I want to add an XP bar in my UI but to show the artifact Power instead, however I can't find any Blizzard API functions to get the current and total values of this status bar. I guess I could just get the min and max values of this status bar without using an API, but I wanted to know of any events that I should be aware of, like when the player gets an artifact weapon and this status bar shows for the first time etc..
You can look at the ArtifactWatchBar in MainMenuBar.xml and the artifact section of MainMenuBar_UpdateExperienceBars() in MainMenuBar.lua for how Blizzard does it.
Note, the actual setting of bar values is handed off to their animation system using AnimatedStatusBarMixin in AnimatedStatusBar.lua.