TotalPricePreferenceKey.swift

import SwiftUI

struct TotalPricePreferenceKey: PreferenceKey {
    static var defaultValue: Double = 0.0
    
    static func reduce(value: inout Double, nextValue: () -> Double) {
        value += nextValue()
    }
}

struct ContentView: View {
    
    @State private var totalPrice: Double = 0
      
      var body: some View {
          NavigationStack {
              VStack {
                  ItemPricesView()
                  Text("Total: ") + Text(totalPrice, format: .currency(code: "USD"))
              }
              .navigationTitle("장바구니 🛒")
          }
          .onPreferenceChange(TotalPricePreferenceKey.self) {
              totalPrice = $0
          }
      }
}

struct ItemPricesView: View {
    
    var body: some View {
        List {
            ForEach(1..<10) { index in
                HStack {
                    Text("Item #\(index)")
                    Spacer()
                    let num = Double.random(in: 10...100)
                    Text(num, format: .currency(code: "USD"))
                        .preference(key: TotalPricePreferenceKey.self, value: num)
                }
            }
        }
    }
}