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doc/gnome: document GIO modules

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In particular, that glib-networking is required for TLS support.
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Jan Tojnar 2021-06-11 14:04:06 +02:00
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doc/languages-frameworks/gnome.section.md
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@ -8,12 +8,30 @@ Programs in the GNOME universe are written in various languages but they all use
[GSettings](https://developer.gnome.org/gio/stable/GSettings.html) 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 `glib-2.0/schemas/gschemas.compiled` files inside the directories of `XDG_DATA_DIRS`.
On Linux, GSettings API is implemented using [dconf](https://wiki.gnome.org/Projects/dconf) backend. You will need to add `dconf` GIO module to `GIO_EXTRA_MODULES` variable, otherwise the `memory` backend will be used and the saved settings will not be persistent.
On Linux, GSettings API is implemented using [dconf](https://wiki.gnome.org/Projects/dconf) backend. You will need to add `dconf` [GIO module](#ssec-gnome-gio-modules) to `GIO_EXTRA_MODULES` variable, otherwise the `memory` backend will be used and the saved settings will not be persistent.
Last you will need the dconf database D-Bus service itself. You can enable it using `programs.dconf.enable`.
Some applications will also require `gsettings-desktop-schemas` for things like reading proxy configuration or user interface customization. This dependency is often not mentioned by upstream, you should grep for `org.gnome.desktop` and `org.gnome.system` to see if the schemas are needed.
### GIO modules {#ssec-gnome-gio-modules}
GLib’s [GIO](https://developer.gnome.org/gio/stable/ch01.html) library supports several [extension points](https://developer.gnome.org/gio/stable/extending-gio.html). Notably, they allow:
* implementing settings backends (already [mentioned](#ssec-gnome-settings))
* adding TLS support
* proxy settings
* virtual file systems
The modules are typically installed to `lib/gio/modules/` directory of a package and you need to add them to `GIO_EXTRA_MODULES` if you need any of those features.
In particular, we recommend:
* adding `dconf.lib` for any software on Linux that reads [GSettings](#ssec-gnome-settings) (even transitivily through e.g. GTK’s file manager)
* adding `glib-networking` for any software that accesses network using GIO or libsoup – glib-networking contains a module that implements TLS support and loads system-wide proxy settings
To allow software to use various virtual file systems, `gvfs` package can be also added. But that is usually an optional feature so we typically use `gvfs` from the system (e.g. installed globally using NixOS module).
### GdkPixbuf loaders {#ssec-gnome-gdk-pixbuf-loaders}
GTK applications typically use [GdkPixbuf](https://developer.gnome.org/gdk-pixbuf/stable/) to load images. But `gdk-pixbuf` package only supports basic bitmap formats like JPEG, PNG or TIFF, requiring to use third-party loader modules for other formats. This is especially painful since GTK itself includes SVG icons, which cannot be rendered without a loader provided by `librsvg`.