Concurrency
Multi-reader single-writer queue
You want a property that several threads can read at the same time while writes happen one at a time. Back in the Objective-C days there was a pattern of dispatch_sync in the getter and dispatch_barrier_async in the setter, and the interviewer asks: "If the getter is sync, that's synchronous, so how can several reads happen at once?" How would you explain that?
How would an interviewer hear this answer? You'll see whether they'd move on, dig in, or say no, and which sentences would trip you up. Your score doesn't change.