How many years are you lived?

.swift

import SwiftUI

struct DaysLived: View {
    
    @State private var userBirthday = ""
    @State private var userDaysLived: Int?
    @State private var birthdate: Date?
    @State private var wrongDateFormat = false
    let alertMessage = """
        Out of range or
        Invalid date format,
        please write yyyy-MM-dd
    """

    var body: some View {
        ZStack {
            Color.cyan.opacity(0.3).ignoresSafeArea()
            VStack {
                TextField("Write your birthday", text: $userBirthday)
                    .font(.system(size: 36))
                    .bold()
                Spacer()
                Text("You have lived").font(.title)
                Text(String(userDaysLived ?? 0))
                    .font(.system(size: 100, weight: .bold))
                    .foregroundColor(.black)
                Text("days until now").font(.title)
                Spacer()
                Button("How many days you lived") {
                    userDaysLived = daysLivedCalculation(userBirthday: userBirthday)
                }
                .buttonStyle(.borderedProminent).font(.title2).foregroundColor(.white)
            }
            .bold()
            .foregroundColor(.blue)
            .fontDesign(.rounded)
            .padding(25)
            .alert(alertMessage, isPresented: $wrongDateFormat) {
                Button("OK", role: .cancel) {
                    self.userDaysLived = 0
                }
            }
        }
        
    }
    
    func daysLivedCalculation(userBirthday: String) -> Int {
        
        // Date Formatter
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd"
        if let birthdate = dateFormatter.date(from: userBirthday) {
            self.birthdate = birthdate
        } else {
            self.userBirthday = ""
            self.wrongDateFormat = true
        }

        // Current Date
        let currentDate = Date()
        let calendar = Calendar.current
        let components = calendar.dateComponents([.day], from: birthdate ?? Date.now, to: currentDate)
        
        if let days = components.day {
            return days
        } else {
            return 0
        }
    }

}

#Preview {
    DaysLived()
}