EnvelopeFactory.kt

/**
 * Complements @Enveloped by performing custom deserialization 
 * for a response that is wrapped in a JSON Object.
 */
internal class EnvelopeFactory : JsonAdapter.Factory {
  companion object {
    val INSTANCE = EnvelopeFactory()
  }
  
  override fun create(
          type: Type,
          annotations: MutableSet<out Annotation>,
          moshi: Moshi): JsonAdapter<*>? {
    val delegateAnnotations = Types.nextAnnotations(
            annotations,
            Enveloped::class.java) ?: return null
    val delegate = moshi.nextAdapter<Any>(
            this,
            type,
            delegateAnnotations)
    return EnvelopeJsonAdapter(delegate)
  }
  
  private class EnvelopeJsonAdapter(val delegate: JsonAdapter<*>) : JsonAdapter<Any>() {
    override fun fromJson(reader: JsonReader): Any? {
      reader.beginObject()
      reader.nextName()
      val envelope = delegate.fromJson(reader)
      reader.endObject()
      return envelope
    }
      
    override fun toJson(writer: JsonWriter?, value: Any?)
            = throw UnsupportedOperationException(
            "@Enveloped is only used to deserialize objects.")
  }
}