Schema conversion

If you are porting your application from GConf, most likely you already have a GConf schema. GIO 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 assistence in some cases.

Example 1. 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 destop_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 a 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 DBus bus name, 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.font-rendering.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 gschema_compile variable for the schema compiler, which can be used in configure.in as follows:

GLIB_GSETTINGS

The corresponding Makefile.am fragment looks like this:

# gsettingsschemadir and gschema_compile are defined by the GLIB_GSETTINGS
# macro in configure.ac
gsettingsschema_DATA = my.app.gschema.xml
# This rule will check your schemas for validity before installation
@GSETTINGS_CHECK_RULE@
if GSETTINGS_SCHEMAS_INSTALL
install-data-hook:
	$(GLIB_COMPILE_SCHEMAS) $(DESTDIR)$(gsettingsschemadir)
endif

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>

        

Note how we used the context attribute to add msgctxt - "18" is not a good string to look up in gettext by itself. Also note that the value 24 is not present in the schema anymore. It has to be added to the gettext catalog for "be" instead.

GSettings schemas have optional <summary> and <description> elements for each key which correspond to the <short> and <long> 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. Translations for these strings will also be handled via gettext, so you should arrange for these strings to be extracted into your gettext catalog.

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.