<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-language-gnome">
 <title>GNOME</title>

 <section xml:id="ssec-gnome-packaging">
  <title>Packaging GNOME applications</title>

  <para>
   Programs in the GNOME universe are written in various languages but they all use GObject-based libraries like GLib, GTK or GStreamer. These libraries are often modular, relying on looking into certain directories to find their modules. However, due to Nix’s specific file system organization, this will fail without our intervention. Fortunately, the libraries usually allow overriding the directories through environment variables, either natively or thanks to a patch in nixpkgs. <link xlink:href="#fun-wrapProgram">Wrapping</link> the executables to ensure correct paths are available to the application constitutes a significant part of packaging a modern desktop application. In this section, we will describe various modules needed by such applications, environment variables needed to make the modules load, and finally a script that will do the work for us.
  </para>

  <section xml:id="ssec-gnome-settings">
   <title>Settings</title>

   <para>
    <link xlink:href="https://developer.gnome.org/gio/stable/GSettings.html">GSettings</link> API is often used for storing settings. GSettings schemas are required, to know the type and other metadata of the stored values. GLib looks for <filename>glib-2.0/schemas/gschemas.compiled</filename> files inside the directories of <envar>XDG_DATA_DIRS</envar>.
   </para>

   <para>
    On Linux, GSettings API is implemented using <link xlink:href="https://wiki.gnome.org/Projects/dconf">dconf</link> backend. You will need to add <literal>dconf</literal> GIO module to <envar>GIO_EXTRA_MODULES</envar> variable, otherwise the <literal>memory</literal> backend will be used and the saved settings will not be persistent.
   </para>

   <para>
    Last you will need the dconf database D-Bus service itself. You can enable it using <option>programs.dconf.enable</option>.
   </para>

   <para>
    Some applications will also require <package>gsettings-desktop-schemas</package> for things like reading proxy configuration or user interface customization. This dependency is often not mentioned by upstream, you should grep for <literal>org.gnome.desktop</literal> and <literal>org.gnome.system</literal> to see if the schemas are needed.
   </para>
  </section>

  <section xml:id="ssec-gnome-icons">
   <title>Icons</title>

   <para>
    When an application uses icons, an icon theme should be available in <envar>XDG_DATA_DIRS</envar> during runtime. The package for the default, icon-less <link xlink:href="https://www.freedesktop.org/wiki/Software/icon-theme/">hicolor-icon-theme</link> (should be propagated by every icon theme) contains <link linkend="ssec-gnome-hooks-hicolor-icon-theme">a setup hook</link> that will pick up icon themes from <literal>buildInputs</literal> and pass it to our wrapper. Unfortunately, relying on that would mean every user has to download the theme included in the package expression no matter their preference. For that reason, we leave the installation of icon theme on the user. If you use one of the desktop environments, you probably already have an icon theme installed.
   </para>

   <para>
    To avoid costly file system access when locating icons, GTK, <link xlink:href="https://woboq.com/blog/qicon-reads-gtk-icon-cache-in-qt57.html">as well as Qt</link>, can rely on <filename>icon-theme.cache</filename> files from the themes’ top-level directories. These files are generated using <command>gtk-update-icon-cache</command>, which is expected to be run whenever an icon is added or removed to an icon theme (typically an application icon into <literal>hicolor</literal> theme) and some programs do indeed run this after icon installation. However, since packages are installed into their own prefix by Nix, this would lead to conflicts. For that reason, <package>gtk3</package> provides a <link xlink:href="#ssec-gnome-hooks-gtk-drop-icon-theme-cache">setup hook</link> that will clean the file from installation. Since most applications only ship their own icon that will be loaded on start-up, it should not affect them too much. On the other hand, icon themes are much larger and more widely used so we need to cache them. Because we recommend installing icon themes globally, we will generate the cache files from all packages in a profile using a NixOS module. You can enable the cache generation using <option>gtk.iconCache.enable</option> option if your desktop environment does not already do that.
   </para>
  </section>

  <section xml:id="ssec-icon-theme-packaging">
    <title>Packaging icon themes</title>

    <para>
      Icon themes may inherit from other icon themes. The inheritance is specified using the <literal>Inherits</literal> key in the <filename>index.theme</filename> file distributed with the icon theme. According to the <link xlink:href="https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html">icon theme specification</link>, icons not provided by the theme are looked for in its parent icon themes. Therefore the parent themes should be installed as dependencies for a more complete experience regarding the icon sets used.
    </para>

