A portal frontend service for Flatpak and possibly other desktop containment frameworks.
xdg-desktop-portal works by exposing a series of D-Bus interfaces known as portals under a well-known name (org.freedesktop.portal.Desktop) and object path (/org/freedesktop/portal/desktop).
The portal interfaces include APIs for file access, opening URIs, printing and others.
Documentation for the available D-Bus interfaces can be found here.
xdg-desktop-portal depends on GLib and Flatpak. To build the documentation, you will need xmlto and the docbook stylesheets.
Flatpak grants sandboxed applications talk access to names in the org.freedesktop.portal.* prefix. One possible way to use the portal APIs is thus just to make D-Bus calls. For many of the portals, toolkits (e.g. GTK+) are expected to support portals transparently if you use suitable high-level APIs.
To actually use most portals, xdg-desktop-portal relies on a backend that provides implementations of the org.freedesktop.impl.portal.* interfaces. One such backend is provided by xdg-desktop-portal-gtk. Another one is in development here: xdg-desktop-portal-kde.