main_loop_unref (loop);
    g_object_unref (conn);
    return 0;    
}