FlightBookingView.swift

import SwiftUI
import Combine
​
struct FlightBookingView: View {
    @ObservedObject var model: FlightBookingViewModel
    
    var body: some View {
        NavigationView {
            Form {
                DatePicker(selection: $model.departureDate, in: model.departureDateRange, displayedComponents: .date) {
                    Text("Departure Date")
                }
                DatePicker(selection: $model.returnDate, in: model.returnDateRange, displayedComponents: .date) {
                    Text("Return Date")
                }
                Text(model.message)
            }
            .listStyle(GroupedListStyle())
            .navigationBarItems(trailing: Button(action: {
                print("")
            })
            { Text("Submit") }
            )
        }
    }
}
​
struct FlightBookingView_Previews: PreviewProvider {
    static var previews: some View {
        FlightBookingView(model: FlightBookingViewModel().connect())
    }
}
​
class FlightBookingViewModel: ObservableObject {
    // Input
    @Published var departureDate: Date = Date()
    @Published var returnDate: Date = Calendar.current.date(byAdding: .day, value: 1, to: Date())!
    
    // Output
    @Published var dates: (Date, Date) = (Date(), Date())
    @Published var message: String = ""
    
    var datesCancellable: AnyCancellable!
    var returnCancellable: AnyCancellable!
    
    var departureDateRange: ClosedRange<Date> {
        return Date()...Date.distantFuture
    }
    
    var returnDateRange: ClosedRange<Date> {
        return departureDate...Date.distantFuture
    }
    
    func debug(_ dates: (Date, Date)) -> String {
        let dateToString = { (date: Date) -> String in
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = "dd-MM-yyyy"
            return dateFormatter.string(from: date)
        }
        let d = dateToString(dates.0)
        let r = dateToString(dates.1)
        return "\(d) - \(r)"
    }
    
    func connect() -> Self {
        self.datesCancellable = Publishers.CombineLatest($departureDate, $returnDate)
            .sink(receiveValue: {_ in
                if self.departureDate > self.returnDate {
                    self.returnDate = self.departureDate
                }
                self.message = self.debug((self.departureDate, self.returnDate))
            })
        return self
    }
}