[Android][Room] 将密封类存储到 Room 数据库中

我花了一些时间，这是正确的行为，密封的类不能实例化.解决方案基于密封类的子类可以实例化的思想工作. 类型转换器成功地从数据库中获取数据(数据库会将其传递给类型转换器)，此时出现问题.

        return Gson().fromJson(json, LibraryEntityDownloadStatus::class.java)

GSON库无法实例化LibraryEntityDownloadStatus.但是，如果该信息可用，它可以实例化密封类的子类.所以我放了一个名为‘tag’的字段(对于每个子元素来说应该是唯一的)，可以用来识别子元素的类型.

sealed class ShareLibraryEntityDownloadStatus(val tag: String) {
class DOWNLOADING(val downloadProgress: Int) :
    ShareLibraryEntityDownloadStatus(tag = "DOWNLOADING")

class DOWNLOADED(val filePath: String) :
    ShareLibraryEntityDownloadStatus(tag = "DOWNLOADED")

class FAILED(val failureReason: DownloadFailureReason) :
    ShareLibraryEntityDownloadStatus(tag = "FAILED")

class NOTDOWNLOADED : ShareLibraryEntityDownloadStatus(tag = "NOTDOWNLOADED")}

并对类型转换器进行如下更改.

@TypeConverter
fun downloadStatusToString(downloadStatus: LibraryEntityDownloadStatus): String {
    return Gson().toJson(downloadStatus)
}

@TypeConverter
fun downloadStatusFromString(json: String): LibraryEntityDownloadStatus {
    var output : LibraryEntityDownloadStatus = LibraryEntityDownloadStatus.NOTDOWNLOADED()
    val obj = JsonParser.parseString(json).asJsonObject
    when(obj["tag"].asString) {
        "NOTDOWNLOADED" -> output = LibraryEntityDownloadStatus.NOTDOWNLOADED()
        "DOWNLOADING" -> output = Gson().fromJson(json, LibraryEntityDownloadStatus.DOWNLOADING::class.java)
        "DOWNLOADED" -> output = Gson().fromJson(json, LibraryEntityDownloadStatus.DOWNLOADED::class.java)
        "DOWNLOADFAILED" -> output = Gson().fromJson(json, LibraryEntityDownloadStatus.FAILED::class.java)
    }
    return output
}

JsonParser和gson来自"Package com.google.gson；"

这显然奏效了，我们知道这是有充分理由的.这是最好的方法吗？让我知道哪种方法效果更好.