Invoke some callbacks before Write Header (for middleware)

[itang]

Invoke some callbacks before Write Header (for middleware)

• Scenes to be used

// XRuntime Middleware
func XRuntime(next echo.HandlerFunc) echo.HandlerFunc {
	return func(ctx echo.Context) (err error) {
		start := time.Now()

		ctx.Response().OnBeforeWriteHeader(func(resp *echo.Response) {
			resp.Header().Set("x-runtime", time.Since(start).String())
		})

		return next(ctx)
	}
}

e := echo.New()
api := e.Group("/api", XRuntime)

• Sample code

Response struct {
	Writer    http.ResponseWriter
	Status    int
	Size      int64
	Committed bool
	echo      *Echo
	writeHeaderCallbacks   []func(resp *Response)
}
	
func (r *Response) OnBeforeWriteHeader(callback func(resp *Response)) {
	r.writeHeaderCallbacks = append(r.writeHeaderCallbacks, callback)
}

func (r *Response) WriteHeader(code int) {
	if r.Committed {
		r.echo.Logger.Warn("response already committed")
		return
	}
	r.Status = code

	for _, callback := range r.writeHeaderCallbacks {
		callback(r)
	}

	r.Writer.WriteHeader(code)
	r.Committed = true
}

Please confirm echo to achieve similar functionality, what is the best practice?

[vishr]

May be related #176