Go JSON Decoder Access Size With Custom Reader

lengthreader.go

type LengthReader struct {
	io.Reader
	Length int
}

func (cw *LengthReader) Read(p []byte) (n int, err error) {
	n, err = cw.Reader.Read(p)
	cw.Length += n
	return n, err
}

v := interface{}
lr := &LengthReader{Reader: r.Body}
decoder := json.NewDecoder(lr).Decode(&v)
fmt.Printf("The size of the payload is %d \n", lr.Length)