from Swift to Solidity fassko www.kristaps.me 

about me geek iOS and  developer crypto 🐈⬛ at 

Mr. Byte 

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Solidity ECMAScript like syntax Static typing Multiple inheritance 

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– swift.org “Swift is designed to make writing and maintaining correct programs easier for the developer.” 

Swift Safe Fast Expressive 

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Deterministic adjective Believing that everything that happens must happen as it does and could not have happened any other way. 

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Multi-party agreement Agreement between 🇺🇦, 🇷🇺 and 🇹🇷 (supported by 🇺🇳). 🇷🇺 will not target ports while shipments are in transit. 🇺🇦 vessels will guide cargo ships through waters that have been mined. 🇹🇷 - supported by the 🇺🇳 - will inspect ships, to allay 🇷🇺 fears of weapons smuggling. 

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Logistics • 🇺🇦 registers shipment • The ship goes from 🇺🇦 to 🇹🇷 through mines • 🇹🇷 checks ship if nothing is being smuggled • The ship leaves 🇹🇷 • Arrives in the destination country, 🇺🇳 approves 

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Agreement address private ukraine; address private russia; address private turkey; address private un; bool private ukraineSigned; bool private russiaSigned; bool private turkeySigned; bool private unSigned; event SignedByUkraine(); event SignedByRussia(); event SignedByTurkey(); event SignedByUN(); event AgreementSigned(); Wallet addresses Signed status Sign event 

Initial setup constructor(address _ukraine, address _russia, address _turkey, address _un) { ukraine = _ukraine; russia = _russia; turkey = _turkey; un = _un; } 

Agreement function sign() external { console.log("Signer address", msg.sender); require(msg.sender == ukraine || msg.sender == russia || msg.sender == turkey || msg.sender == un, "Unknown signer"); if (msg.sender == ukraine) { ukraineSigned = true; emit SignedByUkraine(); } if (msg.sender == russia) { russiaSigned = true; emit SignedByRussia(); } if (msg.sender == turkey) { turkeySigned = true; emit SignedByTurkey(); } if (msg.sender == un) { unSigned = true; emit SignedByUN(); } if (ukraineSigned && russiaSigned && turkeySigned && unSigned) { emit AgreementSigned(); } } Emit event Check wallet address Agreement signed 

Agreement struct ContentView: View { @StateObject var appData = AppData() private var agreementSigned: Bool { appData.ukraineSigned && appData.russiaSigned && appData.turkeySigned && appData.unSigned } var body: some View { NavigationStack { VStack(spacing: 30) { Spacer() CountryButtonView(country: .ukraine) { appData.ukraineSigned = true }.disabled(appData.ukraineSigned) CountryButtonView(country: .russia) { appData.russiaSigned = true }.disabled(appData.russiaSigned) CountryButtonView(country: .turkey) { appData.turkeySigned = true }.disabled(appData.turkeySigned) CountryButtonView(country: .un) { appData.unSigned = true }.disabled(appData.unSigned) Spacer() NavigationLink { ShipmentView() } label: { Text("Next") .font(.title2) } .disabled(!agreementSigned) } } .environmentObject(appData) } } 

🇺🇦 signed CountryButtonView(country: .ukraine) { appData.ukraineSigned = true } .disabled(appData.ukraineSigned) 

Agreement signed @StateObject var appData = AppData() private var agreementSigned: Bool { appData.ukraineSigned && appData.russiaSigned && appData.turkeySigned && appData.unSigned } NavigationLink { ShipmentView() } label: { Text("Next") .font(.title2) } .disabled(!agreementSigned) 

AppData class AppData: ObservableObject { @Published var ukraineSigned: Bool = false @Published var russiaSigned: Bool = false @Published var turkeySigned: Bool = false @Published var unSigned: Bool = false 

Modi fi er modifier agreementSigned() { require(ukraineSigned, "Ukraine not signed"); require(russiaSigned, "Russia not signed"); require(turkeySigned, "Turkey not signed"); require(unSigned, "United Nations not signed"); _; } 

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ShipmentStatus enum ShipmentStatus { InUkraine, // Registered in Ukraine LeftUkraine, // Navigating in Black Sea ArrivedInTurkey, // Arrived in Turkey LeftTurkey, // Left Turkey ArrivedInDestinationCountry // Arrived to the destination country } enum ShipmentStatus { case inUkraine // Registered in Ukraine case leftUkraine // Navigating in Black Sea case arrivedInTurkey // Arrived in Turkey case leftTurkey // Left Turkey case arrivedInDestinationCountry // Arrived to the destination country } 

Shipment struct Shipment { string destination; ShipmentStatus status; } struct Shipment: Identifiable { var id: Int var destination: String var status: ShipmentStatus } 

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Create shipment - Solidity function createShipment(string memory _destination) external agreementSigned { require(msg.sender == ukraine, "Caller is not Ukraine"); currentShipmentId++; shipments[currentShipmentId] = Shipment(_destination, ShipmentStatus.InUkraine); console.log("Shipment created", currentShipmentId); emit ShipmentCreated(currentShipmentId); } 

