After I upgraded to Ubuntu 15.04 Vivid Vervet, the Audacious plugin I used to control a display device I built had a problem. Functionality related to sleep and wake didn't work anymore. That's because UPower doesn't handle sleep and wake anymore. Notification events instead come from systemd-logind. In particular, it is the "PrepareForSleep" signal on the "org.freedesktop.login1.Manager" interface. When its argument is true, the system is preparing to go to sleep, and when it's false, the system is waking up. Here's the new code.
I still need to find a way to determine the last wake time. Formerly I was using the /var/log/pm-suspend.log modification time, but that is also now unavailable because systemd is handling it instead. If the plugin observes a wakeup, it gets the correct time from that, but when started it needs another method.
This illustrates a major difference between Windows and Linux. The Windows version of the plugin can still use the WM_POWERBROADCAST message, with parameters dating from Windows 2000. Linux is a moving target, and things will break unless you keep updating them. This probably also extends to Linux application APIs.