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Async await validation and switching tabs

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This topic is extended from https://www.hackingwithswift.com/forums/swiftui/best-convention-for-fixing-the-pyramid-of-doom/15417, - @Obelix already provided an invaluable response, but I'm creating a new topic with an example that aligns more to the original intent of the question.

I have two questions regarding async functions.

1. Alternatives to if-let pyramid of doom to better validate variables called from async functions.

In ViewA.swift, I have a nested if-let loop that validates if numbers and average are properly instantiated. I am familiar that you could use guard-let to avoid an if-let pyramid of doom, but I am struggling with implementing it in my view.

2. How to switch tabs in TabView while asychronous function is running

In MainView.swift, I have a tabview with ViewA and ViewB as tab items. View A takes ~5 seconds to go from LoadingView to a TextView, so in the mainview the loading view is displayed for a while as well. However, while View A loads its textview, I am unable to switch tabs as if the screen's frozen. Why would this happen and what can I change to avoid it??

Below is a sample code nearly identical to Paul's example in a post about Async functions.

Note: It might not be obvious from this example, but I'm using EnvironmentObject since I want to use the class across multiple views.

NumberManager.swift

class NumberManager: ObservableObject {

    @Published var numbers: [Double]?
    @Published var average: Double?

    func generateNumbers() async  {
        self.numbers = (1...10_000_000).map { _ in Double.random(in: -10...30) }
        // takes about 5 seconds to run...
    }

    func calculateAverageNumber(for numbers: [Double]) async{
        let total = numbers.reduce(0, +)
        let average = total / Double(numbers.count)
        self.average = average
    }
}

MainView.swift

struct MainView: View {

    @StateObject var numberManager = NumberManager()

    var body: some View {
        TabView {
            ViewA()
                .tabItem {
                    Label("ViewA", systemImage: "person.3")
                }
            ViewB()
                .tabItem {
                    Label("ViewA", systemImage: "person.3")
                }
        }
        .environmentObject(numberManager)
    }
}

ViewA.swift

struct ViewA: View {

    @EnvironmentObject var numberManager: NumberManager

    var body: some View {
        if let numbers = numberManager.numbers {
            if let average = numberManager.average {
                Text("Average is: \(average)")
            } else {
                LoadingView(loadingType: "Calculating Average")
                    .task {
                        await numberManager.calculateAverageNumber(for: numbers)
                    }
            }
        } else {
            LoadingView(loadingType: "Generating Numbers")
                .task {
                    await numberManager.generateNumbers()
                }
        }
    }
}

LoadingView.swift

struct LoadingView: View {
    var loadingType: String

    var body: some View {
        Text(loadingType)
    }
}

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