优雅使用Retrofit，在协程时代遨游安卓网络请求（一）下

internal class ApexResponseCallDelegate<T>(private val proxyCall: Call<T>) :
    Call<NetworkResult<T>> {
    override fun enqueue(callback: Callback<NetworkResult<T>>) =
        proxyCall.enqueue(object : Callback<T> {
            override fun onResponse(call: Call<T>, response: Response<T>) {
                //得到响应，将被代理的Call的响应包装成NetworkResult后返回给代理者
                callback.onResponse(
                    this@ApexResponseCallDelegate,
                    Response.success(
                        //将Retrofit中的Response转成NetworkResult
                        response.toNetworkResult()
                    )
                )
            }
            override fun onFailure(call: Call<T>, t: Throwable) {
                //失败了，将被代理的Call的异常包装成NetworkResult后返回给代理者
                callback.onResponse(
                    this@ApexResponseCallDelegate,
                    //将Exception转成NetworkResult
                    Response.success(t.toExceptionResult())
                )
            }
        })
    //下面都是不变的代理方法
    override fun isExecuted(): Boolean = proxyCall.isExecuted
    override fun cancel() = proxyCall.cancel()
    override fun isCanceled(): Boolean = proxyCall.isCanceled
    override fun request(): Request = proxyCall.request()
    override fun timeout(): Timeout = proxyCall.timeout()
    override fun clone(): Call<NetworkResult<T>> =
        ApexResponseCallDelegate(proxyCall.clone())
    override fun execute(): Response<NetworkResult<T>> = throw NotImplementedError()
}
//相关的扩展方法如下：
fun <T> Response<T>.toNetworkResult(): NetworkResult<T> =
   try {
        if (isSuccessful) {
            toSuccessResult()
        } else {
            toServerErrorResult()
        }
    } catch (t: Throwable) {
        t.toExceptionResult()
    }
fun <T> Response<T>.toSuccessResult(): Success<T> {
    return Success(this)
}
fun <T> Response<T>.toServerErrorResult(): ServerError<T> {
    return ServerError(this)
}
fun <T> Throwable.toExceptionResult(): NetworkResult.Failure.Exception<T> {
    return NetworkResult.Failure.Exception(this)
}

class ApexCallAdapterFactory @Inject constructor() : CallAdapter.Factory() {
    class ApexCallAdapter constructor(
        private val resultType: Type
    ) : CallAdapter<Type, Call<NetworkResult<Type>>> {
        override fun responseType() = resultType
        override fun adapt(call: Call<Type>): Call<NetworkResult<Type>> =
            ApexResponseCallDelegate(call)
    }
    override fun get(
        returnType: Type,
        annotations: Array<out Annotation>,
        retrofit: Retrofit
    ): ApexCallAdapter? = when (getRawType(returnType)) {
        Call::class.java -> {
            //判断方法返回值
            val callType = getParameterUpperBound(0, returnType as ParameterizedType)
            //只有返回值是NetworkResult的才会使用本Adapter
            when (getRawType(callType)) {
                NetworkResult::class.java -> {
                    val resultType = getParameterUpperBound(0, callType as ParameterizedType)
                    ApexCallAdapter(resultType)
                }
                else -> null
            }
        }
        else -> null
    }
}

Retrofit.Builder()
    .baseUrl(baseUrl)
    .addCallAdapterFactory(callAdapterFactory)
    .client(okHttpClient)
    .build()
//请求结果
val result=friendService.requestFriend(FriendService.FriendRequestParam(nextPage))
when(result){
    //判断类型
    is Success->{}
    is Exception->{}
}

/**
 * 网络请求成功时操作
 */
inline fun <reified T> NetworkResult<T>.ifSuccess(action: (NetworkResult.Success<T>) -> Unit): NetworkResult<T> {
    if (this is NetworkResult.Success) action(this)
    return this
}
/**
 * 网络请求中，服务器已响应但HTTP协议出现错误时（例如404，502）操作
 */
inline fun <reified T> NetworkResult<T>.ifServerError(action: (NetworkResult.Failure.ServerError<T>) -> Unit): NetworkResult<T> {
    if (this is NetworkResult.Failure.ServerError) action(this)
    return this
}
/**
 * 网络请求中，出现异常时（例如解析JSON异常，超时异常，连接网络失败异常等）操作
 */
inline fun <reified T> NetworkResult<T>.ifException(action: (NetworkResult.Failure.Exception<T>) -> Unit): NetworkResult<T> {
    if (this is NetworkResult.Failure.Exception) action(this)
    return this
}
/**
 * 网络请求失败（包括[ifServerError]和[ifException]两种情况）
 */
inline fun <reified T> NetworkResult<T>.ifFailure(action: (errorMsg: String) -> Unit): NetworkResult<T> {
    ifServerError {
        //errorBodyString()方法在下篇文章中会提到
        action(it.errorBodyString())
    }.ifException {
        //exceptionMessage()方法在下篇文章中会提到
        action(it.exceptionMessage())
    }
    return this
}

searchRepository.search(SearchService.SearchRequestBody(key)).ifSuccess {
    //成功时
}.ifFailure {
    //失败时
}.ifServerError {
    //服务器内部错误时
}.ifException {
    //发生了异常时
}