Concurrency_DispatchBarrier.swift

class ThreadSafeCollection<Element> {
  
    // Concurrent synchronization queue
    private let queue = DispatchQueue(label: "ThreadSafeCollection.queue", attributes: .concurrent)
  
    private var _elements: [Element] = []
  
    var elements: [Element] {
        var result: [Element] = []
        
        queue.sync { // Read
            result = _elements
        }

        return result
    }
  
    func append(_ element: Element) {
        // Write with .barrier
        // This can be performed synchronously or asynchronously not to block calling thread.
        queue.async(flags: .barrier) {
            self._elements.append(element)
        }
    }
}