gettags.go

package main

import (
	"fmt"
	"reflect"
)

type meta struct {
	Lang string `xlsx:"Язык"`
	Gov  string `xlsx:"Гражданство"`
}
type user struct {
	Name string `xlsx:"Имя"`
	Meta meta
	Age  age `xlsx:"Возвраст"`
}

type age uint16

func main() {
	u1 := user{Name: "Max", Age: 14, Meta: meta{Lang: "Russian", Gov: "Russian"}}
	u2 := user{Name: "Alex", Age: 24, Meta: meta{Lang: "Eng", Gov: "USA"}}

	users := []user{u1, u2}
	headers := GetHeaders(&users, `xlsx`)
	fmt.Println(headers)
}

// GetHeaders - geat headers to slice from interface
func GetHeaders(i interface{}, tag string) (headers []string) {
	var E reflect.Type
	T := reflect.TypeOf(i)
	switch T.Kind() {
	case reflect.Ptr:
		E = T.Elem()
	default:
		E = T
	}
	headers = make([]string, 0)
	switch E.Kind() {
	case reflect.Slice:
		headers = append(headers, getTagFromStruct(oneFromSlice(E), tag)...)
	case reflect.Array:
		headers = append(headers, getTagFromStruct(oneFromArray(E), tag)...)
	case reflect.Struct:
		headers = append(headers, getTagFromStruct(E, tag)...)
	}
	return
}

func oneFromSlice(T reflect.Type) reflect.Type {
	return reflect.MakeSlice(T, 1, 1).Index(0).Type()
}

func oneFromArray(T reflect.Type) reflect.Type {
	return reflect.New(T).Elem().Slice(0, 1).Index(0).Type()
}

func getTagFromStruct(T reflect.Type, tag string) (headers []string) {
	fieldsCount := T.NumField()
	headers = make([]string, 0)
	for i := 0; i < fieldsCount; i++ {
		field := T.Field(i)
		switch field.Type.Kind() {
		case reflect.Struct:
			headers = append(headers, getTagFromStruct(field.Type, tag)...)
		}
		if t, ok := field.Tag.Lookup(tag); ok {
			headers = append(headers, t)
		}
	}
	return
}