Migrating from GConf to GSettings

Before you start

Converting individual applications and their settings from GConf to GSettings can be done at will. But desktop-wide settings like font or theme settings often have consumers in multiple modules. Therefore, some consideration has to go into making sure that all users of a setting are converted to GSettings at the same time or that the program responsible for configuring that setting continues to update the value in both places.

It is always a good idea to have a look at how others have handled similar problems before.

Conceptual differences

Conceptually, GConf and GSettings are fairly similar. Both have a concept of pluggable backends. Both keep information about keys and their types in schemas. Both have a concept of mandatory values, which lets you implement lock-down.

There are some differences in the approach to schemas. GConf installs the schemas into the database and has API to handle schema information (gconf_client_get_default_from_schema(), gconf_value_get_schema(), etc). GSettings on the other hand assumes that an application knows its own schemas, and does not provide API to handle schema information at runtime. GSettings is also more strict about requiring a schema whenever you want to read or write a key. To deal with more free-form information that would appear in schema-less entries in GConf, GSettings allows for schemas to be ‘relocatable’.

One difference in the way applications interact with their settings is that with GConf you interact with a tree of settings (ie the keys you pass to functions when reading or writing values are actually paths with the actual name of the key as the last element. With GSettings, you create a GSettings object which has an implicit prefix that determines where the settings get stored in the global tree of settings, but the keys you pass when reading or writing values are just the key names, not the full path.

GConfClient (and GConfBridge) API conversion

Most people use GConf via the high-level GConfClient API. The corresponding API is the GSettings object. While not every GConfClient function has a direct GSettings equivalent, many do:

GConfBridge was a third-party library that used GConf to bind an object property to a particular configuration key. GSettings offers this service itself.

There is a pattern that is sometimes used for GConf, where a setting can have explicit ‘value A’, explicit ‘value B’ or ‘use the system default’. With GConf, ‘use the system default’ is sometimes implemented by unsetting the user value. This is not possible in GSettings, since it does not have API to determine if a value is the default and does not let you unset values. The recommended way (and much clearer) way in which this can be implemented in GSettings is to have a separate ‘use-system-default’ boolean setting.

Change notification

GConf requires you to call gconf_client_add_dir() and gconf_client_notify_add() to get change notification. With GSettings, this is not necessary; signals get emitted automatically for every change.

The GSettings::changed signal is emitted for each changed key. There is also a GSettings::change-event signal that you can handle if you need to see groups of keys that get changed at the same time.

GSettings also notifies you about changes in writability of keys, with the GSettings::writable-changed signal (and the GSettings::writable-change-event signal).

Change sets

GConf has a concept of a set of changes which can be applied or reverted at once: GConfChangeSet (GConf doesn’t actually apply changes atomically, which is one of its shortcomings).

Instead of a separate object to represent a change set, GSettings has a ‘delayed-apply’ mode, which can be turned on for a GSettings object by calling g_settings_delay(). In this mode, changes done to the GSettings object are not applied - they are still visible when calling g_settings_get() on the same object, but not to other GSettings instances or even other processes.

To apply the pending changes all at once (GSettings does atomicity here), call g_settings_apply(). To revert the pending changes, call g_settings_revert() or just drop the reference to the GSettings object.

Schema conversion

If you are porting your application from GConf, most likely you already have a GConf schema. GConf comes with a commandline tool gsettings-schema-convert that can help with the task of converting a GConf schema into an equivalent GSettings schema. The tool is not perfect and may need assistance in some cases.

An example for using gsettings-schema-convert

Running gsettings-schema-convert --gconf --xml --schema-id "org.gnome.font-rendering" --output org.gnome.font-rendering.gschema.xml desktop_gnome_font_rendering.schemas on the following desktop_gnome_font_rendering.schemas file:

<?xml version="1.0"?>
<gconfschemafile>
    <schemalist>
        <schema>
            <key>/schemas/desktop/gnome/font_rendering/dpi</key>
            <applyto>/desktop/gnome/font_rendering/dpi</applyto>
            <owner>gnome</owner>
            <type>int</type>
            <default>96</default>
            <locale name="C">
                <short>DPI</short>
                <long>The resolution used for converting font sizes to pixel sizes, in dots per inch.</long>
            </locale>
        </schema>
    </schemalist>
</gconfschemafile>

produces an org.gnome.font-rendering.gschema.xml file with the following content:

<schemalist>
  <schema id="org.gnome.font-rendering" path="/desktop/gnome/font_rendering/">
    <key name="dpi" type="i">
      <default>96</default>
      <summary>DPI</summary>
      <description>The resolution used for converting font sizes to pixel sizes, in dots per inch.</description>
    </key>
  </schema>
</schemalist>

GSettings schemas are identified at runtime by their id (as specified in the XML source file). It is recommended to use a dotted name as schema id, similar in style to a D-Bus bus name, e.g. “org.gnome.SessionManager”. In cases where the settings are general and not specific to one application, the id should not use StudlyCaps, e.g. “org.gnome.font-rendering”. The filename used for the XML schema source is immaterial, but schema compiler expects the files to have the extension .gschema.xml. It is recommended to simply use the schema id as the filename, followed by this extension, e.g. org.gnome.SessionManager.gschema.xml.

