What event fires when the Viewer has finished loading?

1. A concise explanation of the problem you're experiencing.
I want to set a viewport as soon as (but not before) the Cesium Viewer has finished loading. For Google Maps, I can use an "addListenerOnce(map, "idle" function(){}) call to read the Viewport the very first time the map becomes idle. What is the equivalent in Cesium?

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

I'm coordinating many remote map viewers. Each time a new view is established for a particular "Room" I store that view and make it available to any new users joining the room so they are synced with everyone else. Works with OpenLayers, Leaflet, Google Maps, Bing, but I don't have the right event for the Cesium case

4. The Cesium version you're using, your operating system and browser.
Cesium 1.48 Linux various browsers but mostly Chrome. Cesium Map ,Viewer option not offered if WebGL missing or not working.