To avoid costly file system access when locating icons, GTK, [as well as Qt](https://woboq.com/blog/qicon-reads-gtk-icon-cache-in-qt57.html), can rely on `icon-theme.cache` files from the themes' top-level directories. These files are generated using `gtk-update-icon-cache`, which is expected to be run whenever an icon is added or removed to an icon theme (typically an application icon into `hicolor` 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, `gtk3` provides a [setup hook](#ssec-gnome-hooks-gtk-drop-icon-theme-cache) 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 `gtk.iconCache.enable` option if your desktop environment does not already do that.

### Packaging icon themes {#ssec-icon-theme-packaging}

Icon themes may inherit from other icon themes. The inheritance is specified using the `Inherits` key in the `index.theme` file distributed with the icon theme. According to the [icon theme specification](https://specifications.freedesktop.org/icon-theme-spec/latest), 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.

The package `hicolor-icon-theme` provides a setup hook which makes symbolic links for the parent themes into the directory `share/icons` 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 `hicolor-icon-theme`.

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

### GTK Themes {#ssec-gnome-themes}

Previously, a GTK theme needed to be in `XDG_DATA_DIRS`. This is no longer necessary for most programs since GTK incorporated Adwaita theme. Some programs (for example, those designed for [elementary HIG](https://docs.elementary.io/hig)) might require a special theme like `pantheon.elementary-gtk-theme`.

### GObject introspection typelibs {#ssec-gnome-typelibs}

[GObject introspection](https://gitlab.gnome.org/GNOME/gobject-introspection) allows applications to use C libraries in other languages easily. It does this through `typelib` files searched in `GI_TYPELIB_PATH`.

### Various plug-ins {#ssec-gnome-plugins}

If your application uses [GStreamer](https://gstreamer.freedesktop.org/) or [Grilo](https://gitlab.gnome.org/GNOME/grilo), you should set `GST_PLUGIN_SYSTEM_PATH_1_0` and `GRL_PLUGIN_PATH`, respectively.

## Onto `wrapGApps*` hooks {#ssec-gnome-hooks}

Given the requirements above, the package expression would become messy quickly:

```nix
{
  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
  '';
}
```

Fortunately, we have a [family of hooks]{#ssec-gnome-hooks-wrapgappshook} that automate this. They work in conjunction with other setup hooks that populate environment variables, and will then wrap all executables in `bin` and `libexec` directories using said variables.