https://stackoverflow.com/a/55922473

fieldNameByTag.go

package util

import (
	"reflect"
	"strings"
)

var fieldNamesByTag = make(map[reflect.Type]map[string]map[string]string)

func BuildFieldsByTagMap(s interface{}, key string) {
	rt := reflect.TypeOf(s)
	if rt.Kind() != reflect.Struct {
		panic("bad type")
	}

	if fieldNamesByTag[rt] == nil {
		fieldNamesByTag[rt] = make(map[string]map[string]string)
	}
	if fieldNamesByTag[rt][key] == nil {
		fieldNamesByTag[rt][key] = make(map[string]string)
	}

	for i := 0; i < rt.NumField(); i++ {
		field := rt.Field(i)

		// use split to ignore tag "options"
		tag := strings.Split(field.Tag.Get(key), ",")[0]
		if tag == "" || tag == "-" {
			continue
		}
		fieldNamesByTag[rt][key][tag] = field.Name
	}
}

func FieldValueByName(s interface{}, key, tag string) interface{} {
	rt := reflect.TypeOf(s)

	// panic wrong call
	if rt.Kind() != reflect.Struct {
		panic("bad type")
	} else if tag == "" || tag == "-" {
		panic("bad parameter")
	}

	if typeMap, exists := fieldNamesByTag[rt]; !exists {
		panic("bad type")
	} else if keyMap, exists := typeMap[key]; !exists {
		panic("bad key")
	} else if fieldName, exists := keyMap[tag]; !exists {
		panic("bad tag")
	} else {
		return reflect.ValueOf(s).FieldByName(fieldName).Interface()
	}
}