Codable.swift

struct Person: Codable {
    
    var firstName: String
    var lastName: String
    
    var name: Data? {
        return try? PropertyListEncoder().encode(self)
    }
    
    init?(name: Data) {
        if let newValue = try? PropertyListDecoder().decode(Person.self, from: name) {
            self = newValue
        } else {
            return nil
        }
    }
    
    // found actually use json to encode and decode would be better
    // because json would write the data to json format, which is very nice to read 
    var json: Data? {
        return try? JSONEncoder().encode(self)
    }
    
    init?(json: Data) {
        if let newValue = try? JSONDecoder().decode(Person.self, from: json) {
            self = newValue
        } else {
            return nil
        }
    }

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
        
    }
}

/// Save file
let moi = Person(firstName: "Matt", lastName: "Neuburg")
    
if let moifile = try? FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent("moi.txt"), let name = moi.name{
    do {
        try name.write(to: moifile)
        print(moifile)
        print("saved data successfully")
    } catch let error {
        print("couldn't save \(error)")
    }
}

/// Read file
if let moifile = try? FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false).appendingPathComponent("moi.txt"){
    do {
        let persondata = try Data(contentsOf: moifile)
        let person = Person(name:persondata)
        print(person)
    } catch let error {
        print("couldn't save \(error)")
    }
}