The XML source file for your GSettings schema needs to get installed into $datadir/glib-2.0/schemas, and needs to be compiled into a binary form. At runtime, GSettings looks for compiled schemas in the glib-2.0/schemas subdirectories of all XDG_DATA_DIRS directories, so if you install your schema in a different location, you need to set the XDG_DATA_DIRS environment variable appropriately.

Schemas are compiled into binary form by the glib-compile-schemas utility. GIO provides a glib_compile_schemas variable in its pkg-config file pointing to the schema compiler binary.

Using schemas with Meson

You should use install_data() to install the .gschema.xml file in the correct directory, e.g.

install_data('my.app.gschema.xml', install_dir: get_option('datadir') / 'glib-2.0/schemas')

Schema compilation is done at installation time; if you are using Meson 0.57 or newer, you can use the gnome.post_install() function from the GNOME module:

gnome.post_install(glib_compile_schemas: true)

Alternatively, you can use meson.add_install_script() and the following Python script:

#!/usr/bin/env python3
# build-aux/compile-schemas.py

import os
import subprocess

install_prefix = os.environ['MESON_INSTALL_PREFIX']
schemadir = os.path.join(install_prefix, 'share', 'glib-2.0', 'schemas')

if not os.environ.get('DESTDIR'):
    print('Compiling gsettings schemas...')
    subprocess.call(['glib-compile-schemas', schemadir])

meson.add_install_script('build-aux/compile-schemas.py')

Using schemas with Autotools

GLib provides m4 macros for hiding the various complexities and reduce the chances of getting things wrong.

To handle schemas in your Autotools build, start by adding this to your configure.ac:

GLIB_GSETTINGS

Then add this fragment to your Makefile.am:

# gsettings_SCHEMAS is a list of all the schemas you want to install
gsettings_SCHEMAS = my.app.gschema.xml

# include the appropriate makefile rules for schema handling
@GSETTINGS_RULES@

This is not sufficient on its own. You need to mention what the source of the my.app.gschema.xml file is. If the schema file is distributed directly with your project’s tarball then a mention in EXTRA_DIST is appropriate. If the schema file is generated from another source then you will need the appropriate rule for that, plus probably an item in EXTRA_DIST for the source files used by that rule.

One possible pitfall in doing schema conversion is that the default values in GSettings schemas are parsed by the GVariant parser. This means that strings need to include quotes in the XML. Also note that the types are now specified as GVariant type strings.

<type>string</type>
<default>rgb</default>

becomes

<key name="rgba-order" type="s">
  <default>'rgb'</default> <!-- note quotes -->
</key>

Another possible complication is that GConf specifies full paths for each key, while a GSettings schema has a ‘path’ attribute that contains the prefix for all the keys in the schema, and individual keys just have a simple name. So

<key>/schemas/desktop/gnome/font_rendering/antialiasing</key>

becomes

<schema id="org.gnome.font" path="/desktop/gnome/font_rendering/">
  <key name="antialiasing" type="s">

Default values can be localized in both GConf and GSettings schemas, but GSettings uses gettext for the localization. You can specify the gettext domain to use in the gettext-domain attribute. Therefore, when converting localized defaults in GConf,

<key>/schemas/apps/my_app/font_size</key>
  <locale name="C">
    <default>18</default>
  </locale>
  <locale name="be">
    <default>24</default>
  </locale>
</key>

becomes

<schema id="..." gettext-domain="your-domain">
 ...
<key name="font-size" type="i">
  <default l10n="messages" context="font_size">18</default>
</key>

GSettings uses gettext for translation of default values. The string that is translated is exactly the string that appears inside of the <default> element. This includes the quotation marks that appear around strings. Default values must be marked with the l10n attribute in the <default> tag, which should be set as equal to ‘messages’ or ‘time’ depending on the desired category. An optional translation context can also be specified with the context attribute, as in the example. This is usually recommended, since the string “18” is not particularly easy to translate without context. The translated version of the default value should be stored in the specified gettext-domain. Care must be taken during translation to ensure that all translated values remain syntactically valid; mistakes here will cause runtime errors.

