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Protocols

Slides

Warmup - Review of UIKit

Q1: What view would you use to display information that doesn't fit on the screen? Select all that apply

  1. UIView
  2. UILabel
  3. UIScrollView
  4. UITableView

Q2: Which view or control allows the user to enter a single line of text? Which one allows the user to enter multiple lines of text?

  1. UITextField
  2. UITextView

Q3: Which of the following allows you to execute code for a specific control event? Which allows you to access properties on a specific view or control?

  1. IBAction
  2. IBOutlet

Agenda

  • Intro to Protocols
  • Protocols vs Inheritance
  • Protocols in the Swift Standard Library

Learning Objectives

  • Create and use Protocols
  • Distinguish between Protocol Conformance and Inheritance
  • Use protocols to describe properties and behaviors of a conforming type
  • Identify common protocols in the Swift Standard Library

What are we trying to solve?

We start with class inheritance.

We realize we need more functionality in one class from two other classes.

We need multiple inheritance. Not supported 😟

Passing around classes can cause unexpected behavior. We want to avoid this as much.

Tight coupling.

Protocol

Protocols are another type in Swift. 🙃

But they don't create any instances. Instead, they define a blueprint of methods, properties, and other requirements that other types need to implement.

A protocol can be adopted by a class, structure, or enumeration.

Any type that satisfies the requirements of a protocol is said to conform to that protocol.

Protocols are similar to contracts.

A contract is a guarantee that some terms will be satisfied. ✍🏼

Anything on the contract becomes a set of obligations to whoever "signs" it.

protocol FullName {
  var fullName: String
  func printToConsole(message: String) -> Bool
}
This protocol indicates that any class, struct or enum that conforms to it, needs a property called fullName.

You can declare methods in a protocol. Notice you don't define an implementation. A protocol makes no assumption about the implementation details of any type that conforms to it. A protocol only defines the method name, it's parameters and return value.

struct Person: FullName {
  var fullName: String
  func printToConsole(message: String) -> Bool {
    print(fullName)
    return true
  }
}

var author = Person(fullName: "Haruki Murakami")

struct Person: FullName{
  var firstName: String
  var lastName: String
  var fullName: String {
    return "\(firstName) \(lastName)"
  }
  func printToConsole(message: String) -> Bool {
    print(fullName)
    return true
  }
}

var author = Person(firstName: "Haruki", lastName: "Murakami")
Conforming to a protocol looks like class inheritance but it isn’t. Structs and enumerations can also conform to protocols with the same syntax.

If a class inherits from another and also conforms to a protocol, all can be separated by commas and the inheritance comes first.

Mini challenge

Create a protocol Perimeter that defines a read-only property side of type [Double].

Mini challenge

Implement Perimeter with structs representing Square, and Circle.

Mini challenge

Add a circle and a square to an array, print their perimeters.

Notice, you have two different types but the array sees the type as the protocol so they are compatible!

Walkthrough

Walkthrough

Mini challenge

  1. Create a model of a car, it should have:
  • max speed
  • number of wheels
  • doors
  • model

  1. Generalize the car, create a model for a vehicle which will represent all vehicles, a truck, motorcycle & bus are vehicles

  2. Create a model(struct) and instances of each a truck, motorcycle and bus.

Jigsaw activity 🧩

Swift uses protocols extensively. We won't learn every single aspect of the Swift standard library, it makes no difference in our code's performance. But we can benefit from being familiar with several commonly used protocols.

In groups of X, get together to research about one of the following protocols. One per group.

Make sure everyone understands how they work and individually prepare an example in a playground. (it can be the same for everyone in the group).

Also make sure you can explain in words what it does, it might be helpful to have notes.

It will be thanks to your contribution that others can learn about what you researched.

  • Now split the group. Go and find people who did the other protocols.

  • We should have groups with people who know about all of them.

  • Take turns to have each person explain their protocol and show the example.

  • By the end of the activity you should be familiarized with all 4 protocols thanks to your peers. 😀

Any questions?

Mini challenge

Given the Artist struct below, implement the Equatable protocol. You'll define what makes two instances equal.

// Used by Artist to determine style of Artist
enum Style: String {
    case impressionism
    case surrealism
    case cubism
    case popArt
}

struct Artist {
    let name: String
    let style: Style
    let yearBorn: Int
}

// Example instances of Artists, use for testing your equatable
let monet = Artist(name: "Monet", style: .impressionism, yearBorn: 1840)
let dali = Artist(name: "Salvador Dali", style: .surrealism, yearBorn: 1904)
let andy = Artist(name: "Andy Warhol", style: .popArt, yearBorn: 1928)

After class

Continue working in your final project.

External resources

  1. Iterator
  2. Protocols
  3. Protocols - article
  4. Protocols