Android 无法从Firestore检索对象

我是Firebase和Kotlin的新手，正在尝试使用它进行应用。我有一个名为Announcements的数据类，我已将用于公告的数据保存在Firestore中。但是，当我想从Firestore检索数据时，它总是给我以下错误：

2019-10-09 03:14:13.016 14552-14552/com.example.finalyearproject W/Firestore: (21.1.1) [CustomClassMapper]: No setter/field for Content found on class com.example.finalyearproject.Announcement
2019-10-09 03:14:13.017 14552-14552/com.example.finalyearproject W/Firestore: (21.1.1) [CustomClassMapper]: No setter/field for Title found on class com.example.finalyearproject.Announcement
2019-10-09 03:14:13.017 14552-14552/com.example.finalyearproject W/Firestore: (21.1.1) [CustomClassMapper]: No setter/field for Id found on class com.example.finalyearproject.Announcement
2019-10-09 03:14:13.018 14552-14552/com.example.finalyearproject W/Firestore: (21.1.1) [CustomClassMapper]: No setter/field for Date found on class com.example.finalyearproject.Announcement

以下是我的密码：

val db = FirebaseFirestore.getInstance()
val announcement = Announcement(_aTitle = "Hello Everyone", _aContent = "A new age begins.", _aDate = Date())
announcement.writeToDatabase()

val mAnnouncements: ArrayList<Announcement> = ArrayList()

db.collection(Announcement.ANNOUNCEMENT_COLLECTION).get().addOnSuccessListener {
            mAnnouncements.addAll(it.toObjects(Announcement::class.java))
}

我试图寻找答案，但在科特林似乎没有合适的答案。我曾尝试为每个变量添加自定义getter和setter，但仍然不起作用

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如果有人能解决我的问题，请帮忙。提前谢谢你

如果要将Kotlin数据类与一起使用，则必须将数据类的每个字段设置为可为null的

var

。Firestore SDK不知道如何将文档字段映射到数据类构造函数参数中。如果字段可为空

val

，则Kotlin编译器将为每个字段创建getter和setter，以便Firestore SDK可以发现和调用它们

如果您想要一个包含

val

字段的适当的不可变数据类，您必须手动读取文档中的每个字段，并自己调用数据类构造函数

class Announcement(_aId: UUID = UUID.randomUUID(), _aTitle: String = " ", _aContent: String = " ", _aDate: Date = Date()) {

    var aId = _aId
    var aTitle = _aTitle
    var aContent = _aContent
    var aDate = _aDate

    fun writeToDatabase() {

        val db = FirebaseFirestore.getInstance()

        val write = hashMapOf(
            ANNOUNCEMENT_ID to this.aId,
            ANNOUNCEMENT_TITLE to this.aTitle,
            ANNOUNCEMENT_CONTENT to this.aContent,
            ANNOUNCEMENT_DATE to this.aDate
        )

        db.collection(ANNOUNCEMENT_COLLECTION).document(this.aId.toString()).set(write).addOnSuccessListener { Log.d("Success", "Success") }


    }

    companion object {
        const val ANNOUNCEMENT_COLLECTION = "announcement"
        const val ANNOUNCEMENT_ID = "Id"
        const val ANNOUNCEMENT_TITLE = "Title"
        const val ANNOUNCEMENT_CONTENT = "Content"
        const val ANNOUNCEMENT_DATE = "Date"
    }
}