Object-Oriented Programming: Beyond the Basics

Oct 17 2023 · Swift 5.9, iOS 17, Xcode 15

Lesson 03: Design Patterns

Demo 3

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In this demo, you’ll implement the Observer pattern to allow the contacts book to receive update notifications whenever a contact is updated. Ideally, in a system with a user interface, the contacts book will also notify the UI to update, so it will reflect changes to individual contacts.

public protocol Subject: AnyObject {
}

public protocol Observer: AnyObject {
}

public protocol Observer: AnyObject {
  func subjectUpdated(subject: any Subject)
}

public protocol Subject: AnyObject {
  var observers: [Observer] { get set }
  func addObserver(_ obj: Observer) 
  func removeObserver(_ obj: Observer) 
  func broadcastUpdates() 
}

public extension Subject {
  func addObserver(_ obj: Observer) {
    observers.append(obj)
  }

  func removeObserver(_ obj: Observer) {
    observers.removeAll { item in
      item === obj
    }
  }

  func broadcastUpdates() {
    observers.forEach { observer in
      observer.subjectUpdated(subject: self)
    }
  }
}

public class ContactCard: Subject {
  public var observers: [Observer]
  ...

public init(firstName: String, lastName: String, phoneNumber: String) {
  self.firstName = firstName
  self.lastName = lastName
  self.phoneNumber = phoneNumber
  isCompany = false
  contactID = UUID()
  relatedContacts = []
  observers = [] // new code
}

extension ContactsBook: Observer {
  public func subjectUpdated(subject: Subject) {
    let index = contactsList.firstIndex { contact in
      contact === subject
    }

    guard let index else {
      return
    }
    print("Contact at index \(index) has been updated")
  }
}

ehabContact.set(phone: "333333333333")
ehabContact.set(phone: 44444444444)
timContact.set(firstName: "Nick", lastName: "Fury")

//print(ehabContact.contactInformation())
//print(kodeco.contactInformation())
//print(razeware.contactInformation())

//book1.printContacts()

//print("Contacts Book has \(contactsList.count) entries")

// print("Calling super from Person")

//print("Other contact is a Person too. Add 2-way relationship")

//print("Calling super from Company")

//print("Other contact is a company too. Adding 2-way relationship")

public func saveContact(contact: ContactCard) {
  contactsList.append(contact)
  contact.addObserver(self) // new code
}

public func addRelatedContact(_ contact: ContactCard) {
  relatedContacts.append(contact.contactID)
  broadcastUpdates() // new code
}

public func set(firstName: String, lastName: String) {
  self.firstName = firstName
  self.lastName = lastName
  broadcastUpdates() // new code
}

public func set(phone: String) {
  phoneNumber = phone
  broadcastUpdates() // new code
}

public func set(phone: Double) {
  phoneNumber = "\(phone)"
  broadcastUpdates() // new code
}
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