AsyncReadValue.swift

class ReadValue {
    private var thread : Thread? = nil
    init(with handler: @escaping (String) -> ()) {
        thread = Thread(block: {
            while(true) {
                let value = readLine() ?? ""
                handler(value)
            }
        })
        thread?.start()
    }
    
    func stop() {
        thread?.cancel()
    }
}

main.swift

var asyncRead = { value in
    print(value)
}
print("$", terminator: "")
var read = ReadValue(with: asyncRead)

RunLoop.current.run()