EducaSwift

What is a protocol?

A protocol is a way to require an object to implement a variable, function or init. This allows to identify objects by a protocol, no matter what else they do.

Let's imagine that we have two objects to handle two different type of products, VideoGame and Computer. These two products have different properties, but since they need to be scanned, both will have a barcode property.

struct VideoGame {
    var name: String
    var genre: String

    var barcode: String
}

struct Computer {
    var brand: String
    var model: String

    var barcode: String
}

One of the limitations we have now is that we can't add both product types to the same array.

var re2 = VideoGame(name: "Resident Evil 2", genre: "Horror", barcode: "123")
var re3 = VideoGame(name: "Resident Evil 3", genre: "Horror", barcode: "456")
var macbook = Computer(brand: "Apple", model: "Macbook Pro", barcode: "789")

To solve this we can declare a protocol that defines barcode property.

protocol Scanneable {
    var barcode: String { get set }
}

Then VideoGame and Computer will adopt this protocol (they are already implementing the required barcode property).

struct VideoGame: Scanneable {
    var name: String
    var genre: String

    var barcode: String
}

struct Computer: Scanneable {
    var brand: String
    var model: String

    var barcode: String
}

Now we can create an array of Scanneable products.

var products: [Scanneable] = [re2, re3, macbook]

It's important to understand that, for each item in our array, we can only access to the barcode property, since we are dealing with Scanneable objects.

print(products[0].barcode)

Add functions and inits

We can also define functions and inits in a protocol. To do this we need to add just the definition of it, without a body, that will be added by each object's implementation.

Let's add the function getDescription.

protocol Scanneable {
    var barcode: String { get set }

    func getDescription() -> String
}

Then each model will implement this new function in their own way.

struct VideoGame: Scanneable {
    var name: String
    var genre: String

    var barcode: String

    func getDescription() -> String {
        "\(name) is a \(genre) game"
    }
}

struct Computer: Scanneable {
    var brand: String
    var model: String

    var barcode: String

    func getDescription() -> String {
        "Buy a \(model) from \(brand)"
    }
}

From now, each array item will return a different description according to its product type.

print(products[0].getDescription()) // Resident Evil 2 is a Horror game
print(products[1].getDescription()) // Resident Evil 3 is a Horror game
print(products[2].getDescription()) // Buy a Macbook Pro from Apple

SwiftUI protocols example

Swift is a Protocol-Oriented Programming (POP) language, meaning that we have a wide variety of system protocols.

Let's see an example of how we show a list of objects in our view without using protocols:

struct ContentView: View {

    struct Person {
        var name: String
    }

    var people = [
        Person(name: "Pablo"),
        Person(name: "Andrea"),
        Person(name: "Clara"),
        Person(name: "Mateo"),
        Person(name: "Miguel")
    ]

    var body: some View {
        List {
            ForEach(0..<people.count, id: \.self) { i in
                Text(people[i].name)
            }
        }
    }
}

Now, we are going adopt Identifiable protocol in our Person model. This protocol requires to implement a variable named id. For this example we'll just return the name.

struct ContentView: View {

    struct Person: Identifiable {
        var id: String {
            name
        }
        var name: String
    }

    var people = [
        Person(name: "Pablo"),
        Person(name: "Andrea"),
        Person(name: "Clara"),
        Person(name: "Mateo"),
        Person(name: "Miguel")
    ]

    var body: some View {
        List {
            ForEach(people) { person in
                Text(person.name)
            }
        }
    }
}

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