最近看到有人问Retrofit的原理，然后就研究了一下，发现内部是用java的动态代理实现的，真的很巧妙，动态代理在android端的应用场景不多，retrofit算是一个，就想着能不能自己写一个简化版的Retrofit。

只拿get做演示

定义GET注解：

@Documented
@Target(METHOD)
@Retention(RUNTIME)
public @interface GET {
    String value() default "";
}

定义接口：

interface ServiceApi {
    /**
     * 获取公众号列表
     * 基于 玩android开放api
     */
    @GET("/wxarticle/chapters/json")
    fun getPublicList():Call
}

核心retrofit类，利用建造者模式：

@Suppress("UNCHECKED_CAST")
class Retrofit(var callFactory: Call.Factory, var baseUrl: String) {


    fun <T> create(service: Class<T>): T {
        return Proxy.newProxyInstance(
            service.classLoader, arrayOf(service)
        ) { _, method, _ ->
            //解析注解
            val annotations = method.annotations
            if (annotations.isEmpty()) {
                throw IllegalStateException("annotation required.")
            }
            annotations.forEach {
                Logger.d(it.toString())
                Logger.d(it is GET)
                return@newProxyInstance when (it) {
                    is GET -> {
                        handlerGet(it.value)
                    }
                    is POST -> {
                        //不处理，只模拟GET
                    }
                    else -> {

                    }
                }
            }
        } as T
    }

    /**
     * 处理get请求
     */
    private fun handlerGet(url: String): Call {
        val request = Request.Builder()
            .url(baseUrl + url)
            .get()
            .build()
        //这里没有做返回类型转换，默认都返回call
        return callFactory.newCall(request)
    }


    class Builder {
        private var baseUrl: String? = null
        private var callFactory: Call.Factory? = null
        /**
         * 模拟传入baseUrl
         */
        fun baseUrl(baseUrl: String): Builder {
            this.baseUrl = baseUrl
            return this
        }

        /**
         * 模拟传入client
         */
        fun client(client: OkHttpClient): Builder {
            this.callFactory = client
            return this
        }

        fun build(): Retrofit {
            if (baseUrl == null) {
                throw IllegalStateException("Base URL required.")
            }
            if (callFactory == null) {
                callFactory = OkHttpClient()
            }
            return Retrofit(callFactory!!, baseUrl!!)
        }
    }
}

使用：

 //初始化retrofit
        val api = Retrofit
            .Builder()
            .baseUrl("https://wanandroid.com")
            .client(OkHttpClient())
            .build()
            .create(ServiceApi::class.java)

        //按钮点击（这里利用kotlin的扩展函数，做的防重点）
        bt_net.singleClick {
            api.getPublicList()
                .enqueue(object : Callback {
                    override fun onFailure(call: Call, e: IOException) {
                        Logger.d(e.message)
                    }

                    override fun onResponse(call: Call, response: Response) {
                        val res = response.body()?.source()?.readUtf8()
                        Logger.d(res)
                    }
                })
        }

返回结果：

{
    "data": [{
        "children": [],
        "courseId": 13,
        "id": 408,
        "name": "鸿洋",
        "order": 190000,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 409,
        "name": "郭霖",
        "order": 190001,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 410,
        "name": "玉刚说",
        "order": 190002,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 411,
        "name": "承香墨影",
        "order": 190003,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 413,
        "name": "Android群英传",
        "order": 190004,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 414,
        "name": "code小生",
        "order": 190005,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 415,
        "name": "谷歌开发者",
        "order": 190006,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 416,
        "name": "奇卓社",
        "order": 190007,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 417,
        "name": "美团技术团队",
        "order": 190008,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 420,
        "name": "GcsSloop",
        "order": 190009,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 421,
        "name": "互联网侦察",
        "order": 190010,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 427,
        "name": "susion随心",
        "order": 190011,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 428,
        "name": "程序亦非猿",
        "order": 190012,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }, {
        "children": [],
        "courseId": 13,
        "id": 434,
        "name": "Android达摩院",
        "order": 190013,
        "parentChapterId": 407,
        "userControlSetTop": false,
        "visible": 1
    }],
    "errorCode": 0,
    "errorMsg": ""
}

只是一个简单的模拟，很多深入的功能如果有时间的话再完善~
github传送门

完工 撒花~