Android如何解析异构列表

[
	{
		"msgType" : "text",
		"id" : "1",
		"content" : "Hello world"
	},
	{
		"msgType" : "record",
		"id" : "2",
		"url" : "https://xxxx.mp4",
		"length" : 123450
	},
	{
		"msgType" : "image",
		"id" : "3",
		"url" : "/file/imgs/upload/202211/11/kssyku0ayuv.jpg",
		"size" : "300x300"
	}
]

abstract class BaseMessage(val id: String?, val msgType: String?)

class TextMessage(id: String?, msgType: String?, val content: String?
) : BaseMessage(id, msgType)

class ImageMessage(id: String?, msgType: String?, val url: String?, val size: String?
) : BaseMessage(id, msgType)

class RecordMessage(id: String?, msgType: String?, val url: String?, val length: Long
) : BaseMessage(id, msgType)

class BaseMessageTypeAdapter : TypeAdapter<BaseMessage>() {
 override fun write(out: jsonWriter, value: BaseMessage?) {
 }

 override fun read(`in`: JsonReader): BaseMessage? {
 }
}

class BaseMessageTypeAdapter(private val gson: Gson, 
        private val skipPast: TypeAdapterFactory
) : TypeAdapter<BaseMessage>() {
 override fun read(`in`: JsonReader): BaseMessage? {
  // 1.读取msgType字段
  val jsonObject = Streams.parse(`in`).asJsonObject
  val msgType = jsonObject.get("msgType")?.asString
  // 2.根据msgType获取解析该类型的TypeAdapter
  val adapter = getTypeAdapterByType(msgType)
  // 3.交给对应类型的TypeAdapter解析
  return adapter?.fromJsonTree(jsonObject)
 }
}

class BaseMessageTypeAdapter(private val gson: Gson, 
        private val skipPast: TypeAdapterFactory
) : TypeAdapter<BaseMessage>() {
 override fun write(out: JsonWriter, value: BaseMessage?) {
  if (value == null) {
   out.nullValue()
   return
  }
  getTypeAdapterByType(value.msgType)?.write(out, value)
 }
}

 private fun getTypeAdapterByType(type: String?): TypeAdapter<BaseMessage>? {
  return when (type) {
   "text" -> getTypeAdapter(TextMessage::class.java)
   "image" -> getTypeAdapter(ImageMessage::class.java)
   "record" -> getTypeAdapter(RecordMessage::class.java)
   else -> null
  }
 }

 private fun <R : BaseMessage> getTypeAdapter(clazz: Class<R>): TypeAdapter<BaseMessage> {
  // 获取Gson中该类型对应的TypeAdapter
  return SubTypeAdapterWrapper(clazz, gson.getDelegateAdapter(skipPast, TypeToken.get(clazz)))
 }

class SubTypeAdapterWrapper<T, R : T>(private val clazz: Class<R>,
          private val adapter: TypeAdapter<R>
) : TypeAdapter<T>() {
 override fun write(out: JsonWriter, value: T) {
  if (!clazz.isInstance(value)) {
   throw JsonSyntaxException("Expected a " + clazz.name + " but was " + value)
  }
  adapter.write(out, value as R)
 }

 override fun read(`in`: JsonReader): T {
  return adapter.read(`in`)
 }
}

class BaseMessageTypeAdapterFactory : TypeAdapterFactory {
 override fun <T : Any?> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
  if (!BaseMessage::class.java.isAssignableFrom(type.rawType)) {
   return null
  }
  return BaseMessageTypeAdapter(gson, this) as (TypeAdapter<T>)
 }
}

public class HeterogeneousTypeAdapter<T> extends TypeAdapter<T> {
 private final Gson mGson;
 private final TypeAdapterFactory mSkipPast;
 private final String mFieldName;
 private final Map<String, TypeAdapter<T>> mClassToAdapterMap = new HashMap<>();
 private final Map<String, TypeAdapter<T>> mFieldToAdapterMap = new HashMap<>();

 public HeterogeneousTypeAdapter(Gson gson, TypeAdapterFactory skipPast, String fieldName) {
  mGson = gson;
  mSkipPast = skipPast;
  mFieldName = fieldName;
 }

 public <R extends T> void addSubTypeAdapter(final String fieldValue,
            final Class<R> cls) {
  final TypeAdapter<R> typeAdapter = mGson.getDelegateAdapter(mSkipPast, TypeToken.get(cls));
  addSubTypeAdapter(fieldValue, cls, typeAdapter);
 }

 public <R extends T> void addSubTypeAdapter(final String fieldValue,
            final Class<R> cls,
            final TypeAdapter<R> typeAdapter) {
  final TypeAdapter<T> adapter = new SubTypeAdapterWrapper<>(cls, typeAdapter);
  mClassToAdapterMap.put(cls.getName(), adapter);
  mFieldToAdapterMap.put(fieldValue, adapter);
 }

 @Override
 public void write(JsonWriter out, T value) throws IOException {
  if (value == null) {
   out.nullValue();
   return;
  }
  getTypeAdapterByClass(value.getClass()).write(out, value);
 }

 @Override
 public T read(JsonReader in) throws IOException {
  if (in.peek() == JsonToken.NULL) {
   in.nextNull();
   return null;
  }

  final JsonObject jsonObject = Streams.parse(in).getAsJsonObject();
  final JsonElement fieldElement = jsonObject.get(mFieldName);
  if (fieldElement == null || fieldElement.isJsonNull()) {
   throw new JsonSyntaxException("Field " + mFieldName + " is null or not found");
  }

  final String field = fieldElement.getAsJsonPrimitive().getAsString();
  final TypeAdapter<T> adapter = getTypeAdapterByField(field);
  if (adapter == null) {
   // Unknown field, just skip
   return null;
  }
  return adapter.fromJsonTree(jsonObject);
 }

 private TypeAdapter<T> getTypeAdapterByClass(Class<?> cls) {
  TypeAdapter<T> adapter = mClassToAdapterMap.get(cls.getName());
  if (adapter == null) {
   throw new JsonParseException("Unknown class : " + cls);
  }
  return adapter;
 }

 private TypeAdapter<T> getTypeAdapterByField(String field) {
  return mFieldToAdapterMap.get(field);
 }
}

class BaseMessageTypeAdapterFactory : TypeAdapterFactory {
 override fun <T : Any?> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {
  if (!BaseMessage::class.java.isAssignableFrom(type.rawType)) {
   return null
  }
  val adapter = HeterogeneousTypeAdapter<BaseMessage>(gson, this, "msgType")
  // 注册各种类型
  adapter.addSubTypeAdapter("text", TextMessage::class.java)
  adapter.addSubTypeAdapter("image", ImageMessage::class.java)
  adapter.addSubTypeAdapter("record", RecordMessage::class.java)
  return adapter as (TypeAdapter<T>)
 }
}