scala - 使用Akka HTTP处理PATCH请求并绕过可空字段

这是我做这种事情的方式：

import io.circe.{Decoder, Encoder, FailedCursor, Json}
import java.util.UUID

sealed trait UpdateOrDelete[+A]

case object Missing                      extends UpdateOrDelete[Nothing]
case object Delete                       extends UpdateOrDelete[Nothing]
final case class UpdateWith[A](value: A) extends UpdateOrDelete[A]

object UpdateOrDelete {
  implicit def decodeUpdateOrDelete[A](
    implicit decodeA: Decoder[A]
  ): Decoder[UpdateOrDelete[A]] = Decoder.withReattempt {
    // We're trying to decode a field but it's missing.
    case c: FailedCursor if !c.incorrectFocus => Right(Missing)
    case c => Decoder.decodeOption[A].tryDecode(c).map {
      case Some(a) => UpdateWith(a)
      case None    => Delete
    }
  }

  // Random UUID to _definitely_ avoid collisions
  private[this] val marker: String   = s"$$marker-${UUID.randomUUID()}-marker$$"
  private[this] val markerJson: Json = Json.fromString(marker)

  implicit def encodeUpdateOrDelete[A](
    implicit encodeA: Encoder[A]
  ): Encoder[UpdateOrDelete[A]] = Encoder.instance {
    case UpdateWith(a) => encodeA(a)
    case Delete        => Json.Null
    case Missing       => markerJson
  }

  def filterMarkers[A](encoder: Encoder.AsObject[A]): Encoder.AsObject[A] =
    encoder.mapJsonObject(
      _.filter {
        case (_, value) => value != markerJson
      }
    )
}

接着：

import io.circe.generic.semiauto._

case class UserPatch(
  id: Long,
  firstName: UpdateOrDelete[String],
  lastName: UpdateOrDelete[String]
)

object UserPatch {
  implicit val decodeUserPatch: Decoder[UserPatch] = deriveDecoder
  implicit val encodeUserPatch: Encoder.AsObject[UserPatch] =
    UpdateOrDelete.filterMarkers(deriveEncoder[UserPatch])
}

接着：

scala> import io.circe.syntax._
import io.circe.syntax._

scala> UserPatch(101, Missing, Delete).asJson
res0: io.circe.Json =
{
  "id" : 101,
  "lastName" : null
}

scala> UserPatch(101, UpdateWith("Foo"), Missing).asJson
res1: io.circe.Json =
{
  "id" : 101,
  "firstName" : "Foo"
}

scala> io.circe.jawn.decode[UserPatch]("""{"id":1}""")
res2: Either[io.circe.Error,UserPatch] = Right(UserPatch(1,Missing,Missing))

这种方法使您可以更清晰地建模意图，同时仍然可以使用通用派生来避免编写编解码器的大部分样板。