Reversing view models in Swift

ViewModel.swift

import Foundation
import UIKit

struct CreditCard {
	let number: String
	let expirationDate: Date
}

class CreditCardView {
	@IBOutlet private weak var numberTextView: UITextView!
	@IBOutlet private weak var monthTextField: UITextField!
	@IBOutlet private weak var yearTextField: UITextField!
	
	var viewModel: ViewModel? {
		didSet {
			numberTextView.text = viewModel?.number
			monthTextField.text = viewModel?.month
			yearTextField.text = viewModel?.year
		}
	}
}

extension CreditCardView {
	struct ViewModel {
		let number: String
		let month: String
		let year: String
	}
}

extension CreditCardView.ViewModel {
	init(creditCard: CreditCard) {
		number = creditCard.number.accountNumberFormatting
		month = "\(creditCard.expirationDate.month)"
		year = "\(creditCard.expirationDate.year)"
	}
	
	var creditCard: CreditCard {
		let number = self.number.trimmingCharacters(in: .whitespaces)
		let date = Date.from(month: Int(month)!, year: Int(year)!)
		return CreditCard(number: number, expirationDate: date)
	}
}

extension String {
	var accountNumberFormatting: String {
		var formattedString = ""
		for (index, character) in enumerated() {
			if index % 4 < 3 || index == count - 1 {
				formattedString.append(character)
			} else {
				formattedString.append("\(character) ")
			}
		}
		return formattedString
	}
}

extension Date {
	static func from(month: Int, year: Int) -> Date {
		let calendar = NSCalendar.current
		var components = DateComponents()
		components.month = month
		components.year = year
		return calendar.date(from: components)!
	}
	
	var month: Int {
		let calendar = NSCalendar.current
		return calendar.component(.month, from: self)
	}
	
	var year: Int {
		let calendar = NSCalendar.current
		return calendar.component(.year, from: self)
	}
}