IceCreamPreferenceKey.swift

import SwiftUI

enum ShopState: Int {
    case iceCream, flowers
}

struct ShopStatePreferenceKey: PreferenceKey {
    static var defaultValue: ShopState = .iceCream
    
    static func reduce(value: inout ShopState, nextValue: () -> ShopState) {
        value = nextValue()
    }
}

struct ContentView: View {
    @State var shopState: ShopState = .iceCream
    
    var body: some View {
        ZStack {
            switch shopState {
            case .iceCream:
                IceCreamView()
            case .flowers:
                Text("Here are flowers")
            }
        }
        .onPreferenceChange(ShopStatePreferenceKey.self) { value in
            self.shopState = value
        }
    }
}

struct IceCreamView: View {
    @State var shopState: ShopState = .iceCream
    
    var body: some View {
        VStack(spacing: 15.0) {
            Text("Here is ice cream: 🍦 Wanna go see flowers next?")
            
            Button {
                self.shopState = .flowers
            } label: {
                Text("See Flowers")
            }
            .preference(key: ShopStatePreferenceKey.self, value: shopState)
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Just move the onPreferenceChange method just under the IceCreamView inside the switch case.