Nice app man, I noticed that you're storing user data in cache directory, this doesn't seem right because an app like Optimizer can wipe the cache and erase it (which happened to me), I suggest that it should be saved directly on home or inside a directory for the application starting with an . to hide it as described here.
This is a bit obvious but worth noting that you may need to move the content from cache to home.
Or like other apps move to ~/.config/
You're right, seems that using ~/ to store data was the before freedesktop standard.
But I don't think ~/.config is the best place either because it is meant for configuration files so it isn't that safe also (backup tools might ignore these, users can delete it, etc.), the most appropriate place seems to be $XDG_DATA_HOME that is ~/.local/share/
Thank you very much I will take it into account in a next release
I found the database under ~/.var/app/com.github.alainm23.planner/data/com.github.alainm23.planner
has it been moved to ~/.local/share? I wish the app itself would give some kind of indication where data is stored, so it could be synced and backed up easily without having to google
Coming in the next release.
We have not made any changes to the database location yet but it will change for v2.4 thanks to @romixch. The idea is that the installation of Planner from App Center or Flathub have the same database location.
Database current location:
AppCenter
~/.local/share/com.github.alainm23.planner
Flatpak
~/.var/app/com.github.alainm23.planner/data/com.github.alainm23.planner

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You're right, seems that using
~/to store data was the before freedesktop standard.But I don't think
~/.configis the best place either because it is meant for configuration files so it isn't that safe also (backup tools might ignore these, users can delete it, etc.), the most appropriate place seems to be$XDG_DATA_HOMEthat is~/.local/share/