GSettings schemas have optional <summary> and <description> elements for each key which correspond to the <short> and <long> elements in the GConf schema and can be used in the same way by a GUI editor, so you should use the same conventions for them: The summary is just a short label with no punctuation, the description can be one or more complete sentences. If multiple paragraphs are desired for the description, the paragraphs should be separated by a completely empty line.

Translations for these strings will also be handled via gettext, so you should arrange for these strings to be extracted into your gettext catalog. Gettext supports GSettings schemas natively since version 0.19, so all you have to do is add the XML schema file to the list of translatable files inside your POTFILES.in.

GSettings is a bit more restrictive about key names than GConf. Key names in GSettings can be at most 32 characters long, and must only consist of lowercase characters, numbers and dashes, with no consecutive dashes. The first character must not be a number or dash, and the last character cannot be ‘-‘.

If you are using the GConf backend for GSettings during the transition, you may want to keep your key names the same they were in GConf, so that existing settings in the users GConf database are preserved. You can achieve this by using the --allow-any-name with the glib-compile-schemas schema compiler. Note that this option is only meant to ease the process of porting your application, allowing parts of your application to continue to access GConf and parts to use GSettings. By the time you have finished porting your application you must ensure that all key names are valid.

Data conversion

GConf comes with a GSettings backend that can be used to facility the transition to the GSettings API until you are ready to make the jump to a different backend (most likely dconf). To use it, you need to set the GSETTINGS_BACKEND to ‘gconf’, e.g. by using

g_setenv ("GSETTINGS_BACKEND", "gconf", TRUE);

early on in your program. Note that this backend is meant purely as a transition tool, and should not be used in production.

GConf also comes with a utility called gsettings-data-convert, which is designed to help with the task of migrating user settings from GConf into another GSettings backend. It can be run manually, but it is designed to be executed automatically, every time a user logs in. It keeps track of the data migrations that it has already done, and it is harmless to run it more than once.

To make use of this utility, you must install a keyfile in the directory /usr/share/GConf/gsettings which lists the GSettings keys and GConf paths to map to each other, for each schema that you want to migrate user data for.

Here is an example:

[org.gnome.fonts]
antialiasing = /desktop/gnome/font_rendering/antialiasing
dpi = /desktop/gnome/font_rendering/dpi
hinting = /desktop/gnome/font_rendering/hinting
rgba-order = /desktop/gnome/font_rendering/rgba_order

[apps.myapp:/path/to/myapps/]
some-odd-key1 = /apps/myapp/some_ODD-key1

The last key demonstrates that it may be necessary to modify the key name to comply with stricter GSettings key name rules. Of course, that means your application must use the new key names when looking up settings in GSettings.

The last group in the example also shows how to handle the case of ‘relocatable’ schemas, which don’t have a fixed path. You can specify the path to use in the group name, separated by a colon.

There are some limitations: gsettings-data-convert does not do any transformation of the values. And it does not handle complex GConf types other than lists of strings or integers.

Don’t forget to require GConf 2.31.1 or newer in your configure script if you are making use of the GConf backend or the conversion utility.

If, as an application developer, you are interested in manually ensuring that gsettings-data-convert has been invoked (for example, to deal with the case where the user is logged in during a distribution upgrade or for non-XDG desktop environments which do not run the command as an autostart) you may invoke it manually during your program initialisation. This is not recommended for all application authors — it is your choice if this use case concerns you enough.

Internally, gsettings-data-convert uses a keyfile to track which settings have been migrated. The following code fragment will check that keyfile to see if your data conversion script has been run yet and, if not, will attempt to invoke the tool to run it. You should adapt it to your application as you see fit.

static void
ensure_migrated (const gchar *name)
{
  gboolean needed = TRUE;
  GKeyFile *kf;
  gchar **list;
  gsize i, n;

  kf = g_key_file_new ();

  g_key_file_load_from_data_dirs (kf, "gsettings-data-convert",
                                  NULL, G_KEY_FILE_NONE, NULL);
  list = g_key_file_get_string_list (kf, "State", "converted", &n, NULL);

  if (list)
    {
      for (i = 0; i < n; i++)
        if (strcmp (list[i], name) == 0)
          {
            needed = FALSE;
            break;
          }

      g_strfreev (list);
    }

  g_key_file_free (kf);

  if (needed)
    g_spawn_command_line_sync ("gsettings-data-convert",
                               NULL, NULL, NULL, NULL);
}

Although there is the possibility that the gsettings-data-convert script will end up running multiple times concurrently with this approach, it is believed that this is safe.