1  /*
2   * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
3   */
4  
5  package kotlinx.serialization.json
6  
7  import kotlinx.serialization.encoding.*
8  
9  /**
10   * Encoder used by [Json] during serialization.
11   * This interface can be used to inject desired behaviour into a serialization process of [Json].
12   *
13   * Typical example of the usage:
14   * ```
15   * // Class representing Either<Left|Right>
16   * sealed class Either {
17   *     data class Left(val errorMsg: String) : Either()
18   *     data class Right(val data: Payload) : Either()
19   * }
20   *
21   * // Serializer injects custom behaviour by inspecting object content and writing
22   * object EitherSerializer : KSerializer<Either> {
23   *     override val descriptor: SerialDescriptor = buildSerialDescriptor("package.Either", PolymorphicKind.SEALED) {
24   *          // ..
25   *      }
26   *
27   *     override fun deserialize(decoder: Decoder): Either {
28   *         val input = decoder as? JsonDecoder ?: throw SerializationException("This class can be decoded only by Json format")
29   *         val tree = input.decodeJsonElement() as? JsonObject ?: throw SerializationException("Expected JsonObject")
30   *         if ("error" in tree) return Either.Left(tree["error"]!!.jsonPrimitive.content)
31   *         return Either.Right(input.json.decodeFromJsonElement(Payload.serializer(), tree))
32   *     }
33   *
34   *     override fun serialize(encoder: Encoder, value: Either) {
35   *         val output = encoder as? JsonEncoder ?: throw SerializationException("This class can be encoded only by Json format")
36   *         val tree = when (value) {
37   *           is Either.Left -> JsonObject(mapOf("error" to JsonPrimitve(value.errorMsg)))
38   *           is Either.Right -> output.json.encodeToJsonElement(Payload.serializer(), value.data)
39   *         }
40   *         output.encodeJsonElement(tree)
41   *     }
42   * }
43   * ```
44   *
45   * ### Not stable for inheritance
46   *
47   * `JsonEncoder` interface is not stable for inheritance in 3rd party libraries, as new methods
48   * might be added to this interface or contracts of the existing methods can be changed.
49   * Accepting this interface in your API methods, casting [Encoder] to [JsonEncoder] and invoking its
50   * methods is considered stable.
51   */
52  public interface JsonEncoder : Encoder, CompositeEncoder {
53      /**
54       * An instance of the current [Json].
55       */
56      public val json: Json
57  
58      /**
59       * Appends the given JSON [element] to the current output.
60       * This method is allowed to invoke only as the part of the whole serialization process of the class,
61       * calling this method after invoking [beginStructure] or any `encode*` method will lead to unspecified behaviour
62       * and may produce an invalid JSON result.
63       * For example:
64       * ```
65       * class Holder(val value: Int, val list: List<Int>())
66       *
67       * // Holder serialize method
68       * fun serialize(encoder: Encoder, value: Holder) {
69       *     // Completely okay, the whole Holder object is read
70       *     val jsonObject = JsonObject(...) // build a JsonObject from Holder
71       *     (encoder as JsonEncoder).encodeJsonElement(jsonObject) // Write it
72       * }
73       *
74       * // Incorrect Holder serialize method
75       * fun serialize(encoder: Encoder, value: Holder) {
76       *     val composite = encoder.beginStructure(descriptor)
77       *     composite.encodeSerializableElement(descriptor, 0, Int.serializer(), value.value)
78       *     val array = JsonArray(value.list)
79       *     // Incorrect, encoder is already in an intermediate state after encodeSerializableElement
80       *     (composite as JsonEncoder).encodeJsonElement(array)
81       *     composite.endStructure(descriptor)
82       *     // ...
83       * }
84       * ```
85       */
encodeJsonElementnull86      public fun encodeJsonElement(element: JsonElement)
87  }
88