Gio.Settings (original) (raw)

The GSettings class provides a convenient API for storing and retrieving application settings.

Reads and writes can be considered to be non-blocking. Reading settings with GSettings is typically extremely fast: on approximately the same order of magnitude (but slower than) aGHashTable lookup. Writing settings is also extremely fast in terms of time to return to your application, but can be extremely expensive for other threads and other processes. Many settings backends (including dconf) have lazy initialisation which means in the common case of the user using their computer without modifying any settings a lot of work can be avoided. For dconf, the D-Bus service doesn’t even need to be started in this case. For this reason, you should only ever modify GSettings keys in response to explicit user action. Particular care should be paid to ensure that modifications are not made during startup — for example, when setting the initial value of preferences widgets. The built-in g_settings_bind()functionality is careful not to write settings in response to notify signals as a result of modifications that it makes to widgets.

When creating a GSettings instance, you have to specify a schema that describes the keys in your settings and their types and default values, as well as some other information.

Normally, a schema has a fixed path that determines where the settings are stored in the conceptual global tree of settings. However, schemas can also be ‘relocatable’, i.e. not equipped with a fixed path. This is useful e.g. when the schema describes an ‘account’, and you want to be able to store a arbitrary number of accounts.

Paths must start with and end with a forward slash character (/) and must not contain two sequential slash characters. Paths should be chosen based on a domain name associated with the program or library to which the settings belong. Examples of paths are/org/gtk/settings/file-chooser/ and /ca/desrt/dconf-editor/. Paths should not start with /apps/, /desktop/ or /system/ as they often did in GConf.

Unlike other configuration systems (like GConf), GSettings does not restrict keys to basic types like strings and numbers. GSettings stores values as GVariant, and allows any GVariantType for keys. Key names are restricted to lowercase characters, numbers and -. Furthermore, the names must begin with a lowercase character, must not end with a -, and must not contain consecutive dashes.

Similar to GConf, the default values in GSettings schemas can be localized, but the localized values are stored in gettext catalogs and looked up with the domain that is specified in thegettext-domain attribute of the <schemalist> or <schema>elements and the category that is specified in the l10n attribute of the <default> element. The string which is translated includes all text in the <default> element, including any surrounding quotation marks.

The l10n attribute must be set to messages or time, and sets thelocale category for translation. The messages category should be used by default; use time for translatable date or time formats. A translation comment can be added as anXML comment immediately above the <default> element — it is recommended to add these comments to aid translators understand the meaning and implications of the default value. An optional translation contextattribute can be set on the <default> element to disambiguate multiple defaults which use the same string.

For example:

 <default l10n='messages' context='Banned words'>['bad', 'words']</default>

Translations of default values must remain syntactically valid serializedGVariants (e.g. retaining any surrounding quotation marks) or runtime errors will occur.

GSettings uses schemas in a compact binary form that is created by the glib-compile-schemasutility. The input is a schema description in an XML format.

A DTD for the gschema XML format can be found here:gschema.dtd

The glib-compile-schemas tool expects schema files to have the extension .gschema.xml.

At runtime, schemas are identified by their ID (as specified in theid attribute of the <schema> element). The convention for schema IDs is to use a dotted name, similar in style to a D-Bus bus name, e.g. org.gnome.SessionManager. In particular, if the settings are for a specific service that owns a D-Bus bus name, the D-Bus bus name and schema ID should match. For schemas which deal with settings not associated with one named application, the ID should not use StudlyCaps, e.g. org.gnome.font-rendering.

In addition to GVariant types, keys can have types that have enumerated types. These can be described by a <choice>,<enum> or <flags> element, as seen in the second example below. The underlying type of such a key is string, but you can use g_settings_get_enum(),g_settings_set_enum(), g_settings_get_flags(),g_settings_set_flags() access the numeric values corresponding to the string value of enum and flags keys.

An example for default value:

<key name="greeting" type="s">
  <default l10n="messages">"Hello, earthlings"</default>
  <summary>A greeting</summary>
  <description>
    Greeting of the invading martians
  </description>
</key>

<key name="box" type="(ii)">
  <default>(20,30)</default>
</key>

<key name="empty-string" type="s">
  <default>""</default>
  <summary>Empty strings have to be provided in GVariant form</summary>
</key>

An example for ranges, choices and enumerated types:

<key name="key-with-range" type="i">
  <range min="1" max="100"/>
  <default>10</default>
</key>

<key name="key-with-choices" type="s">
  <choices>
    <choice value='Elisabeth'/>
    <choice value='Annabeth'/>
    <choice value='Joe'/>
  </choices>
  <aliases>
    <alias value='Anna' target='Annabeth'/>
    <alias value='Beth' target='Elisabeth'/>
  </aliases>
  <default>'Joe'</default>
</key>

<key name='enumerated-key' enum='org.gtk.Test.myenum'>
  <default>'first'</default>
</key>

<key name='flags-key' flags='org.gtk.Test.myflags'>
  <default>["flag1","flag2"]</default>
</key>

Vendor overrides

Default values are defined in the schemas that get installed by an application. Sometimes, it is necessary for a vendor or distributor to adjust these defaults. Since patching the XML source for the schema is inconvenient and error-prone,glib-compile-schemas reads so-called ‘vendor override’ files. These are keyfiles in the same directory as the XMLschema sources which can override default values. The schema ID serves as the group name in the key file, and the values are expected in serialized GVariant form, as in the following example:

[org.gtk.Example] key1='string' key2=1.5

glib-compile-schemas expects schema files to have the extension.gschema.override.

