How to Save List> to Room Database?

Question

How to save List<List<Bitmap>> to room database? Want to save List<List<Bitmap>> to room database, but it gives error:

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

How to add a type converter for it?

@Entity(tableName = "items")
data class Item(
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,
    var image: List<List<Bitmap>>? = null,
)

Answer 1

This won't be effective for large images. You don't want megabytes of data in each row of your database. If the images are large, save them as files and put just the file names in your database.

But assuming these are tiny images:

Follow the instructions here for using type converters with your database. Based on the answer here and translating it, we get the following functions.

class Converters {
  @TypeConverter
  fun stringToBitmap(value: String?): Bitmap? {
    value ?: return null
    val bytes = Base64.decode(value, Base64.DEFAULT)
    val bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.size)
    if (bitmap == null) {
        Log.e("stringToBitmap converter", "Failed to decode image.")
    }
    return bitmap
  }

  @TypeConverter
  fun bitmapToString(bitmap: Bitmap?): String? {
    bitmap ?: return null
    val stream = ByteArrayOutputStream()
    if (!bitmap.compress(Bitmap.CompressFormat.PNG, 0, stream)) {
        Log.e("bitmapToString converter", "Failed to compress image.")
        return null
    }
    return Base64.encodeToString(stream.toByteArray(), Base64.DEFAULT)
  }
}