背景 在返回响应报文到客户端时，需要保存整个记录。

Golang 标准库有实现：src/net/http/httptest/recorder.go
ResponseRecorder，但是此不符合需求，会拦截 response ，不返回给客户端。想要的效果是返回给客户端的同时取得简单的响应的 response, 需要做简单的修改

ResponseRecorder

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package xhttp

import (
    "bytes"
    "net/http"
)

type ResponseRecorder struct {
    W          http.ResponseWriter
    StatusCode int
    Body       *bytes.Buffer
}

func NewRecorder(w http.ResponseWriter) *ResponseRecorder {
    return &ResponseRecorder{
        W:          w,
        StatusCode: http.StatusOK,
        Body:       new(bytes.Buffer),
    }
}

func (rw *ResponseRecorder) WriteHeader(statusCode int) {
    rw.StatusCode = statusCode
    rw.W.WriteHeader(statusCode)
}

func (rw *ResponseRecorder) Write(buf []byte) (int, error) {
    _, _ = rw.Body.Write(buf)
    return rw.W.Write(buf)
}

func (rw *ResponseRecorder) Header() http.Header {
    return rw.W.Header()
}

其中一个使用例子如下

package main

import (
    "log"
    "net/http"

    "github.com/gorilla/mux"
    "github.com/vnotes/xhttp"
)

func recordMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        rec := xhttp.NewRecorder(w)

        next.ServeHTTP(rec, r)
        log.Printf("response is %s\n", rec.Body)
        log.Printf("response header is %d\n", rec.StatusCode)
    })
}

func main() {
    r := mux.NewRouter()
    r.HandleFunc("/api", hello)
    r.Use(recordMiddleware)
    _ = http.ListenAndServe(":11111", r)
}

func hello(w http.ResponseWriter, r *http.Request) {
    _, _ = w.Write([]byte("hello"))
    w.WriteHeader(http.StatusOK)
}

curl -X GET http://127.0.0.1:11111/api 结果为 hello。假若使用 httptest.NewRecorder() 则结果为空。
当然，都同时会有日志打印

2020/04/04 17:40:15 response is hello
2020/04/04 17:40:15 response header is 200

coresHandlerInfoMiddleware

另外 https://github.com/gorilla/mux 典型的 middlewares 使用为 r.Use(mux.CORSMethodMiddleware(r))
例如在每次的接口调用中打印函数名：相对应的 middlewares 为

// 获取函数名
func getFunctionName(i interface{}) string {
    return runtime.FuncForPC(reflect.ValueOf(i).Pointer()).Name()
}

func getHandlerName(r *mux.Router, req *http.Request) {
    _ = r.Walk(func(route *mux.Route, router *mux.Router, ancestors []*mux.Route) error {
        if !route.Match(req, &mux.RouteMatch{}) {
            return nil
        }
        _handler := route.GetHandler()
        fName := getFunctionName(_handler)
        log.Printf("function name %s", fName)
        return nil
    })
}
// handler info middleware
func coresHandlerInfoMiddleware(r *mux.Router) mux.MiddlewareFunc {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
            getHandlerName(r, req)
            next.ServeHTTP(w, req)
        })
    }
}

r.Use(coresHandlerInfoMiddleware(r))

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