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Gson 数据解析

0x01 Kotlin Gson 解析 data class 两条黄金法则：

1、 String 必须是可空类型 String?

2、 需要使用默认值，则全部字段都必须给予默认值，以满足kotlin对象有空的构造函数

0x02 手动解析Gson基础字段

1、msg 可空String解析 jsonReader.peek() == JsonToken.NULL

import com.google.gson.Gson
import com.google.gson.JsonSyntaxException
import com.google.gson.TypeAdapter
import com.google.gson.reflect.TypeToken
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonToken
import okhttp3.ResponseBody
import retrofit2.Converter
import java.lang.reflect.ParameterizedType
import java.lang.reflect.Type

class ResponseBodyConverter(val gson: Gson, val type: Type) :
    Converter<ResponseBody, BaseResponse<Any>> {
    override fun convert(value: ResponseBody): BaseResponse<Any> {
        val dataType = GenericsUtils.getParameterUpperBound(
            0,
            type as ParameterizedType
        )
        val baseResponse = BaseResponse<Any>()
        val jsonReader = JsonReader(value.charStream())
        try {
            jsonReader.beginObject()
            while (jsonReader.hasNext()) {
                val name: String = jsonReader.nextName()
                when (name) {
                    "code" -> {
                        baseResponse.code = jsonReader.nextString()
                    }
                    "msg" -> {
                        // this works, but do not do this.
                        if (jsonReader.peek() == JsonToken.NULL) {
                            jsonReader.nextNull()
                            baseResponse.msg = null
                        } else {
                            baseResponse.msg = jsonReader.nextString()
                        }
                    }
                    "data" -> {
                        val mapped: TypeAdapter<*>? = gson.getAdapter(TypeToken.get(dataType))
                        baseResponse.data = mapped?.read(jsonReader)
                    }
                    else -> {
                        jsonReader.skipValue()
                    }
                }
            }
        } catch (e: IllegalStateException) {
            throw JsonSyntaxException(e)
        } catch (e: IllegalAccessException) {
            throw AssertionError(e)
        }
        jsonReader.endObject()
        return baseResponse
    }
}