Retrofit 2.0 Gson解析数据的特殊处理

Retrofit 2.0的使用及封装见 Retrofit 2.0的封装与异常处理

开发过程中有时会遇到这种需求，即后台返回的数据，我们需要做进一步处理，比如字段的替换、筛选或者排序，结合上一篇Retrofit 2.0的使用，通常情况下我们在onResponse回调后获得后台返回的model，然后再执行一些特殊处理，如果是耗时操作，我们还需要开个线程...这种方式既不灵活也不优雅。

我们能不能在http获取数据后，onResponse回调之前，即Retrofit2.0 Gson解析时“顺便”把数据处理了？这样既保证了在子线程又能保持onResponse回调中代码的清爽。

下面看我的解决方案：

interface ResponseConverter <T>{
    fun onGsonResponseBody(value: T): T
}

1. 先定义一个接口，如过某个请求需要特殊处理，实现该接口并在onGsonResponseBody回调中进行处理，这里不需要考虑线程，因为该回调本身就是在子线程中执行的，这个后面会讲，示例代码如下：

class MarketResponseConverter : ResponseConverter<HttpResponse<Market>> {
    override fun onGsonResponseBody(httpResponse: HttpResponse<Market>): HttpResponse<Market>{
        httpResponse.msg = "this is test message"
        return httpResponse
    }
}

2. 再定义一个注解，该注解是加到Retrofit2.0定义的请求上，好让GsonResponseBodyConverter知道我这个请求需要单独做处理，注解的属性即上面我们定义的ResponseConverter的Class

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class ResponseProcess(val clazz: KClass<out ResponseConverter<*>>)

interface ApiService {
    @POST("market/getMarket")
    @ResponseProcess(MarketResponseConverter::class)
    fun getMarket(): Call<HttpResponse<Market>>

3. 重点，自定义GsonConverterFactory与GsonResponseBodyConverter , Gson解析完后不直接返回数据，而是判断请求是否有加ResponseProcess注解，如果有ResponseProcess注解则通过反射直接获取该注解属性的ResponseConverter数据处理类，执行onGsonResponseBody后再返回数据。

class CustomGsonConverterFactory(val gson: Gson) : Converter.Factory() {
    companion object {
        fun create(): CustomGsonConverterFactory {
            return create(Gson())
        }

        private fun create(gson: Gson?): CustomGsonConverterFactory {
            if (gson == null) throw NullPointerException("gson == null")
            return CustomGsonConverterFactory(gson)
        }
    }

    override fun responseBodyConverter(type: Type?, annotations: Array<out Annotation>?, retrofit: Retrofit?): Converter<ResponseBody, *>? {
        /** 直接将annotations作为参数传递给CustomGsonResponseBodyConverter**/
        return CustomGsonResponseBodyConverter(gson, annotations, gson.getAdapter(TypeToken.get(type)))
    }

    override fun requestBodyConverter(type: Type?, parameterAnnotations: Array<out Annotation>?,
                                      methodAnnotations: Array<out Annotation>?, retrofit: Retrofit?): Converter<*, RequestBody>? {
        return CustomGsonRequestBodyConverter(gson, gson.getAdapter(TypeToken.get(type)))
    }
}

class CustomGsonResponseBodyConverter<T>(private val gson: Gson, private val annotations: Array<out Annotation>?, private val adapter: TypeAdapter<T>) : Converter<ResponseBody, T> {

    override fun convert(value: ResponseBody): T {
        val response = value.string()
        val httpStatus = gson.fromJson(response, HttpStatus::class.java)
        /** 本节无关，异常统一处理，见上篇《Retrofit 2.0的封装与异常处理》**/
        if (!httpStatus.isSuccess()) {
            value.close()
            throw ApiException(httpStatus.code, httpStatus.msg)
        }
        val contentType = value.contentType()
        val charset = contentType?.charset(UTF_8) ?: UTF_8
        val inputStream = ByteArrayInputStream(response.toByteArray())
        val reader = InputStreamReader(inputStream, charset!!)
        val jsonReader = gson.newJsonReader(reader)

        value.use { _ ->
            val result = adapter.read(jsonReader)
            if (jsonReader.peek() != JsonToken.END_DOCUMENT) {
                throw JsonIOException("JSON document was not fully consumed.")
            }

            /** 遍历annotations判断该请求是否存在ResponseProcess注释 **/
            if (annotations != null) {
                for (annotation: Annotation in annotations) {
                    if (annotation is ResponseProcess) {
                        @Suppress("UNCHECKED_CAST")
                        /** 反射获取ResponseConverter的实例 **/
                        val instance = annotation.clazz.createInstance() as ResponseConverter<T>
                        /** 数据经过onGsonResponseBody处理后再返回 **/
                        return instance.onGsonResponseBody(result)
                    }
                }
            }
            return result
        }
    }
}

测试后你会发现该请求的HttpResponse的msg已经被我们替换成"this is test message"。