Create shipment - iOS struct AddShipmentView: View { @Environment(\.presentationMode) var presentationMode @EnvironmentObject var appData: AppData @State private var destination: String = "" var body: some View { NavigationStack { Form { Section { TextField("Destination", text: $destination) } Section { Button("Save", action: saveShipment) } } .toolbar { ToolbarItem(placement: .navigationBarTrailing) { Button("Cancel", action: closeModal) } } } } } 

Create shipment - iOS func saveShipment() { let shipment = Shipment( id: Int(Date().timeIntervalSince1970), destination: destination, status: .inUkraine ) Task { @MainActor in appData.shipments.append(shipment) } closeModal() } 

Left Ukraine - Solidity function leftUkraine(uint256 shipmentId) external agreementSigned { require(msg.sender == ukraine, "Caller is not Ukraine"); shipments[shipmentId].status = ShipmentStatus.LeftUkraine; console.log("Shipment left Ukraine", shipmentId); emit ShipmentLeftUkraine(shipmentId); } 

Left Ukraine - iOS .sheet(item: $selectedShipment) { shipment in ChangeShipmentStatusView(shipment: shipment) .presentationDetents([.height(200)]) } 

Left Ukraine - iOS struct ChangeShipmentStatusView: View { @EnvironmentObject var appData: AppData @Environment(\.presentationMode) var presentationMode @State private var showSelectCountryAlert = false var shipment: Shipment private var nextStatus: ShipmentStatus { switch shipment.status { case .inUkraine: return .leftUkraine case .leftUkraine: return .arrivedInTurkey case .arrivedInTurkey: return .leftTurkey case .leftTurkey: return .arrivedInDestinationCountry case .arrivedInDestinationCountry: return .arrivedInDestinationCountry } } 

Left Ukraine - iOS var body: some View { NavigationStack { VStack { Button { changeStatusFor(to: nextStatus) } label: { Text(nextStatus.description) .font(.title3) .padding(10) } .buttonStyle(.borderedProminent) } } .toolbar { ToolbarItem(placement: .navigationBarTrailing) { Button("Cancel", action: closeModal) } } .alert(isPresented: $showSelectCountryAlert) { Alert( title: Text("Please select correct country"), message: Text("To continue please select correct country") ) } } } 

Arrives in Turkey - Solidity function arrivesInTurkey(uint256 shipmentId) external agreementSigned { require(msg.sender == turkey, "Caller is not Turkey"); shipments[shipmentId].status = ShipmentStatus.ArrivedInTurkey; console.log("Shipment arrived in Turkey", shipmentId); emit ShipmentArrivedInTurkey(shipmentId); } 

Arrives in Turkey - Swift func changeStatusFor(to status: ShipmentStatus) { var shipment = shipment switch status { case .inUkraine: return /// case .arrivedInTurkey: guard checkCountryIfSelectedIs(.turkey) else { showSelectCountryAlert = true return } shipment.status = .arrivedInTurkey 

Arrives in Turkey - Swift .sheet(isPresented: $showCountrySwitch) { VStack { CountryButtonView(country: .ukraine, font: .body) { appData.activeCountry = .ukraine closeSheet() } CountryButtonView(country: .russia, font: .body) { appData.activeCountry = .russia closeSheet() } CountryButtonView(country: .turkey, font: .body) { appData.activeCountry = .turkey closeSheet() } CountryButtonView(country: .un, font: .body) { appData.activeCountry = .un closeSheet() } } .presentationDetents([.medium]) .padding(.vertical, 40) } 

Arrives in Turkey - Swift case .arrivedInTurkey: guard checkCountryIfSelectedIs(.turkey) else { showSelectCountryAlert = true return } shipment.status = .arrivedInTurkey 

Examined - Solidity function arrivesInTurkey(uint256 shipmentId) external agreementSigned { require(msg.sender == turkey, "Caller is not Turkey"); shipments[shipmentId].status = ShipmentStatus.ArrivedInTurkey; console.log("Shipment arrived in Turkey", shipmentId); emit ShipmentArrivedInTurkey(shipmentId); } 

Examined - Swift case .leftTurkey: guard checkCountryIfSelectedIs(.turkey) else { showSelectCountryAlert = true return } shipment.status = .leftTurkey 

Arrived in destination - Solidity function arrivedInDestination(uint256 shipmentId) external agreementSigned { require(msg.sender == un, "Caller is not United Nations"); shipments[shipmentId].status = ShipmentStatus.ArrivedInDestinationCountry; console.log("Shipment", shipmentId, "arrived in", shipments[shipmentId].destination); emit ShipmentArrivedInDestination(shipmentId, shipments[shipmentId].destination); } 

Arrived in destination - Swift case .arrivedInDestinationCountry: guard checkCountryIfSelectedIs(.un) else { showSelectCountryAlert = true return } shipment.status = .arrivedInDestinationCountry 

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Similarities Statistically typed Data structures Inheritance - contract can extend from multiple contracts 

Solidity Absence of null Primitive No Strings attached 

Swift Modern Expressive Sugar 

Blockchain ⛓ Immutable Transparency Security 

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🤨 & 🤔 

fassko www.kristaps.me 

Materials Solidity app: github.com/fassko/grain-shipment-solidity Swift app: github.com/fassko/grain-shipments-ios