Android如何解析异构列表

前言

开发业务需求时，遇到了列表中包含完全不同类型的数据结构。这种列表我们称为异构列表。以聊天记录列表为例

[
	{
		"msgType" : "text",
		"id" : "1",
		"content" : "Hello world"
	},
	{
		"msgType" : "record",
		"id" : "2",
		"url" : "https://xxxx.mp4",
		"length" : 123450
	},
	{
		"msgType" : "image",
		"id" : "3",
		"url" : "http://img.558idc.com/uploadfile/allimg/Android/xxxx.jpg",
		"size" : "300x300"
	}
]

要想解析上面的JSON，手动解析不是不行，但肯定不推荐。如果直接使用解析工具，比如用Gson来解析的话，无论定义什么样的数据结构好像都不符合上面的列表元素。

那可不可以做到，我们告诉Gson列表中各个元素分别是什么样的数据类型，这样它不就知道该怎么解析了吗？接下来我们通过Gson的TypeAdapter来实现自定义解析。

实现方案

先定义好各种数据类型，与msgType字段一一对应

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)

接着自定义一个TypeAdapter。

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

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

可以看到里面有两个方法：write()负责序列化，read()负责反序列化。我们先重点关注read()的实现

实现read()的基本思路如下

1. 读取msgType字段
2. 根据msgType判断对应的数据类型
3. 根据该数据类型获取解析该类型的TypeAdapter
4. 交给对应类型的TypeAdapter解析
   

依照上述思路，可以写出read()的基本实现代码。当然这是比较粗糙的实现，实际上还有其他情况要考虑

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)
 }
}

write()方法没什么好说的，直接交给对应类型的TypeAdapter序列化

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)
 }
}

接着就是实现getTypeAdapterByType()方法。

 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)))
 }

逻辑也比较简单。需要注意的是，在getTypeAdapter()方法中，要将TypeAdapter<out BaseMessage>转换成TypeAdapter<BaseMessage>。接下来我们看看SubTypeAdapterWrapper是怎样实现的

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`)
 }
}

其实就是一个包装类。将解析R类型的TypeAdapter包装成解析T类型的TypeAdapter。

最后就是实现一个TypeAdapterFactory，并将其注册到Gson

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>)
 }
}

写个测试用例测试一下

抽象封装

为了更好的复用，接下来我们要对BaseMessageTypeAdapter改造一下。

定义一个新的TypeAdapter子类。将类型和TypeAdapter的关系用Map来存储，并提供方法给外部调用。

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);
 }
}

使用方式

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>)
 }
}

总结

通过自定义TypeAdapter，我们实现了解析异构列表的功能。免除手动解析的繁琐工作，避免出现不必要的错误。

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