    <para>
      The package <package>hicolor-icon-theme</package> provides a setup hook which makes symbolic links for the parent themes into the directory <filename>share/icons</filename> of the current theme directory in the nix store, making sure they can be found at runtime. For that to work the packages providing parent icon themes should be listed as propagated build dependencies, together with <package>hicolor-icon-theme</package>.
    </para>

    <para>
      Also make sure that <filename>icon-theme.cache</filename> is installed for each theme provided by the package, and set <code>dontDropIconThemeCache</code> to <code>true</code> so that the cache file is not removed by the <package>gtk3</package> setup hook.
    </para>

  </section>

  <section xml:id="ssec-gnome-themes">
   <title>GTK Themes</title>

   <para>
    Previously, a GTK theme needed to be in <envar>XDG_DATA_DIRS</envar>. This is no longer necessary for most programs since GTK incorporated Adwaita theme. Some programs (for example, those designed for <link xlink:href="https://elementary.io/docs/human-interface-guidelines#human-interface-guidelines">elementary HIG</link>) might require a special theme like <package>pantheon.elementary-gtk-theme</package>.
   </para>
  </section>

  <section xml:id="ssec-gnome-typelibs">
   <title>GObject introspection typelibs</title>

   <para>
    <link xlink:href="https://wiki.gnome.org/Projects/GObjectIntrospection">GObject introspection</link> allows applications to use C libraries in other languages easily. It does this through <literal>typelib</literal> files searched in <envar>GI_TYPELIB_PATH</envar>.
   </para>
  </section>

  <section xml:id="ssec-gnome-plugins">
   <title>Various plug-ins</title>

   <para>
    If your application uses <link xlink:href="https://gstreamer.freedesktop.org/">GStreamer</link> or <link xlink:href="https://wiki.gnome.org/Projects/Grilo">Grilo</link>, you should set <envar>GST_PLUGIN_SYSTEM_PATH_1_0</envar> and <envar>GRL_PLUGIN_PATH</envar>, respectively.
   </para>
  </section>
 </section>

 <section xml:id="ssec-gnome-hooks">
  <title>Onto <package>wrapGAppsHook</package></title>

  <para>
   Given the requirements above, the package expression would become messy quickly:
<programlisting>
preFixup = ''
  for f in $(find $out/bin/ $out/libexec/ -type f -executable); do
    wrapProgram "$f" \
      --prefix GIO_EXTRA_MODULES : "${getLib dconf}/lib/gio/modules" \
      --prefix XDG_DATA_DIRS : "$out/share" \
      --prefix XDG_DATA_DIRS : "$out/share/gsettings-schemas/${name}" \
      --prefix XDG_DATA_DIRS : "${gsettings-desktop-schemas}/share/gsettings-schemas/${gsettings-desktop-schemas.name}" \
      --prefix XDG_DATA_DIRS : "${hicolor-icon-theme}/share" \
      --prefix GI_TYPELIB_PATH : "${lib.makeSearchPath "lib/girepository-1.0" [ pango json-glib ]}"
  done
'';
</programlisting>
   Fortunately, there is <package>wrapGAppsHook</package>, that does the wrapping for us. In particular, it works in conjunction with other setup hooks that will populate the variable:
   <itemizedlist>
    <listitem xml:id="ssec-gnome-hooks-wrapgappshook">
     <para>
      <package>wrapGAppsHook</package> itself will add the package’s <filename>share</filename> directory to <envar>XDG_DATA_DIRS</envar>.
     </para>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-glib">
     <para>
      <package>glib</package> setup hook will populate <envar>GSETTINGS_SCHEMAS_PATH</envar> and then <package>wrapGAppsHook</package> will prepend it to <envar>XDG_DATA_DIRS</envar>.
     </para>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-gtk-drop-icon-theme-cache">
     <para>
      One of <package>gtk3</package>’s setup hooks will remove <filename>icon-theme.cache</filename> files from package’s icon theme directories to avoid conflicts. Icon theme packages should prevent this with <code>dontDropIconThemeCache = true;</code>.
     </para>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-dconf">
     <para>
      <package>dconf.lib</package> is a dependency of <package>wrapGAppsHook</package>, which then also adds it to the <envar>GIO_EXTRA_MODULES</envar> variable.
     </para>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-hicolor-icon-theme">
     <para>
      <package>hicolor-icon-theme</package>’s setup hook will add icon themes to <envar>XDG_ICON_DIRS</envar> which is prepended to <envar>XDG_DATA_DIRS</envar> by <package>wrapGAppsHook</package>.
     </para>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-gobject-introspection">
     <para>
      <package>gobject-introspection</package> setup hook populates <envar>GI_TYPELIB_PATH</envar> variable with <filename>lib/girepository-1.0</filename> directories of dependencies, which is then added to wrapper by <package>wrapGAppsHook</package>. It also adds <filename>share</filename> directories of dependencies to <envar>XDG_DATA_DIRS</envar>, which is intended to promote GIR files but it also <link xlink:href="https://github.com/NixOS/nixpkgs/issues/32790">pollutes the closures</link> of packages using <package>wrapGAppsHook</package>.
     </para>
     <warning>
      <para>
       The setup hook <link xlink:href="https://github.com/NixOS/nixpkgs/issues/56943">currently</link> does not work in expressions with <literal>strictDeps</literal> enabled, like Python packages. In those cases, you will need to disable it with <code>strictDeps = false;</code>.
      </para>
     </warning>
    </listitem>
    <listitem xml:id="ssec-gnome-hooks-gst-grl-plugins">
     <para>
      Setup hooks of <package>gst_all_1.gstreamer</package> and <package>gnome3.grilo</package> will populate the <envar>GST_PLUGIN_SYSTEM_PATH_1_0</envar> and <envar>GRL_PLUGIN_PATH</envar> variables, respectively, which will then be added to the wrapper by <literal>wrapGAppsHook</literal>.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   You can also pass additional arguments to <literal>makeWrapper</literal> using <literal>gappsWrapperArgs</literal> in <literal>preFixup</literal> hook:
<programlisting>
preFixup = ''
  gappsWrapperArgs+=(
    # Thumbnailers
    --prefix XDG_DATA_DIRS : "${gdk-pixbuf}/share"
    --prefix XDG_DATA_DIRS : "${librsvg}/share"
    --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"
  )
'';
</programlisting>
  </para>
 </section>

