Modernizing Checkout at Scale: From a 2,700-Line Monolith to Modular SwiftUI

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import SwiftUI
 
// MARK: - Pattern 1: Section View as a Pure Micro-Assembler (<100 LoC)
// Completely decoupled from ViewControllers and global mutable singletons
public struct OrderItemsSection: View {
  let state: CartSectionState
  let onQuantityChanged: (String, Int) -> Void
  let onPromotionTapped: (String) -> Void
 
  public var body: some View {
    VStack(alignment: .leading, spacing: 16) {
      // 1. Isolated Fulfillment Badge Molecule
      FulfillmentHeaderRow(badge: state.fulfillmentBadge)
 
      // 2. Composable Product Rows with Granular Mutation Boundaries
      ForEach(state.items) { item in
        VStack(spacing: 8) {
          ProductItemSummaryRow(item: item)
 
          if let promo = item.promotionHighlight {
            PromotionTagBadge(promo: promo) { onPromotionTapped(item.id) }
          }
 
          HStack {
            UnitPriceBreakdownView(price: item.price, subtotal: item.subtotal)
            Spacer()
            // Surgical Mutation: Touching stepper only invalidates this subview
            QuantityStepperView(quantity: item.quantity) { newQty in
              onQuantityChanged(item.id, newQty)
            }
          }
        }
      }
    }
    .padding(16)
    .background(Color(.secondarySystemBackground))
    .cornerRadius(16)
  }
}