ViewModel.swift

extension ContentView {
    final class ViewModel: ObservableObject {
        private let package: Package
        private var cancellables = [AnyCancellable]()
        
        init(package: Package) {
            self.package = package
            
            // Model -> View Model
            package.recipient.publisher(for: \.firstName)
                .assign(to: &$recipientFirstName)
            package.recipient.publisher(for: \.lastName)
                .assign(to: &$recipientLastName)
            
            package.recipient.publisher(for: \.postalAddress)
                .notifyObjectWillChange(objectWillChange)
                .store(in: &cancellables)
            package.publisher(for: \.contents)
                .notifyObjectWillChange(objectWillChange)
                .store(in: &cancellables)
            
            // View Model -> Model
            $recipientFirstName.dropFirst()
                .removeDuplicates()
                .assign(to: \.firstName, on: package.recipient)
                .store(in: &cancellables)
            $recipientLastName.dropFirst()
                .removeDuplicates()
                .assign(to: \.lastName, on: package.recipient)
                .store(in: &cancellables)
        }
        
        // Example of using published property
        @Published var recipientFirstName: String = ""
        @Published var recipientLastName: String = ""
        
        // Example of using bindings for propagating values
        var street: Binding<String> {
            let package = self.package
            return Binding<String>(
                get: {
                    package.recipient.postalAddress.street
                },
                set: { newValue in
                    let postalAddress = package.recipient.postalAddress.mutableCopy() as! CNMutablePostalAddress
                    postalAddress.street = newValue
                    package.recipient.postalAddress = postalAddress
                }
            )
        }
        
        var summary: String {
            let contents = package.contents
                .map({ "\($0.title) \($0.weight)" })
                .joined(separator: ", ")
            return """
            Recipient: \(package.recipient.firstName) \(package.recipient.lastName)
            Postal address: \(CNPostalAddressFormatter().string(from: package.recipient.postalAddress))
            Contents: \(contents)
            """
        }
        
        func addRandomItem() {
            let weight = Int.random(in: 200...300)
            let item = PackageContent(title: "Light bulb", weight: weight)
            package.contents.append(item)
        }
    }
}