let today = Date()
let otherDate = ...
if today.compare(otherDate) == .orderedAscending {
    print("Today is earlier.")
}

let prefs = UserDefaults.standard
prefs.set("sherlock.holmes", forKey: "username")
prefs.set(10, forKey: "booksInList")
prefs.synchronize()

if let username = prefs.string(forKey: "username") {
    print(username)
}

import CoreData

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
    var managedObjectContext: NSManagedObjectContext!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        let appDelegate = UIApplication.shared.delegate as! AppDelegate
        managedObjectContext = appDelegate.persistentContainer.viewContext
    }
    
    func loadBooks() -> [Book] {
        let fetchRequest: NSFetchRequest<Book> = Book.fetchRequest()
        do {
            return try managedObjectContext.fetch(fetchRequest)
        } catch {
            NSLog("Error fetching books: %@", error as NSError)
            return []
        }
    }
    
    @IBAction func addNew(_ sender: Any) {
        let book = NSEntityDescription.insertNewObject(forEntityName: "Book", into: managedObjectContext) as! Book
        book.title = "New Book \(loadBooks().count)"
        do {
            try managedObjectContext.save()
        } catch {
            NSLog("Error saving context: %@", error as NSError)
        }
        tableView.reloadData()
    }
}

protocol RandomNumberGenerator {
    var mustBeSettable: Int { get set }
    var doesNotNeedToBeSettable: Int { get }
    func random() -> Double
}

protocol GuessDelegate {
    func userDidFinish(_ controller: GuessInputViewController, guess: String)
}

class GuessInputViewController: UIViewController {
    var delegate: GuessDelegate?
    var previousGuess: String = ""
    
    @IBOutlet weak var guessLabel: UILabel!
    @IBOutlet weak var guessTextField: UITextField!
    
    @IBAction func saveGuess(_ sender: AnyObject) {
        if let delegate = delegate, let guessText = guessTextField.text {
            delegate.userDidFinish(self, guess: guessText)
        }
    }
}

class AnotherClass: GuessDelegate {
    func userDidFinish(_ controller: GuessInputViewController, guess: String) {
        print("Received guess: \(guess)")
    }
}