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docs: Move migrating-gconf.xml to Markdown

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Matthias Clasen 2023-10-09 23:20:38 +01:00 committed by Philip Withnall
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1
docs/reference/gio/gio-docs.xml
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@ -271,7 +271,6 @@
<title>Migrating to GIO</title> <title>Migrating to GIO</title>
<xi:include href="xml/migrating-posix.xml"/> <xi:include href="xml/migrating-posix.xml"/>
<xi:include href="xml/migrating-gnome-vfs.xml"/> <xi:include href="xml/migrating-gnome-vfs.xml"/>
<xi:include href="xml/migrating-gconf.xml"/>
<xi:include href="xml/migrating-gdbus.xml"/> <xi:include href="xml/migrating-gdbus.xml"/>
</part> </part>

1
docs/reference/gio/gio.toml.in
View File

@ -40,6 +40,7 @@ show_class_hierarchy = true
urlmap_file = "urlmap.js" urlmap_file = "urlmap.js"
# The same order will be used when generating the index # The same order will be used when generating the index
content_files = [ content_files = [
"migrating-gconf.md",
] ]
content_images = [ content_images = [
"menu-example.png", "menu-example.png",

3
docs/reference/gio/meson.build
View File

@ -164,7 +164,6 @@ if get_option('gtk_doc')
'overview.xml', 'overview.xml',
'migrating-posix.xml', 'migrating-posix.xml',
'migrating-gnome-vfs.xml', 'migrating-gnome-vfs.xml',
'migrating-gconf.xml',
'migrating-gdbus.xml', 'migrating-gdbus.xml',
'gio-querymodules.xml', 'gio-querymodules.xml',
'glib-compile-schemas.xml', 'glib-compile-schemas.xml',
@ -201,7 +200,6 @@ if get_option('gtk_doc')
'overview.xml', 'overview.xml',
'migrating-posix.xml', 'migrating-posix.xml',
'migrating-gnome-vfs.xml', 'migrating-gnome-vfs.xml',
'migrating-gconf.xml',
'migrating-gdbus.xml', 'migrating-gdbus.xml',
'gdbus-codegen.xml', 'gdbus-codegen.xml',
], ],
@ -237,6 +235,7 @@ endif
# gi-docgen version # gi-docgen version
expand_content_files = [ expand_content_files = [
'migrating-gconf.md',
] ]
gio_gir = meson.current_source_dir() / 'Gio-2.0.gir' gio_gir = meson.current_source_dir() / 'Gio-2.0.gir'

466
docs/reference/gio/migrating-gconf.md Normal file
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@ -0,0 +1,466 @@
Title: Migrating from GConf to GSettings
SPDX-License-Identifier: LGPL-2.1-or-later
SPDX-FileCopyrightText: 2010, 2012 Matthias Clasen
SPDX-FileCopyrightText: 2010 Allison Lortie
SPDX-FileCopyrightText: 2011 Ray Strode
# 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 [class@Gio.Settings] object. While not every
`GConfClient` function has a direct GSettings equivalent, many do:
| GConfClient | GSettings |
|-------------|-----------|
| `gconf_client_get_default()` | no direct equivalent, instead you call [`ctor@Gio.Settings.new`] for the schemas you use |
| `gconf_client_set()` | [`method@Gio.Settings.set`] |
| `gconf_client_get()` | `g_settings_get()` |
| `gconf_client_get_bool()` | `g_settings_get_boolean()` |
| `gconf_client_set_bool()` | `g_settings_set_boolean()` |
| `gconf_client_get_int()` | `g_settings_get_int()` |
| `gconf_client_set_int()` | `g_settings_set_int()` |
| `gconf_client_get_float()` | `g_settings_get_double()` |
| `gconf_client_set_float()` | `g_settings_set_double()` |
| `gconf_client_get_string()` | `g_settings_get_string()` |
| `gconf_client_set_string()` | `g_settings_set_string()` |
| `gconf_client_get_list()` | for string lists, see `g_settings_get_strv()`, else see `g_settings_get_value()` and GVariant API |
| `gconf_client_set_list()` | for string lists, see `g_settings_set_strv()`, else see `g_settings_set_value()` and GVariant API |
| `gconf_entry_get_is_writable()` | `g_settings_is_writable()` |
| `gconf_client_notify_add()` | not required, the “changed” signal is emitted automatically |
| `gconf_client_add_dir()` | not required, each GSettings instance automatically watches all keys in its path |
| `GConfChangeSet` | `g_settings_delay()`, `g_settings_apply()` |
| `gconf_client_get_default_from_schema()` | no equivalent, applications are expected to know their schema |
| `gconf_client_all_entries()` | no equivalent, applications are expected to know their schema, and GSettings does not allow schema-less entries |
| `gconf_client_get_without_default()` | no equivalent |
| `gconf_bridge_bind_property()` | `g_settings_bind()` |
| `gconf_bridge_bind_property_full()` | `g_settings_bind_with_mapping()` |
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 [signal@Gio.Settings::changed] signal is emitted for each changed key.
There is also a [`signal@Gio.Settings::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
[signal@Gio.Settings::writable-changed] signal (and the
[signal@Gio.Settings::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 [class@Gio.Settings]
object by calling [method@Gio.Settings.delay]. In this mode, changes done to
the GSettings object are not applied - they are still visible when calling
[method@Gio.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 [method@Gio.Settings.apply]. To revert the pending changes, call
[method@Gio.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
destop_gnome_font_rendering.schemas` on the following
`desktop_gnome_font_rendering.schemas` file:
```xml
<?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:
```xml
<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:
```py
#!/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.
```xml
<type>string</type>
<default>rgb</default>
```
becomes
```xml
<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
```xml
<key>/schemas/desktop/gnome/font_rendering/antialiasing</key>
```
becomes
```xml
<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,
```xml
<key>/schemas/apps/my_app/font_size</key>
<locale name="C">
<default>18</default>
</locale>
<locale name="be">
<default>24</default>
</locale>
</key>
```
becomes
```xml
<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
```c
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.
```c
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.

515
docs/reference/gio/migrating-gconf.xml
View File

@ -1,515 +0,0 @@
<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.
</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 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
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.
</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. Since intltool 0.50.1, schema files are
supported, so all you have to do is to add your .gschema.xml
files to <filename>POTFILES.in</filename> with a line like
<programlisting>
[type: gettext/gsettings]data/org.foo.MyApp.gschema.xml
</programlisting>
</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>