前言

開發(fā)業(yè)務(wù)需求時，遇到了列表中包含完全不同類型的數(shù)據(jù)結(jié)構(gòu)。這種列表我們稱為異構(gòu)列表。以聊天記錄列表為例

[{'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，手動解析不是不行，但肯定不推薦。如果直接使用解析工具，比如用Gson來解析的話，無論定義什么樣的數(shù)據(jù)結(jié)構(gòu)好像都不符合上面的列表元素。

那可不可以做到，我們告訴Gson列表中各個元素分別是什么樣的數(shù)據(jù)類型，這樣它不就知道該怎么解析了嗎？接下來我們通過Gson的TypeAdapter來實現(xiàn)自定義解析。

實現(xiàn)方案

先定義好各種數(shù)據(jù)類型，與msgType字段一一對應(yīng)

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)

接著自定義一個TypeAdapter。

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

可以看到里面有兩個方法：write()負(fù)責(zé)序列化，read()負(fù)責(zé)反序列化。我們先重點關(guān)注read()的實現(xiàn)

實現(xiàn)read()的基本思路如下

讀取msgType字段 根據(jù)msgType判斷對應(yīng)的數(shù)據(jù)類型 根據(jù)該數(shù)據(jù)類型獲取解析該類型的TypeAdapter 交給對應(yīng)類型的TypeAdapter解析

依照上述思路，可以寫出read()的基本實現(xiàn)代碼。當(dāng)然這是比較粗糙的實現(xiàn)，實際上還有其他情況要考慮

class BaseMessageTypeAdapter(private val gson: Gson, private val skipPast: TypeAdapterFactory) : TypeAdapter<BaseMessage>() { override fun read(`in`: JsonReader): BaseMessage? { // 1.讀取msgType字段 val jsonObject = Streams.parse(`in`).asJsonObject val msgType = jsonObject.get('msgType')?.asString // 2.根據(jù)msgType獲取解析該類型的TypeAdapter val adapter = getTypeAdapterByType(msgType) // 3.交給對應(yīng)類型的TypeAdapter解析 return adapter?.fromJsonTree(jsonObject) }}

write()方法沒什么好說的，直接交給對應(yīng)類型的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) }}

接著就是實現(xiàn)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> { // 獲取Gson中該類型對應(yīng)的TypeAdapter return SubTypeAdapterWrapper(clazz, gson.getDelegateAdapter(skipPast, TypeToken.get(clazz))) }

邏輯也比較簡單。需要注意的是，在getTypeAdapter()方法中，要將TypeAdapter<out BaseMessage>轉(zhuǎn)換成TypeAdapter<BaseMessage>。接下來我們看看SubTypeAdapterWrapper是怎樣實現(xiàn)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`) }}

其實就是一個包裝類。將解析R類型的TypeAdapter包裝成解析T類型的TypeAdapter。

最后就是實現(xiàn)一個TypeAdapterFactory，并將其注冊到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>) }}

寫個測試用例測試一下

抽象封裝

為了更好的復(fù)用，接下來我們要對BaseMessageTypeAdapter改造一下。

定義一個新的TypeAdapter子類。將類型和TypeAdapter的關(guān)系用Map來存儲，并提供方法給外部調(diào)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') // 注冊各種類型 adapter.addSubTypeAdapter('text', TextMessage::class.java) adapter.addSubTypeAdapter('image', ImageMessage::class.java) adapter.addSubTypeAdapter('record', RecordMessage::class.java) return adapter as (TypeAdapter<T>) }}總結(jié)

通過自定義TypeAdapter，我們實現(xiàn)了解析異構(gòu)列表的功能。免除手動解析的繁瑣工作，避免出現(xiàn)不必要的錯誤。

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