This macOS application gives you advance notice of when your screensaver is about to start. For stock macOS installations, this application is not necessary because you can use the grace period provided by System Preferences / Security & Privacy / Require Password 1 minute after screen saver begins. However, if you work in an environment where this option has been disabled, your screensaver will often start at the most inopportune, and then this application has some value.
To install, download the binary *.dmg archive file (below). Right click on the *.dmg file in Finder and select "Open." Drag-and-drop the "Harbinger" icon to your "Applications" directory (e.g. "$HOME/Applications"). After the Harbinger application bundle has been copied, right click on the "release/Harbinger" disk image to eject it.
You can add Harbinger to the list of applications that run automatically when you login by changing the list under System Preferences / Users & Groups / Login Items.
Harbinger-1.0.0.dmg is the binary distribution package for macOS.
Harbinger-1.0.0.tar.gz is the source which you should be able to build yourself if you have Qt installed.