c++ gdbus - 如何使用 gdbus 库获取接口的属性？

Question

我想知道使用 c++ 代码 sn-p 从 dbus 接口获取属性的方法/代码 sn-p。

我尝试了以下错误方法。

方法#1使用 g_dbus_proxy_get_cached_property 但它总是返回 null

ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                       flags,
                                       NULL,
                                       "org.freedesktop.NetworkManager",
                                       "org/freedesktop/NetworkManager/Device/0",
                                       "org.freedesktop.NetworkManager.Device",
                                       NULL, 
                                       &error);
ret = g_dbus_proxy_get_cached_property(ifproxy, "State")

方法#2 g_dbus_proxy_call_sync - 这个说“org.freedesktop.networkmanager”未导出（或可能不存在），无法访问属性“接口”

ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                             G_DBUS_PROXY_FLAGS_NONE,
                                             NULL,
                                             "org.freedesktop.NetworkManager",
                                            "org/freedesktop/NetworkManager/Device/0",
                                             "org.freedesktop.DBus.Properties",
                                             NULL, NULL);
    g_assert (ifproxy);

    /* Get the object path of the Connection details */
    ret = g_dbus_proxy_call_sync (ifproxy,
                                  "Get",
                                  g_variant_new ("(ss)",
                                                "org/freedesktop/NetworkManager/Device/0",
                                                 "Interface"),
                                  G_DBUS_CALL_FLAGS_NONE, -1,
                                  NULL, &error);
    if (!ret) {
        g_dbus_error_strip_remote_error (error);
        g_warning ("Failed to get property: %s\n",
                   error->message);
        g_error_free (error);
        return;
    }

    name = g_variant_get_string(ret, NULL);

    //g_assert(ret != NULL);

    g_variant_get (ret, "s", &name);
    g_variant_unref (ret);

Answer 1

至少第一种形式应该可以工作（假设您在实际代码中处理错误），但是您的 D-Bus 对象路径存在问题。

首先，有效的对象路径以“/”开头，因此您可能想要/org/freedesktop/NetworkManager/Device/0...，但它似乎不是 NetworkManager 使用的路径。

查看他们的 API 参考，您可能真的想要 /org/freedesktop/NetworkManager/Devices/N（注意复数“Devices”），但请注意，您无法确定路径 (N) 的最后一部分将是什么.在正确的代码中，您应该从org.freedesktop.NetworkManager 获取设备对象路径，但为了调试，您可能只需使用 d-feet 之类的工具来查找可用的对象路径并使用它们。

Answer 2

非常感谢@jku。

我能够解决编码问题。 为其他人分享下面的工作代码。

static void
list_connections (GDBusProxy *proxy)
{
    int i;
    GError *error = NULL;
    GVariant *ret, *ret1;
    char **paths;
    gchar *name;
    GDBusProxy *ifproxy;
    GDBusProxyFlags flags;

    /* Call ListConnections D-Bus method */
    ret = g_dbus_proxy_call_sync (proxy,
                                  "GetDevices",
                                  NULL,
                                  G_DBUS_CALL_FLAGS_NONE, -1,
                                  NULL, &error);
    if (!ret) {
        g_dbus_error_strip_remote_error (error);
        g_print ("ListConnections failed: %s\n", error->message);
        g_error_free (error);
        return;
    }

    g_variant_get (ret, "(^ao)", &paths);
    g_variant_unref (ret);

    flags = static_cast<GDBusProxyFlags> (G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES | G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START);

    for (i = 0; paths[i]; i++)
    {
        g_print ("%s\n", paths[i]);
        /*ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                           flags,
                                           NULL,
                                           "org.freedesktop.NetworkManager",
                                           paths[i],//"/org/freedesktop/NetworkManager/Devices/0"
                                           "org.freedesktop.NetworkManager.Device",
                                           NULL, 
                                           &error);

        //name = g_dbus_proxy_get_interface_name(ifproxy);
        ret = g_dbus_proxy_get_cached_property(ifproxy, "FirmwareVersion");*/

        ifproxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                                 G_DBUS_PROXY_FLAGS_NONE,
                                                 NULL,
                                                 "org.freedesktop.NetworkManager",
                                                 "/org/freedesktop/NetworkManager/Devices/0",
                                                 "org.freedesktop.DBus.Properties",
                                                 NULL, NULL);
        g_assert (ifproxy);

        ret = g_dbus_proxy_call_sync (ifproxy,
                                      "Get",
                                      g_variant_new ("(ss)",
                                                    "org.freedesktop.NetworkManager.Device",
                                                     "Interface"),
                                      G_DBUS_CALL_FLAGS_NONE, -1,
                                      NULL, &error);
        if (!ret) {
            g_dbus_error_strip_remote_error (error);
            g_warning ("Failed to get property: %s\n",
                       error->message);
            g_error_free (error);
            return;
        }

         g_print("\nType String of Variant:- %s\n", g_variant_get_type_string (ret));

        g_variant_get (ret, "(v)", &ret1);
        g_variant_unref (ret);

         g_print("\nType String of Variant:- %s\n", g_variant_get_type_string (ret1));
        g_variant_get (ret1, "s", &name);
        g_variant_unref (ret1);


        g_print ("Interface name:- %s\n", name);
    }
    g_strfreev (paths);
}

int
main (int argc, char *argv[])
{
    GDBusProxy *proxy;
    GDBusProxyFlags flags;
    GError *error = NULL;
    flags = static_cast<GDBusProxyFlags> (G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES | G_DBUS_PROXY_FLAGS_DO_NOT_AUTO_START);

#if !GLIB_CHECK_VERSION (2, 35, 0)
    /* Initialize GType system */
    g_type_init ();
#endif

    proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
                                           flags,
                                           NULL, /* GDBusInterfaceInfo */
                                           "org.freedesktop.NetworkManager",
                                           "/org/freedesktop/NetworkManager",
                                           "org.freedesktop.NetworkManager",
                                           NULL, /* GCancellable */
                                           &error);
    g_assert (proxy != NULL);

    /* List connections of system settings service */
    list_connections (proxy);

    g_object_unref (proxy);

    return 0;
}