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glib/docs/reference/gio/migrating-gconf.xml
Ray Strode dc8b03027d gsettings: Update documentation on schema naming convention 
The existing docs are a bit inconsistent in that they say to follow
the dbus convention, but then give an example that doesn't.

This commit changes things to be how Ryan says they should be.
2011-01-17 17:31:14 -05:00

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<chapter>
<title>Migrating from GConf to GSettings</title>
<section>
<title>Before you start</title>
<para>
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.
</para>
<para>
It is always a good idea to have a look at how others have handled
similar problems before. An examplaric conversion can be found e.g.
in the <ulink url="http://git.gnome.org/browse/gnome-utils/log/?h=gsettings-tutorial">gsettings-tutorial</ulink> branch of gnome-utils.
</para>
</section>
<section>
<title>Conceptual differences</title>
<para>
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.
</para>
<para>
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'.
</para>
<para>
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.
</para>
</section>
<section>
<title>GConfClient (and GConfBridge) API conversion</title>
<para>
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:
<table id="gconf-client-vs-gsettings">
<tgroup cols="2">
<thead>
<row><entry>GConfClient</entry><entry>GSettings</entry></row>
</thead>
<tbody>
<row><entry>gconf_client_get_default()</entry><entry>no direct equivalent,
instead you call g_settings_new() for the schemas you use</entry></row>
<row><entry>gconf_client_set()</entry><entry>g_settings_set()</entry></row>
<row><entry>gconf_client_get()</entry><entry>g_settings_get()</entry></row>
<row><entry>gconf_client_get_bool()</entry><entry>g_settings_get_boolean()</entry></row>
<row><entry>gconf_client_set_bool()</entry><entry>g_settings_set_boolean()</entry></row>
<row><entry>gconf_client_get_int()</entry><entry>g_settings_get_int()</entry></row>
<row><entry>gconf_client_set_int()</entry><entry>g_settings_set_int()</entry></row>
<row><entry>gconf_client_get_float()</entry><entry>g_settings_get_double()</entry></row>
<row><entry>gconf_client_set_float()</entry><entry>g_settings_set_double()</entry></row>
<row><entry>gconf_client_get_string()</entry><entry>g_settings_get_string()</entry></row>
<row><entry>gconf_client_set_string()</entry><entry>g_settings_set_string()</entry></row>
<row><entry>gconf_client_get_list()</entry><entry>for string lists, see g_settings_get_strv(), else see g_settings_get_value() and #GVariant API</entry></row>
<row><entry>gconf_client_set_list()</entry><entry>for string lists, see g_settings_set_strv(), else see g_settings_set_value() and #GVariant API</entry></row>
<row><entry>gconf_entry_get_is_writable()</entry><entry>g_settings_is_writable()</entry></row>
<row><entry>gconf_client_notify_add()</entry><entry>not required, the #GSettings::changed signal is emitted automatically</entry></row>
<row><entry>gconf_client_add_dir()</entry><entry>not required, each GSettings instance automatically watches all keys in its path</entry></row>
<row><entry>#GConfChangeSet</entry><entry>g_settings_delay(), g_settings_apply()</entry></row>
<row><entry>gconf_client_get_default_from_schema()</entry><entry>no equivalent, applications are expected to know their schema</entry></row>
<row><entry>gconf_client_all_entries()</entry><entry>no equivalent, applications are expected to know their schema, and GSettings does not allow schema-less entries</entry></row>
<row><entry>gconf_client_get_without_default()</entry><entry>no equivalent</entry></row>
<row><entry>gconf_bridge_bind_property()</entry><entry>g_settings_bind()</entry></row>
<row><entry>gconf_bridge_bind_property_full()</entry><entry>g_settings_bind_with_mapping()</entry></row>
</tbody>
</tgroup>
</table>
</para>
<para>
GConfBridge was a third-party library that used GConf to bind an object property
to a particular configuration key. GSettings offers this service itself.
</para>
<para>
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.
</para>
<para>
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.
