glib/docs/reference/gio/migrating.xml

<part id="migrating">
  <title>Migrating to GIO</title>

  <chapter>
    <title>Migrating from POSIX to GIO</title>
  
    <table id="posix-vs-gio">
      <title>Comparison of POSIX and GIO concepts</title>
      <tgroup cols="2">
        <thead>
          <row><entry>POSIX</entry><entry>GIO</entry></row>
        </thead>
        <tbody>
          <row><entry>char *path</entry><entry>GFile *file</entry></row>
          <row><entry>struct stat *buf</entry><entry>GFileInfo *info</entry></row>
          <row><entry>struct statvfs *buf</entry><entry>GFileInfo *info</entry></row>
          <row><entry morerows="1">int fd</entry><entry>GInputStream *in</entry></row>
          <row><entry>GOutputStream *out</entry></row>
          <row><entry>DIR *</entry><entry>GFileEnumerator *enum</entry></row>
          <row><entry>fstab entry</entry><entry>GUnixMountPoint *mount_point</entry></row>
          <row><entry>mtab entry</entry><entry>GUnixMountEntry *mount_entry</entry></row>
        </tbody>
      </tgroup>
    </table>  

  </chapter>

  <chapter>
    <title>Migrating from GnomeVFS to GIO</title>

    <table id="gnome-vfs-vs-gio">
      <title>Comparison of GnomeVFS and GIO concepts</title>
      <tgroup cols="2">
        <thead>
          <row><entry>GnomeVFS</entry><entry>GIO</entry></row>
        </thead>
        <tbody>
          <row><entry>GnomeVFSURI</entry><entry>GFile</entry></row>
          <row><entry>GnomeVFSFileInfo</entry><entry>GFileInfo</entry></row>
          <row><entry>GnomeVFSResult</entry><entry>GError, with G_IO_ERROR values</entry></row>
          <row><entry>GnomeVFSHandle &amp; GnomeVFSAsyncHandle</entry><entry>GInputStream or GOutputStream</entry></row>
          <row><entry>GnomeVFSDirectoryHandle</entry><entry>GFileEnumerator</entry></row>
          <row><entry>mime type</entry><entry>content type</entry></row>
          <row><entry>GnomeVFSMonitor</entry><entry>GFileMonitor</entry></row>
          <row><entry>GnomeVFSVolumeMonitor</entry><entry>GVolumeMonitor</entry></row>
          <row><entry>GnomeVFSVolume</entry><entry>GMount</entry></row>
          <row><entry>GnomeVFSDrive</entry><entry>GVolume</entry></row>
          <row><entry>-</entry><entry>GDrive</entry></row>
          <row><entry>GnomeVFSContext</entry><entry>GCancellable</entry></row>
          <row><entry>gnome_vfs_async_cancel</entry><entry>g_cancellable_cancel</entry></row>
        </tbody>
      </tgroup>
    </table>

    <section>
      <title>Trash handling</title>

      <para>
        The handling of trashed files has been changed in GIO, compared
        to gnome-vfs. gnome-vfs has a home-grown trash implementation that 
        predates the freedesktop.org <ulink url="http://www.freedesktop.org/wiki/Specifications/trash-spec">Desktop Trash Can</ulink> specification
        that is implemented in GIO. The location for storing trashed files 
        has changed from <filename>$HOME/.Trash</filename> to 
        <filename>$HOME/.local/share/Trash</filename> (or more correctly
        <filename>$XDG_DATA_HOME/Trash</filename>), which means that 
        there is a need for migrating files that have been trashed by 
        gnome-vfs to the new location.
      </para>
      <para>
        In gnome-vfs, the <filename>trash://</filename> scheme offering a 
        merged view of all trash directories was implemented in nautilus,
        and trash-handling applications had to find and monitor all trash 
        directories themselves. With GIO, the <filename>trash://</filename>
        implementation has been moved to gvfs and applications can simply
        monitor that location:
      </para>
<informalexample><programlisting>
static void
file_changed (GFileMonitor      *file_monitor,
              GFile             *child,
              GFile             *other_file,
              GFileMonitorEvent  event_type,
              gpointer           user_data)
{
  switch (event_type)
  {
  case G_FILE_MONITOR_EVENT_DELETED:
    g_print ("'%s' removed from trash\n", g_file_get_basename (child));
    break;
  case G_FILE_MONITOR_EVENT_CREATED:
    g_print ("'%s' added to trash\n", g_file_get_basename (child));
    break;
  default: ;
  }
}

static void
start_monitoring_trash (void)
{
  GFile *file;
  GFileMonitor *monitor;

  file = g_file_new_for_uri ("trash://");
  monitor = g_file_monitor_directory (file, 0, NULL, NULL);
  g_object_unref (file);

  g_signal_connect (monitor, "changed", G_CALLBACK (file_changed), NULL);

  /* ... */

}       
</programlisting></informalexample> 
      <para>
        GIO exposes some useful metadata about trashed files. There are
        trash::orig-path and trash::deletion-date attributes. The 
        standard::icon attribute of the <filename>trash://</filename> 
        itself provides a suitable icon for displaying the trash can on 
        the desktop. If you are using this icon, make sure to monitor
        this attribute for changes, since the icon may be updated to
        reflect that state of the trash can.
      </para>
      <para>
        Moving a file to the trash is much simpler with GIO. Instead of
        using gnome_vfs_find_directory() with %GNOME_VFS_DIRECTORY_KIND_TRASH 
        to find out where to move the trashed file, just use the g_file_trash()
        function.
      </para>
    </section>

    <section>
      <title>Operations on multiple files</title>

      <para>
        gnome-vfs has the dreaded gnome_vfs_xfer_uri_list() function which
        has tons of options and offers the equivalent of cp, mv, ln, mkdir
        and rm at the same time. 
      </para>
      <para>
        GIO offers a much simpler I/O scheduler functionality instead, that
        lets you schedule a function to be called in a separate thread, or
        if threads are not available, as an idle in the mainloop.
        See g_io_scheduler_push_job(). 
      </para>

    </section>

    <section>
      <title>Mime monitoring</title>

      <para>
        gnome-vfs offered a way to monitor the association between mime types
        and default handlers for changes, with the #GnomeVFSMIMEMonitor object.
        GIO does not offer a replacement for this functionality at this time,
        since we have not found a compelling use case where
        #GnomeVFSMIMEMonitor was used. If you think you have such a use
        case, please report it at
        <ulink url="http://bugzilla.gnome.org">bugzilla.gnome.org</ulink>.
      </para>
    </section>
  </chapter>

  <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 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_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>
            </tbody>
          </tgroup>
        </table>
      </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>
        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, so
        <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[
<key name="font-size" type="i">
  <default l10n="messages" context="font_size">18</default>
</key>
]]>
        </programlisting>
        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.
      </para>
      <para>
        GSettings schemas have more stringent restrictions on key names
        than GConf. Key names in GSettings are restricted to at most 32
        characters, 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 '-'.
        The <link linkend="gschema-compile">gschema-compile</link> schema
        compiler has a <option>--allow-any-name</option> that lets you
        ignore these restrictions. 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>
      <para>
        GIO 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>
    </section>

    <section><title>Data conversion</title>
      <para>
        GConf 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 GSetting. <command>gsettings-data-convert</command> can be run
        manually, but it is designed to be run automatically, every time a user
        logs in. It keeps track of the conversion 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
        <filename>/usr/share/gsettings-data-convert</filename> which lists the GSettings
        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]
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 make the corresponding adjustments.
      </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>
    </section>
  </chapter>

</part>