I used the sabre core libraries to create a slack-backed CardDAV server for a hackathon. Was super-fun to get it working, and the demo was quite powerful - you scan a QR code to OAuth via Slack, and download a iOS mobileconfig that adds the CardDAV profile to your iPhone.
Unfortunately, Android not supporting CardDAV made it a half-way solution, so it wasn't as seamless as you'd like. It was super fun to build using sabre, but it was limited by the CardDAV decisions. You can't make a read-only CardDAV server that easily, for eg - the client will optimistically make local changes, and assume that server is just temporarily down when it returns a non-2xx code.
I might still polish it and release it, since it was a super-cool tool, if only for iOS users.
Unfortunately, Android not supporting CardDAV made it a half-way solution, so it wasn't as seamless as you'd like. It was super fun to build using sabre, but it was limited by the CardDAV decisions. You can't make a read-only CardDAV server that easily, for eg - the client will optimistically make local changes, and assume that server is just temporarily down when it returns a non-2xx code.
I might still polish it and release it, since it was a super-cool tool, if only for iOS users.