在自定义UnmarshalXML函数中正确使用XML批注，字段和结构

Question

Consider the following struct:

type MyStruct struct {
    Name string
    Meta map[string]interface{}
}

Which has the following UnmarshalXML function:

func (m *MyStruct) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
    var v struct {
        XMLName xml.Name //`xml:"myStruct"`
        Name    string   `xml:"name"`
        Meta    struct {
            Inner []byte `xml:",innerxml"`
        } `xml:"meta"`
    }

    err := d.DecodeElement(&v, &start)
    if err != nil {
        return err
    }

    m.Name = v.Name
    myMap := make(map[string]interface{})

    // ... do the mxj magic here ... -

    temp := v.Meta.Inner

    prefix := "<meta>"
    postfix := "</meta>"
    str := prefix + string(temp) + postfix
    //fmt.Println(str)
    myMxjMap, err := mxj.NewMapXml([]byte(str))
    myMap = myMxjMap

    // fill myMap
    //m.Meta = myMap
    m.Meta = myMap["meta"].(map[string]interface{})
    return nil
}

My problem with this code is these lines:

prefix := "<meta>"
postfix := "</meta>"
str := prefix + string(temp) + postfix
myMxjMap, err := mxj.NewMapXml([]byte(str))
myMap = myMxjMap
//m.Meta = myMap
m.Meta = myMap["meta"].(map[string]interface{})

My question is how I make the correct use of the xml annotations (,innerxml etc), fields and structs, so I don't have to manually pre-/append the <meta></meta> tags afterwards to get the whole Meta field as a single map.

The full code example is here: http://play.golang.org/p/Q4_tryubO6

Answer 1

xml package doesn't provide a way to unmarshal XML into map[string]interface{} because there is no single way to do it and in some cases it is not possible. A map doesn't preserve order of the elements (that is important in XML) and doesn't allow elements with duplicate keys.

mxj package that you used in your example has some rules how to unmarshal arbitrary XML into Go map. If your requirements do not conflict with these rules you can use mxj package to do all parsing and do not use xml package at all:

// I am skipping error handling here
m, _ := mxj.NewMapXml([]byte(s))
mm := m["myStruct"].(map[string]interface{})
myStruct.Name = mm["name"].(string)
myStruct.Meta = mm["meta"].(map[string]interface{})

Full example: http://play.golang.org/p/AcPUAS0QMj