 <section xml:id="ssec-gnome-updating">
  <title>Updating GNOME packages</title>

  <para>
   Most GNOME package offer <link linkend="var-passthru-updateScript"><literal>updateScript</literal></link>, it is therefore possible to update to latest source tarball by running <command>nix-shell maintainers/scripts/update.nix --argstr package gnome3.nautilus</command> or even en masse with <command>nix-shell maintainers/scripts/update.nix --argstr path gnome3</command>. Read the package’s <filename>NEWS</filename> file to see what changed.
  </para>
 </section>

 <section xml:id="ssec-gnome-common-issues">
  <title>Frequently encountered issues</title>

  <variablelist>
   <varlistentry xml:id="ssec-gnome-common-issues-no-schemas">
    <term>
     <computeroutput>GLib-GIO-ERROR **: <replaceable>06:04:50.903</replaceable>: No GSettings schemas are installed on the system</computeroutput>
    </term>
    <listitem>
     <para>
      There are no schemas avalable in <envar>XDG_DATA_DIRS</envar>. Temporarily add a random package containing schemas like <package>gsettings-desktop-schemas</package> to <literal>buildInputs</literal>. <link linkend="ssec-gnome-hooks-glib"><package>glib</package></link> and <link linkend="ssec-gnome-hooks-wrapgappshook"><package>wrapGAppsHook</package></link> setup hooks will take care of making the schemas available to application and you will see the actual missing schemas with the <link linkend="ssec-gnome-common-issues-missing-schema">next error</link>. Or you can try looking through the source code for the actual schemas used.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id="ssec-gnome-common-issues-missing-schema">
    <term>
     <computeroutput>GLib-GIO-ERROR **: <replaceable>06:04:50.903</replaceable>: Settings schema ‘<replaceable>org.gnome.foo</replaceable>’ is not installed</computeroutput>
    </term>
    <listitem>
     <para>
      Package is missing some GSettings schemas. You can find out the package containing the schema with <command>nix-locate <replaceable>org.gnome.foo</replaceable>.gschema.xml</command> and let the hooks handle the wrapping as <link linkend="ssec-gnome-common-issues-no-schemas">above</link>.
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id="ssec-gnome-common-issues-double-wrapped">
    <term>
     When using <package>wrapGAppsHook</package> with special derivers you can end up with double wrapped binaries.
    </term>
    <listitem>
     <para>
      This is because derivers like <function>python.pkgs.buildPythonApplication</function> or <function>qt5.mkDerivation</function> have setup-hooks automatically added that produce wrappers with <package>makeWrapper</package>. The simplest way to workaround that is to disable the <package>wrapGAppsHook</package> automatic wrapping with <code>dontWrapGApps = true;</code> and pass the arguments it intended to pass to <package>makeWrapper</package> to another.
     </para>
     <para>
      In the case of a Python application it could look like:
<programlisting>
python3.pkgs.buildPythonApplication {
  pname = "gnome-music";
  version = "3.32.2";

  nativeBuildInputs = [
    wrapGAppsHook
    gobject-introspection
    ...
  ];

