editButtonItem对象如何实例化？

Question

I'm following a tutorial on using the Keychain and I prefer to understand how the code works before diving into the tutorial. I noticed that in this class called MasterViewController, it assigns a UIBarButtonItem with a style of edit to the navigationItem.leftBarButtonItem variable. The code doesn't instantiate this edit button anywhere in the codebase so I'm curious to know how does this work? When I try to do the following, it doesn't work:

navigationItem.rightBarButtonItem = camera

Why is this?

The tutorial I'm following is found here on the Ray Weinerlich Website :

Here is the code for the MasterViewController:

    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
    }
}

Answer 1

It's a built-in UIBarButton item available readily for users to use. You can directly use it and it'll toggle between edit and done.

See the documentation below.

If you have a doubt with an object or variable, just Command + click on that variable and object and you could check more about it.

Answer 2

I think you are asking about this line:

navigationItem.leftBarButtonItem = editButtonItem

editButtonItem is from UIViewController . Note that this is a read-only property. The implementation is probably a lazy computed property that creates the button the first time you access the property. It's part of the UIViewController implementation.