Android如何解析异构列表
时间：2023-01-07 09:28:14|栏目：Android代码|点击： 次
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" : "https://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
- 读取msgType字段
- 根据msgType判断对应的数据类型
- 根据该数据类型获取解析该类型的TypeAdapter
- 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。
