在swift中保存对象内的对象数组

Question

我是swift的新手，我正在努力坚持数据。我创建了一个名为Students的类，其中存储了学生的基本属性，例如名称和计划对象数组。该计划是另一个包含每个计划实例的计数器的类。

class Students: NSObject, NSCoding{

var name: String = ""
var schedule = [Schedule]()

static let DocumentsDirectory = NSFileManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask).first!
static let ArchiveURL = DocumentsDirectory.URLByAppendingPathComponent("students")

struct PropertyKey {
    static let nameKey = "name"
    static let scheduleKey = "schedule"
}



init(name: String, schedule: [Schedule]){
    self.name = name
    self.schedule = schedule

    super.init()
}

init(name: String)
{
    self.name = name

    super.init()
}


func addClass(className: String){
    let scheduleItem = Schedule()
    scheduleItem.scheduleName = className
    schedule.append(scheduleItem)
}

func encodeWithCoder(aCoder: NSCoder) {
    aCoder.encodeObject(name,forKey: PropertyKey.nameKey)
    aCoder.encodeObject(schedule, forKey: PropertyKey.scheduleKey)
}

required convenience init?(coder aDecoder: NSCoder){
    let name = aDecoder.decodeObjectForKey(PropertyKey.nameKey) as! String
    let schedule = aDecoder.decodeObjectForKey(PropertyKey.scheduleKey) as! [Schedule]
    self.init(name: name, schedule: schedule)
}

}

class Schedule{

    var counter = 0
    var scheduleName: String = ""

    func increase()
    {
        counter += 1
    }

    func reset()
    {
        counter = 0
    }

}

我的问题是，当我调用我的保存方法时

func save(){
    let isSuccessfulSave = NSKeyedArchiver.archiveRootObject(students, toFile: Students.ArchiveURL.path!)

    if !isSuccessfulSave {
        print("Failed to save meals...")
    }
}

我从第

行收到错误

aCoder.encodeObject(schedule, forKey: PropertyKey.scheduleKey)

为什么不让我保存计划对象数组？