doublench21

class ViewController: UIViewController {
  
  @IBOutlet var button: UIButton!
  
  
  var alert: UIAlertController?
  
  func displayActivityIndicatorAlert() {
    alert = UIAlertController(title: "Deleting from black list...", message: nil, preferredStyle: .alert)
    let activityIndicator = UIActivityIndicatorView(style: .medium)
    activityIndicator.translatesAutoresizingMaskIntoConstraints = false
    activityIndicator.isUserInteractionEnabled = false
    activityIndicator.startAnimating()
    
    alert!.view.addSubview(activityIndicator)
    alert!.view.heightAnchor.constraint(equalToConstant: 95).isActive = true
    
    activityIndicator.centerXAnchor.constraint(equalTo: alert!.view.centerXAnchor, constant: 0).isActive = true
    activityIndicator.bottomAnchor.constraint(equalTo: alert!.view.bottomAnchor, constant: -20).isActive = true
    
    present(alert!, animated: true)
  }
  
  func dismissActivityIndicatorAlert() {
    alert?.dismiss(animated: true)
    alert = nil
  }
  
  var blackList = [Int](0...9)
  let lock = NSLock()
  
  func asyncDeleteBlackList() {
    var tasks = [URLSessionTask]()
    tasks.reserveCapacity(10)
    
    var results = [Int: String]()
    results.reserveCapacity(10)
    
    let group = DispatchGroup()
    
    displayActivityIndicatorAlert()
    
    for item in blackList {
      group.enter()
      
      let url = URL(string: "https://.../api/v1/today/batch")!
      let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
        if let error = error {
          // Что-то делаем с ошибкой на клиенте...
          return
        }
        
        guard let httpResponse = response as? HTTPURLResponse,
          (200...299).contains(httpResponse.statusCode) else
        {
          // Что-то делаем с ошибкой на сервере...
          return
        }
        
        if let mimeType = httpResponse.mimeType,
          mimeType == "application/json",
          let data = data,
          let string = String(data: data, encoding: .utf8)
        {
          print("Запрос №\(item) завершён.")
          self.lock.lock()
          results[item] = string
          self.lock.unlock()
          group.leave()
        }
      }
      
      tasks.append(task)
      task.resume()
    }
    
    group.notify(queue: .main) {
      print("\nЗадача завершена.")
      self.dismissActivityIndicatorAlert()
    }
  }
  
  override func viewDidLoad() {
    button.addTarget(self, action: #selector(pressed(sender:)), for: .touchUpInside)
  }
  
  @objc
  func pressed(sender: UIButton) {
    asyncDeleteBlackList()
  }
}

"adflksdlfdfddddflsdkflsdkflsdfZ"
  .lazy
  .reduce(into: [Character:Int]()) { $0[$1] = $0[$1, default: 0] + 1 }
  .sorted { $0.value < $1.value }
  .last // (key: "d", value: 10)