</para>
</section>
<section>
<title>Change notification</title>
<para>
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.
</para>
<para>
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.
</para>
<para>
GSettings also notifies you about changes in writability of keys,
with the #GSettings::writable-changed signal (and the
#GSettings::writable-change-event signal).
</para>
</section>
<section><title>Change sets</title>
<para>
GConf has a 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).
</para>
<para>
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()
<emphasis>on the same object</emphasis>, but not to other GSettings instances
or even other processes.
</para>
<para>
To apply the pending changes all at once (GSettings <emphasis>does</emphasis>
atomicity here), call g_settings_apply(). To revert the pending changes,
call g_settings_revert() or just drop the reference to the #GSettings object.
</para>
</section>
<section>
<title>Schema conversion</title>
<para>
If you are porting your application from GConf, most likely you already
have a GConf schema. GConf comes with a commandline tool
<link linkend="gsettings-schema-convert">gsettings-schema-convert</link>
that can help with the task of converting a GConf schema into
an equivalent GSettings schema. The tool is not perfect and
may need assistence in some cases.
</para>
<example><title>An example for using gsettings-schema-convert</title>
<para>Running <userinput>gsettings-schema-convert --gconf --xml --schema-id "org.gnome.font-rendering" --output org.gnome.font-rendering.gschema.xml destop_gnome_font_rendering.schemas</userinput> on the following <filename>desktop_gnome_font_rendering.schemas</filename> file:
<programlisting>
<![CDATA[
<?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>
]]>
</programlisting>
produces a <filename>org.gnome.font-rendering.gschema.xml</filename> file with the following content:
<programlisting>
<![CDATA[
<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>
]]>
</programlisting>
</para>
</example>
<para>
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
<filename>.gschema.xml</filename>. It is recommended to simply
use the schema id as the filename, followed by this extension,
e.g. <filename>org.gnome.SessionManager.gschema.xml</filename>.
</para>
<para>
The XML source file for your GSettings schema needs to get installed
into <filename>$datadir/glib-2.0/schemas</filename>, and needs to be
compiled into a binary form. At runtime, GSettings looks for compiled
schemas in the <filename>glib-2.0/schemas</filename> subdirectories
of all <envar>XDG_DATA_DIRS</envar> directories, so if you install
your schema in a different location, you need to set the
<envar>XDG_DATA_DIRS</envar> environment variable appropriately.
</para>
<para>
Schemas are compiled into binary form by the
<link linkend="glib-compile-schemas">glib-compile-schemas</link> utility.
GIO provides a <literal>glib_compile_schemas</literal>
variable for the schema compiler.
</para>
<para>
You can ignore all of this by using the provided m4 macros. To
do this, add to your <filename>configure.ac</filename>:
<programlisting>
GLIB_GSETTINGS
</programlisting>
The corresponding <filename>Makefile.am</filename> fragment looks like
this:
<programlisting>
# 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@
</programlisting>
</para>
<para>
This is not sufficient on its own. You need to mention what the source
of the <filename>my.app.gschema.xml</filename> file is. If the schema
file is distributed directly with your project's tarball then a mention
in <varname>EXTRA_DIST</varname> 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 <varname>EXTRA_DIST</varname> for the
source files used by that rule.
</para>
<para>
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.
<programlisting>
<![CDATA[
<type>string</type>
<default>rgb</default>
]]>
</programlisting>
becomes
<programlisting>
<![CDATA[
<key name="rgba-order" type="s">
<default>'rgb'</default> <!-- note quotes -->
</key>
]]>
</programlisting>
</para>
<para>
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
<programlisting>
<![CDATA[
<key>/schemas/desktop/gnome/font_rendering/antialiasing</key>
]]>
</programlisting>
becomes
<programlisting>
<![CDATA[
<schema id="org.gnome.font" path="/desktop/gnome/font_rendering/">
<key name="antialiasing" type="s">
]]>
</programlisting>
</para>
<para>
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 <tag class="attribute">gettext-domain</tag>
attribute. Therefore, when converting localized defaults in GConf,
<programlisting>
<![CDATA[
<key>/schemas/apps/my_app/font_size</key>
<locale name="C">
<default>18</default>
</locale>
<locale name="be">
<default>24</default>
</locale>
</key>
]]>
</programlisting>
becomes
<programlisting>
<![CDATA[
<schema id="..." gettext-domain="your-domain">
...
