Observation framework runs the onChange only once, which means you should call it recursively to observe changes constantly, and onChange closure runs before the actual change applies. That’s why we postpone the onChange action by starting a new task. 
https://swiftwithmajid.com/2023/10/03/mastering-observable-framework-in-swift/

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 We can use DispatchQueue.concurrentPerform function to verify the thread safety of our types using unit tests. Don't wait for strange crashes from production, invest your time earlier into safe types.
https://swiftwithmajid.com/2023/09/05/thread-safety-in-swift-with-locks/

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