android - 如何将 Room TypeConverter 应用于实体的单个字段？

Question

我一直在尝试将 TypeConverter 应用于 Room 数据库实体的单个字段的不同解决方案，但出现错误

Cannot figure out how to save this field into database. You can consider adding a type converter for it.

当我像这样将转换器应用于实体时:

@Entity(tableName = DBKey.calendarDayTable)
@TypeConverters(DateStringConverter::class)
data class CalendarDay(

    @PrimaryKey
    val date: Date
)

一切都按预期工作，但是当我将它应用于这样的字段时:

@Entity(tableName = DBKey.calendarDayTable)
data class CalendarDay(

    @PrimaryKey
    @TypeConverters(DateStringConverter::class)
    val date: Date
)

我收到上面提到的错误。

DateStringConverter 类是:

class DateStringConverter {
    private val formatter = SimpleDateFormat("yyyy-MM-dd")

    @TypeConverter
    fun dateFromString(value: String): Date {
        return formatter.parse(value)!!
    }

    @TypeConverter
    fun dateToString(date: Date): String {
        return formatter.format(date)
    }
}

我正在使用 Room 版本 2.2.5 并使用 Kotlin 编写应用程序。 Room 的依赖项是:

implementation "androidx.room:room-runtime:$room_version"
kapt "androidx.room:room-compiler:$room_version"
implementation "androidx.room:room-ktx:$room_version"

有没有一种方法可以将 DateStringConverter 仅应用于 CalendarDay 实体的 date 字段，或者我是否必须应用它对整个实体？

Answer 1

您必须指定注释应用于该字段

@field:TypeConverters(DateStringConverter::class)
val date: Date

If you don't specify a use-site target, the target is chosen according to the @Target annotation of the annotation being used. If there are multiple applicable targets, the first applicable target from the following list is used:

• param (constructor parameter);
• property;
• field.

https://kotlinlang.org/docs/reference/annotations.html#annotation-use-site-targets