API in godoc

The trick played here is that context is hiden in ResponseWriter. So, one can write standard http.Handler as usual and get the context as need, in this way:

func HelloWorld(w http.ResponseWriter, r *http.Request) {
	if cw, ok := w.(middleware.ContextResponseWriter); ok {
		value := cw.Value("key")
		...
	}
}

As elegancy as gorilla while global lock is unnecessary.

A complete demo:

package main

import (
	"github.com/tiancaiamao/middleware"
	"io"
	"net/http"
)

type MyContextResponseWriter struct {
	http.ResponseWriter
	key, value interface{}
}

func (w *MyContextResponseWriter) Value(key interface{}) interface{} {
	if w.key == key {
		return w.value
	}
	return nil
}

func HelloWorld(w http.ResponseWriter, r *http.Request) {
	if ctx, ok := w.(middleware.ContextResponseWriter); ok {
		valueFromContext := ctx.Value("xxx")
		io.WriteString(w, "hello, "+valueFromContext.(string))
		return
	}
	
	io.WriteString(w, "hello world")
}

type MiddleWareDemo struct{}

func (demo MiddleWareDemo) Chain(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		cw := &MyContextResponseWriter{
			ResponseWriter: w,
			key:            "xxx",
			value:          "demo",
		}
		
		h.ServeHTTP(cw, r)
	})
}

func main() {
	handler := middleware.New(http.HandlerFunc(HelloWorld), MiddleWareDemo{})
	http.ListenAndServe(":8080", handler)
}

Tips:

1. golang/x/net/context can be used as ContextResponseWriter's Value if you want.
2. the code is deadly simple, so you may copy one in your project rather then import this package.