Delay-apply mode

By default, values set on a GSettings instance immediately start to be written to the backend (although these writes may not complete by the time that g_settings_set()) returns; see g_settings_sync()).

In order to allow groups of settings to be changed simultaneously and atomically, GSettings also supports a ‘delay-apply’ mode. In this mode, updated values are kept locally in the GSettings instance until they are explicitly applied by calling g_settings_apply().

For example, this could be useful for a preferences dialog where the preferences all need to be applied simultaneously when the user clicks ‘Save’.

Switching a GSettings instance to ‘delay-apply’ mode is a one-time irreversible operation: from that point onwards, all changes made to thatGSettings have to be explicitly applied by callingg_settings_apply(). The ‘delay-apply’ mode is also propagated to any child settings objects subsequently created usingg_settings_get_child().

At any point, the set of unapplied changes can be queried usingGSettings:has-unapplied, and discarded by callingg_settings_revert().

Binding

A very convenient feature of GSettings lets you bind GObjectproperties directly to settings, using g_settings_bind(). Once aGObject property has been bound to a setting, changes on either side are automatically propagated to the other side. GSettings handles details like mapping between GObject and GVarianttypes, and preventing infinite cycles.

This makes it very easy to hook up a preferences dialog to the underlying settings. To make this even more convenient, GSettings looks for a boolean property with the name sensitivity and automatically binds it to the writability of the bound setting. If this ‘magic’ gets in the way, it can be suppressed with theG_SETTINGS_BIND_NO_SENSITIVITY flag.

Relocatable schemas

A relocatable schema is one with no path attribute specified on its<schema> element. By using g_settings_new_with_path(), a GSettingsobject can be instantiated for a relocatable schema, assigning a path to the instance. Paths passed to g_settings_new_with_path() will typically be constructed dynamically from a constant prefix plus some form of instance identifier; but they must still be valid GSettings paths. Paths could also be constant and used with a globally installed schema originating from a dependency library.

For example, a relocatable schema could be used to store geometry information for different windows in an application. If the schema ID wasorg.foo.MyApp.Window, it could be instantiated for paths/org/foo/MyApp/main/, /org/foo/MyApp/document-1/,/org/foo/MyApp/document-2/, etc. If any of the paths are well-known they can be specified as <child> elements in the parent schema, e.g.:

<schema id="org.foo.MyApp" path="/org/foo/MyApp/"> <child name="main" schema="org.foo.MyApp.Window"/> </schema>

Build system integration

Meson

GSettings is natively supported by Meson’s GNOME module.

You can install the schemas as any other data file:

install_data( 'org.foo.MyApp.gschema.xml', install_dir: get_option('datadir') / 'glib-2.0/schemas', )

You can use gnome.post_install() function to compile the schemas on installation:

gnome = import('gnome') gnome.post_install( glib_compile_schemas: true, )

If an enumerated type defined in a C header file is to be used in a GSettings schema, it can either be defined manually using an <enum> element in the schema XML, or it can be extracted automatically from the C header. This approach is preferred, as it ensures the two representations are always synchronised. To do so, you will need to use the gnome.mkenums() function with the following templates:

schemas_enums = gnome.mkenums('org.foo.MyApp.enums.xml', comments: '', fhead: '<schemalist>', vhead: ' <@type@ id="org.foo.MyApp.@EnumName@">', vprod: ' <value nick="@valuenick@" value="@valuenum@"/>', vtail: ' </@type@>', ftail: '</schemalist>', sources: enum_sources, install_header: true, install_dir: get_option('datadir') / 'glib-2.0/schemas', )

It is recommended to validate your schemas as part of the test suite for your application:

test('validate-schema', find_program('glib-compile-schemas'), args: ['--strict', '--dry-run', meson.current_source_dir()], )

If your application allows running uninstalled, you should also use thegnome.compile_schemas() function to compile the schemas in the current build directory:

Autotools

GSettings comes with autotools integration to simplify compiling and installing schemas. To add GSettings support to an application, add the following to your configure.ac:

In the appropriate Makefile.am, use the following snippet to compile and install the named schema:

`gsettings_SCHEMAS = org.foo.MyApp.gschema.xml EXTRA_DIST = $(gsettings_SCHEMAS)

@GSETTINGS_RULES@ `

gsettings_ENUM_NAMESPACE = org.foo.MyApp gsettings_ENUM_FILES = my-app-enums.h my-app-misc.h

gsettings_ENUM_NAMESPACE specifies the schema namespace for the enum files, which are specified in gsettings_ENUM_FILES. This will generate aorg.foo.MyApp.enums.xml file containing the extracted enums, which will be automatically included in the schema compilation, install and uninstall rules. It should not be committed to version control or included inEXTRA_DIST.

Localization

No changes are needed to the build system to mark a schema XML file for translation. Assuming it sets the gettext-domain attribute, a schema may be marked for translation by adding it to POTFILES.in, assuming gettext 0.19 or newer is in use (the preferred method for translation):

data/org.foo.MyApp.gschema.xml

Alternatively, if intltool 0.50.1 is in use:

[type: gettext/gsettings]data/org.foo.MyApp.gschema.xml

GSettings will use gettext to look up translations for the <summary> and<description> elements, and also any <default> elements which have al10n attribute set.

Translations must not be included in the .gschema.xml file by the build system, for example by using a rule to generate the XML file from a template.