Correctly initializing observable classes in modern SwiftUI

Question

Created 1d

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I have two @Observable manager classes, which share a reference to a third class. I initialize this setup using a custom init in my App struct, like so:

@main
struct MyApp: App {    
    private let managerA: ManagerA
    private let managerB: ManagerB
    
    init() {
        let managerC = ManagerC()
        self.managerA = ManagerA(managerC: managerC)
        self.managerB = ManagerB(managerC: managerC)
    }

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

I've been using this pattern for some time and it has been working fine. However, I just today discovered that @Observable objects are supposed to be initialized as @State vars, as shown in Apple's documentation here. This means I shoud be doing the following:

@main
struct MyApp: App {    
    @State private var managerA: ManagerA
    @State private var managerB: ManagerB
    
    init() {
        let managerC = ManagerC()
        self.managerA = ManagerA(managerC: managerC)
        self.managerB = ManagerB(managerC: managerC)
    }

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

I've also seen some examples where the @State vars are initialized manually like this:

@main
struct MyApp: App {    
    @State private var managerA: ManagerA
    @State private var managerB: ManagerB
    
    init() {
        let managerC = ManagerC()
        let managerA = ManagerA(managerC: managerC)
        let managerB = ManagerB(managerC: managerC)
        
        self._managerA = State(initialValue: managerA)
        self._managerB = State(initialValue: managerB)
    }

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

ChatGPT tells me the third approach is the correct one, but I don't understand why and ChatGPT can't produce a convincing explanation. The compiler doesn't produce any errors or warnings under each approach, and as far as I can tell, they all behave identically with no discernible difference in performance.

Does it matter which pattern I use? Is there a "correct" way?

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Answer 1

DTS Engineer OP

Apple

2m

If you’re performing any significant work on your state’s default value or instantiating Observable objects, we recommend handling that differently for optimal performance.

For example:

struct MyView: View {
    @State private var object = MyObject()

    var body: some View {
        ContentView(object)
    }
}

That's not the best for performance, because MyObject will be instantiated eagerly every time MyView is initialized, which is expensive since it allocates a new object,

Instead defer the creation of the object using callbacks like, ,task, .onAppear or .onChange(of:). For example:

struct MyView: View {
    @State private var object: MyObject?

    var body: some View {
        ContentView(object)
            .task { object = MyObject() }
    }
}

If the model needs to be part and owned by the view, then use @State. If the model needs to be global to your appliction use @Environment. Please review WWDC23 Session: Discover Observation in SwiftUI and Managing model data in your app

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