[Unit]
Description=Auto lock
PartOf=graphical-session.target

[Service]
Type=simple
# You must arrange to run `systemctl --user import-environment
# XDG_SESSION_ID` in your X environment prior to launching the user
# session, e.g. in your `.xsessionrc` or similar.
ExecStart=/usr/bin/xss-lock -l -s ${XDG_SESSION_ID} -- i3lock

[Install]
WantedBy=graphical-session.target