AFAICT, XDG_DOCUMENTS_DIR isn't set for Sailfish OS apps. Besides, when those are tested for compliance with Jolla Store rules, testers insist that ~/Documents is the place where data files should be stored.

In any case it is what it is, and moving the files isn't worth the trouble at this point, I think.

And since foil.key file is shared by all Foil apps - that one certainly isn't going anywhere.

Well, most apps, including Sailfish apps or more to the point, Jolla apps, obviously use the XDG user directories, because none of them has created a ~/Documents folder until now, but used the ones set by the user under ~/.config/user-dirs.dirs.

You should look into it more closely.

Aha, I didn't know that we do have ~/.config/user-dirs.dirs on Sailfish OS. Anyway, it says XDG_DOCUMENTS_DIR="$HOME/Documents" so apparently I'm doing the right thing. It might make sense to use Qt or glib API to fetch that path instead of hardcoding it but functionally it won't change anything - data files will still go to ~/Documents.

XDG_DOCUMENTS_DIR="$HOME/Documents" is just the default value, and it's in no way the recommended one -- most users I know change those values first chance they get. You're right, using Qt for that does make more sense, as it's probably the way Jolla does it for their apps as well. And data files should go to whatever folder the user has set for documents, don't you think. Why have two folders for documents when you don't need to, its an additional nuisance.

~/Documents/FoilAuth is not a directory where a user is supposed to drop arbitrary files and expect something reasonable to happen. It's a representation of app's internal data model - it could be e.g. an sqlite database or a single file or whatever, it's just a format that I chose. One user, one app, one model, one representation in the internal storage. I don't see any reason to complicate that, really. It's not a desktop where you can start multiple instances of the app, open different files in each instance, etc.

... It's a representation of app's internal data model ...

XDG_DOCUMENTS_DIR is a "well known" user directory (https://www.freedesktop.org/wiki/Software/xdg-user-dirs/) therefore to not be in the way of the user I suggest ~/.config/foilauth/ (https://www.freedesktop.org/software/systemd/man/file-hierarchy.html). Maybe foil/foil.key should be placed also there, thought ...