SwiftUI: Why Is an Extension Used for a Custom Struct (Apple iOS App Dev Tutorial)

Question

In the Apple [iOS App Dev Tutorial] (as of Jan 22)(https://developer.apple.com/tutorials/app-dev-training), under the Creating a Navigation Hierarchy section, we extend a struct that we have defined.

I have tried reading the Swift documentation about extensions, but don't really understand why this is done here. The documentation talks about extending system types such as adding extra properties to the system type Double, but not adding extra properties to something that we have full control over, such as our own structure.

I am sure it is a best practice, as this is an Apple tutorial, but they don't really do a good job of explaining it.

This is an example of the code they want you to write:

import SwiftUI

struct DailyScrum: Identifiable  {
    let id: UUID
    var title: String
    var attendees: [Attendee]
    var lengthInMinutes: Int
    var theme: Theme
    
    init(id: UUID = UUID(), title: String, attendees: [String], lengthInMinutes: Int, theme: Theme) {
        self.id = id
        self.title = title
        self.attendees = attendees.map { Attendee(name: $0) }
        self.lengthInMinutes = lengthInMinutes
        self.theme = theme
    }
}

extension DailyScrum {
    struct Attendee: Identifiable {
        let id: UUID
        var name: String
        
        init(id: UUID = UUID(), name: String) {
            self.id = id
            self.name = name
        }
    }
}

I am uncertain why the Attendee structure has to be defined within an extension. For example, this works too:

import SwiftUI

struct Attendee: Identifiable {
    let id: UUID
    var name: String
    
    init(id: UUID = UUID(), name: String) {
        self.id = id
        self.name = name
    }
}

struct DailyScrum: Identifiable  {
    let id: UUID
    var title: String
    var attendees: [Attendee]
    var lengthInMinutes: Int
    var theme: Theme
    
    init(id: UUID = UUID(), title: String, attendees: [String], lengthInMinutes: Int, theme: Theme) {
        self.id = id
        self.title = title
        self.attendees = attendees.map { Attendee(name: $0) }
        self.lengthInMinutes = lengthInMinutes
        self.theme = theme
    }
}

I'm just not sure why you would do one over the other.

user_12738750 · Answer

It is styling and also focuses the code in this case

Your type as originally written is

DailyScrum.Attendee

If you change it to the second version you will have 2 separate independent types.

Attendee

and

DailyScrum

Without looking at the tutorial I would assume that there is another type of Attendee. Such as..

Conference.Attendee

or

WeeklyScrum.Attendee

But as you pointed out this would only make sense if they are different like a Homonym. Some samples are...

Bird.Crane
Construction.Crane

or

Dog.Bark
Tree.Bark

If all you are using is an id and a name there is no point to have a separate data structure.

user_5456919 · Answer

In the tutorial, I believe that it is a pattern choice so that the data model more closely mirrors the SwiftUI layout for ease of understanding.

Attendees only appear as a child view of the detailView using ForEach(...) to generate a VStack. Therefore, in this case, where MVVM isn't being used, it makes sense to clearly map the data model to the view, and make it explicit in the code pattern that the Attendee type only occurs within an instance of a DailyScrum, just as attendees details only appear within the detailView.

SwiftUI: Why Is an Extension Used for a Custom Struct (Apple iOS App Dev Tutorial)

2 Answers