editButtonItem对象是如何实例化的?

我遵循的是一个关于使用钥匙链的教程,我更喜欢在进入教程之前理解代码的工作原理。我注意到在这个名为MasterViewController的类中,它为navigationItem.leftBarButtonItem变量指定了一个具有编辑样式的UIBarButtonItem。代码在代码库的任何地方都没有实例化这个编辑按钮,所以我很想知道它是如何工作的?当我尝试执行以下操作时,它不起作用:

navigationItem.rightBarButtonItem = camera

我下面的教程可以在 Ray Weinerlich Website :

以下是MasterViewController的代码:

    class MasterViewController: UIViewController, UITableViewDelegate {

  // MARK: - IBOutlets
  @IBOutlet var tableView: UITableView!

  // MARK: - Properties
  var detailViewController: DetailViewController?
  var managedObjectContext: NSManagedObjectContext?
  var isAuthenticated = false
  var didReturnFromBackground = false
  var _fetchedResultsController: NSFetchedResultsController<Note>?

  // MARK: - View Life Cycle
  override func viewDidLoad() {
    super.viewDidLoad()

    navigationItem.leftBarButtonItem = editButtonItem


    let addButton = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(insertNewObject(_:)))
    navigationItem.rightBarButtonItem = addButton

    if let split = splitViewController {
      let controllers = split.viewControllers
      detailViewController = (controllers[controllers.count-1] as! UINavigationController).topViewController as? DetailViewController
    }
}