使用NSCoding Swift保存对象时出现异常

我在尝试使用NSCoding保存/归档对象时遇到了一些问题。我基本上有一个定义问答的QA类,我还有一个包含多个属性的类卷,包括QA对象数组。

我的卷类继承自NSObject和NSCoding,QA对象仅继承自NSObject。

引发的异常是:。。。编码器:无法识别的选择器 发送到实例0x600000077c80

static func save() {
    print("starting save...")
    let DocumentsDirectory = FileManager().urls(for: .documentDirectory, in: .userDomainMask).first!
    let ArchiveURL = DocumentsDirectory.appendingPathComponent("volumesData")
    NSKeyedArchiver.archiveRootObject(VolumeTableViewController.volumesArray, toFile: ArchiveURL.path)
    print("Ending save...")
}

我的音量等级是:

import Foundation
class Volume: NSObject, NSCoding {
    let volumeNumber: Int
    var completed: Bool
    var questionsData: [QA]

    init (volumeNumber: Int, completed: Bool, questionsData: [QA]) {
        self.volumeNumber = volumeNumber
        self.completed = completed
        self.questionsData = questionsData
    }

    // MARK: NSCoding
    public convenience required init?(coder aDecoder: NSCoder) {

        let volumeNumber = aDecoder.decodeObject(forKey: "volumeNumber") as! Int
        let completed = aDecoder.decodeObject(forKey: "completed") as! Bool
        let questionsData = aDecoder.decodeObject(forKey: "questionsData") as! [QA]

        self.init(volumeNumber: volumeNumber, completed: completed, questionsData: questionsData)
    }

    func encode(with aCoder: NSCoder) {
        aCoder.encode(volumeNumber, forKey: "volumeNumber")
        aCoder.encode(completed, forKey: "completed")
        aCoder.encode(questionsData, forKey: "questionsData")
    }
}

我的QA课程是:

 import Foundation
class QA: NSObject {
    let questionsText: String
    let answerText: [String]
    let correctAnswer: [Bool]
    var selectedAnswer: [Bool?]


    // Create standard initiator
    init(questionsText:String, answerText: [String], correctAnswer: [Bool], selectedAnswer: [Bool?]) {
        self.questionsText = questionsText
        self.answerText = answerText
        self.correctAnswer = correctAnswer
        self.selectedAnswer = selectedAnswer
    }
}