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[X11] Locking on suspend with xlock and systemd

In the file /etc/systemd/system/xlock@.service, put the following. 

[Unit]
Description=Lock X session using xlock for user %i
Before=sleep.target

[Service]
User=%i
Environment=DISPLAY=:0
ExecStartPre=/usr/bin/xset dpms force suspend
ExecStart=/usr/bin/xlock

[Install]
WantedBy=sleep.target

Now, to activate locking on suspend, do systemctl enable xlock@user where user is your user.

Note: Pretty much the same as the Archlinux wiki's slock entry, but for xlock.