An important aspect of xdg-app is application sandboxing, which will require application changes to use sandbox-specific APIs. However, xdg-app is also a good way to deploy and run non-sandboxed (or partially sandboxed) regular applications.
A very interesting usecase for this is to have an image-based operating system, for instance a Workstation spin of Fedora Atomic. Such a system would have a basic workstation installation with a read-only /usr, and atomic updates/rollback. However, installing an application is painful, and customizing yor install in that way undoes many of the advantages of an image-based OS.
With xdg-app you can install apps into /var (or $HOME) and have them fully integrate with the system, while still being isolated from changes to the host. This makes for a great combination, just like atomic + docker is a good combination for the server space.
I’ve spent some time recently making a prototype runtime based on the Fedora packages, as reported on the desktop list. This is kind of interesting as it lets you test applications from rawhide on fedora 21 or 22. Just install xdg-app from fedora-updates and then install the runtime:
$ xdg-app add-remote --no-gpg-verify --user fedora http://fedorapeople.org/~alexl/repo/ $ xdg-app install-runtime --user fedora org.fedoraproject.Platform 23
And then you can try gedit 3.17.0:
$ xdg-app install-app --user fedora org.gnome.gedit $ xdg-app run org.gnome.gedit
Or evince 3.17.2:
$ xdg-app install-app --user fedora org.gnome.evince $ xdg-app run org.gnome.evince
Once installed you can also just start them from the desktop environment as usual. They should be there like any regular application as the desktop files and icons are exported to the host.