How to subscribe to state changes of a projection

I’m interested in receiving events whenever the state of a particular projection (or the state of one of it’s partitions) changes. I believe that I read something about that a while ago, but I can’t find it anymore.
Could anyone give me a hint where to look for information?



The state stream is just a stream based on naming convention. You can see the stream/streams that get created (depends on projection type).