Using NSUserDefaults on arrays

You need to convert it to NSData using NSKeyedArchiver before storing it to NSUserDefaults, try like this:

update: Xcode 11.4 • Swift 5.2 or later

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let list = List(name: "Student")
        list.answers = [Answer(english: "english answer", chinese: "中文回答")]
        let data = (try? NSKeyedArchiver.archivedData(withRootObject: [list], requiringSecureCoding: false)) ?? Data()
        UserDefaults.standard.set(data, forKey: "listData")
        guard
            let loadedData = UserDefaults.standard.data(forKey: "listData"),
            let loadedArray = try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(loadedData) as? [List]
            else { return }
        print(loadedData.count)
        print(loadedArray.first ?? "none")
        print(loadedArray.first?.name ?? "no name")
        print(loadedArray.first?.answers.first?.english ?? "no english")
        print(loadedArray.first?.answers.first?.chinese ?? "no chinese")
    }
}

---

class Answer: NSObject, NSCoding {
    let english: String
    let chinese: String
    init(english: String, chinese: String) {
        self.english = english
        self.chinese = chinese
    }
    required init(coder decoder: NSCoder) {
        self.english = decoder.decodeString(forKey: "english")
        self.chinese = decoder.decodeString(forKey: "chinese")
    }
    func encode(with coder: NSCoder) {
        coder.encode(english, forKey: "english")
        coder.encode(chinese, forKey: "chinese")
    }
}

---

class List: NSObject, NSCoding {
    let name: String
    fileprivate var data = Data()
    var answers: [Answer] {
        get {
            (try? NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(data)) as? [Answer] ?? []
        }
        set {
            data = (try? NSKeyedArchiver.archivedData(withRootObject: newValue, requiringSecureCoding: false)) ?? Data()
        }
    }
    init(name: String) {
        self.name = name
    }
    required init(coder decoder: NSCoder) {
        self.data = decoder.decodeData(forKey: "answersData")
        self.name = decoder.decodeString(forKey: "name")
    }
    func encode(with coder: NSCoder) {
        coder.encode(data, forKey: "answersData")
        coder.encode(name, forKey: "name")
    }
}

---

extension NSCoder {
    func decodeString(forKey key: String) -> String {
        return decodeObject(forKey: key) as? String ?? ""
    }
    func decodeData(forKey key: String) -> Data {
        return decodeObject(forKey: key) as? Data ?? Data()
    }
}