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by buildPython*
  preFixup = ''
    makeWrapperArgs+=("''${gappsWrapperArgs[@]}")
  '';
}
</programlisting>
      And for a QT app like:
<programlisting>
mkDerivation {
  pname = "calibre";
  version = "3.47.0";

  nativeBuildInputs = [
    wrapGAppsHook
    qmake
    ...
  ];

  dontWrapGApps = true;

  # Arguments to be passed to `makeWrapper`, only used by qt5’s mkDerivation
  preFixup = ''
    qtWrapperArgs+=("''${gappsWrapperArgs[@]}")
  '';
}
</programlisting>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id="ssec-gnome-common-issues-unwrappable-package">
    <term>
     I am packaging a project that cannot be wrapped, like a library or GNOME Shell extension.
    </term>
    <listitem>
     <para>
      You can rely on applications depending on the library setting the necessary environment variables but that is often easy to miss. Instead we recommend to patch the paths in the source code whenever possible. Here are some examples:
      <itemizedlist>
       <listitem xml:id="ssec-gnome-common-issues-unwrappable-package-gnome-shell-ext">
        <para>
         <link xlink:href="https://github.com/NixOS/nixpkgs/blob/7bb8f05f12ca3cff9da72b56caa2f7472d5732bc/pkgs/desktops/gnome-3/core/gnome-shell-extensions/default.nix#L21-L24">Replacing a <envar>GI_TYPELIB_PATH</envar> in GNOME Shell extension</link> – we are using <function>substituteAll</function> to include the path to a typelib into a patch.
        </para>
       </listitem>
       <listitem xml:id="ssec-gnome-common-issues-unwrappable-package-gsettings">
        <para>
         The following examples are hardcoding GSettings schema paths. To get the schema paths we use the functions
         <itemizedlist>
          <listitem>
           <para>
            <function>glib.getSchemaPath</function> Takes a nix package attribute as an argument.
           </para>
          </listitem>
          <listitem>
           <para>
            <function>glib.makeSchemaPath</function> Takes a package output like <literal>$out</literal> and a derivation name. You should use this if the schemas you need to hardcode are in the same derivation.
           </para>
          </listitem>
         </itemizedlist>
        </para>
        <para xml:id="ssec-gnome-common-issues-unwrappable-package-gsettings-vala">
         <link xlink:href="https://github.com/NixOS/nixpkgs/blob/7bb8f05f12ca3cff9da72b56caa2f7472d5732bc/pkgs/desktops/pantheon/apps/elementary-files/default.nix#L78-L86">Hard-coding GSettings schema path in Vala plug-in (dynamically loaded library)</link> – here, <function>substituteAll</function> cannot be used since the schema comes from the same package preventing us from pass its path to the function, probably due to a <link xlink:href="https://github.com/NixOS/nix/issues/1846">Nix bug</link>.
        </para>
        <para xml:id="ssec-gnome-common-issues-unwrappable-package-gsettings-c">
         <link xlink:href="https://github.com/NixOS/nixpkgs/blob/29c120c065d03b000224872251bed93932d42412/pkgs/development/libraries/glib-networking/default.nix#L31-L34">Hard-coding GSettings schema path in C library</link> – nothing special other than using <link xlink:href="https://github.com/NixOS/nixpkgs/pull/67957#issuecomment-527717467">Coccinelle patch</link> to generate the patch itself.
        </para>
       </listitem>
      </itemizedlist>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry xml:id="ssec-gnome-common-issues-weird-location">
    <term>
     I need to wrap a binary outside <filename>bin</filename> and <filename>libexec</filename> directories.
    </term>
    <listitem>
     <para>
      You can manually trigger the wrapping with <function>wrapGApp</function> in <literal>preFixup</literal> phase. It takes a path to a program as a first argument; the remaining arguments are passed directly to <function xlink:href="#fun-wrapProgram">wrapProgram</function> function.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
</section>
