What does the error "Exceeded maximum allowed number of Trackers" mean?

Question

I'm following this tutorial to implement object tracking on iOS 11. I'm able to track objects perfectly, until a certain point, then this error appears in the console.

Throws: Error Domain=com.apple.vis Code=9 "Internal error: Exceeded maximum allowed number of Trackers for a tracker type: VNObjectTrackerType" UserInfo={NSLocalizedDescription=Internal error: Exceeded maximum allowed number of Trackers for a tracker type: VNObjectTrackerType}

Am I using the API incorrectly, or perhaps Vision has trouble handling too many consecutive object tracking tasks? Curious if anyone has insight into why this is happening.

Answer 1

It appears that you hit the limit on the number of trackers that can be active in the system. First thing to note is that a new tracker is created every time a new observation, with new -uuid property is used. You should be recycling the initial observation you use when you started the tracker, until you no longer want to use it, by feeding what you got from “results” for time T into the subsequent request you make for time T+1. When you no longer want to use that tracker (maybe the confidence score gets too low), there is a “lastFrame” property that can be set, which lets the Vision framework know that you are done with that tracker. Trackers also get released when the sequence request handler is released.