<key name="font-size" type="i">
<default l10n="messages" context="font_size">18</default>
</key>
]]>
</programlisting>
</para>
<para>
GSettings uses gettext for translation of default values.
The string that is translated is exactly the string that appears
inside of the <tag class='starttag'>default</tag> element. This
includes the quotation marks that appear around strings.
Default values must be marked with the <varname>l10n</varname>
attribute in the <tag class='starttag'>default</tag> tag, which
should be set as equal to <literal>'messages'</literal> or
<literal>'time'</literal> depending on the desired category. An
optional translation context can also be specified with the
<varname>context</varname> attribute, as in the example. This
is usually recommended, since the string "<literal>18</literal>"
is not particularly easy to translate without context. The
translated version of the default value should be stored in the
specified <varname>gettext-domain</varname>. Care must be taken
during translation to ensure that all translated values remain
syntactically valid; mistakes here will cause runtime errors.
</para>
<para>
GSettings schemas have optional <tag class="starttag">summary</tag> and
<tag class="starttag">description</tag> elements for each key which
correspond to the <tag class="starttag">short</tag> and
<tag class="starttag">long</tag> elements in the GConf schema and
will be used in similar ways by a future gsettings-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.
</para>
<para>
Translations for these strings will also be handled
via gettext, so you should arrange for these strings to be
extracted into your gettext catalog. One way to do that is to use
intltool. For that, you use <tag class="starttag">_summary</tag>
and <tag class="starttag">_description</tag> elements in a
.gschema.xml.in file and use
<literal>@<!-- -->INTLTOOL_XML_NOMERGE_RULE<!-- -->@</literal>
in your Makefile.am to produce the .gschema.xml file. The
<literal>NOMERGE</literal> part of the rule instructs intltool
to extract translatable strings, but not merge the translations
back into the generated xml file.
</para>
<para>
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 '-'.
</para>
<para>
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
<option>--allow-any-name</option> with the
<link linkend="glib-compile-schemas">glib-compile-schemas</link> 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.
</para>
</section>
<section><title>Data conversion</title>
<para>
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 <envar>GSETTINGS_BACKEND</envar>
to 'gconf', e.g. by using
<programlisting>
g_setenv ("GSETTINGS_BACKEND", "gconf", TRUE);
</programlisting>
early on in your program. Note that this backend is meant purely
as a transition tool, and should not be used in production.
</para>
<para>
GConf also comes with a utility called
<command>gsettings-data-convert</command>, 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.
</para>
<para>
To make use of this utility, you must install a keyfile in the
directory <filename>/usr/share/GConf/gsettings</filename> which
lists the GSettings keys and GConf paths to map to each other, for
each schema that you want to migrate user data for.
</para>
<para>
Here is an example:
<programlisting>
<![CDATA[
[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
]]>
</programlisting>
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.
</para>
<para>
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.
</para>
<para>
There are some limitations: <command>gsettings-data-convert</command>
does not do any transformation of the values. And it does not handle
complex GConf types other than lists of strings or integers.
</para>
<para>
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.
</para>
<para>
If, as an application developer, you are interested in manually
ensuring that <command>gsettings-data-convert</command> 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.
</para>
<para>
Internally, <command>gsettings-data-convert</command> 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.
</para>
<para>
<programlisting>
<![CDATA[
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);
}
]]>
</programlisting>
</para>
<para>
Although there is the possibility that the
<command>gsettings-data-convert</command> script will end up
running multiple times concurrently with this approach, it is
believed that this is safe.
</para>
</section>
</chapter>
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