Applying Protocol-Oriented Programming in Development

Demo

Now, it’s time to see how to implement a MediaCollection. Open the starter project playground for this lesson in Xcode. MediaItem and MediaCollection are already defined for you.

A few media types have also been defined for you, such as Movie, TVShow and VideoGame. These all conform to MediaItem and have a title and a price property. Some also have an additional duration, where appropriate. There’s also a CardGame for you to use later in the lesson.

First, create a collection for your movies:

struct MovieCollection: MediaCollection {

}

This collection will contain Movies as the Item. To satisfy the compiler, use typealias to define the associated type:

typealias Item = Movie

Then, define the array of items:

var items: [Movie] = []

Finally, implement getDescription():

func getDescription() -> String {
    "The movie collection contains \(items.count) movies"
}

Because you specify the type of Item in the items array, you can even remove the type alias; the compiler will infer it for you.

[Remove type alias and show working]

Most of the time, you want to keep the type alias, however, so your code is more readable.

[Undo the change]

Finally, create a couple of movies and add them to the collection, then print the description:

let bourneIdentity = Movie(title: "The Bourne Identity", price: 3.99, duration: 113)
let oppenheimer = Movie(title: "Oppenheimer", price: 17.99, duration: 180)

var movieCollection = MovieCollection()
movieCollection.items.append(bourneIdentity)
movieCollection.items.append(oppenheimer)

print(movieCollection.getDescription())

In this demo, you implemented a MovieCollection conforming to the MediaCollection protocol, demonstrating the practical application of associated types. Here are the steps you followed:

• First, you defined the MovieCollection struct to represent a collection of movies, specifying Movie as the associated Item type using a typealias.
• Then, you created an array of Movie items to hold the collection’s content.
• Finally, you implemented the getDescription() function, which counts the movies in the collection and returns a descriptive message.

This demo showcases how associated types allow you to work with generic data types while preserving the safety and structure of protocols.