SOLVED: @State vs @StateObject in Apple's SwiftUI Landmark Tutorial

@bzamora

Mar 7

Hello!

I've read Hacking with Swift's excellent description of the differences between @State and @StateObject here:

https://www.hackingwithswift.com/quick-start/swiftui/whats-the-difference-between-observedobject-state-and-environmentobject

Ofcourse, shortly after, I found this example from Apple's Swift UI Landmark Tutorial that shook my confidence in the understanding of the guidance 😫

See Section 5 - Step 6

https://developer.apple.com/tutorials/swiftui/handling-user-input#Adopt-the-model-object-in-your-views

ModelData is an Observable class:

@Observable
class ModelData {
    var landmarks: [Landmark] = load("landmarkData.json")
}

and yet, the examples uses @State which I thought was for simple types, should be private only the View, etc...

struct LandmarksApp: App {
    @State private var modelData = ModelData()

    var body: some Scene {
        WindowGroup {
            ContentView()
                .environment(modelData)
        }
    }
}

I would have expected instead to see @StateObject (i.e.):

@StateObject private var modelData = ModelData()

So yeah...can anyone help me understand what I am missing?

@SwiftDevJournal

Mar 7

The reason the Apple tutorial uses @State is because the tutorial is using the Observation framework Apple added at WWDC 2023. The @Observable before the class indicates that the model is using the Observation framework.

@Observable
class ModelData {
    var landmarks: [Landmark] = load("landmarkData.json")
}

The Observation framework does not use @StateObject. The Observation framework uses @State for both structs and classes.

If the tutorial was not using the Observation framework, then it would use @StateObject for a class, like you would have expected to see.

@bzamora

Mar 8

Aha! This is the first time I've come across the Observation framework in my learning. I think I've been using/reading the ObservableObject protocol and Observable macro interchangeably without realizing they are different...

I will check it out, thank you for the help!

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