在Go中将<tag value =“ val” />解组为Tag字符串

Question

Say that I've got a Go struct defined as follows:

type MyType struct {
    FieldA string
    FieldB string
    FIeldC string
}

and XML that corresponds to it that looks like this:

<obj>
    <fieldA value="apple"/>
    <fieldB value="banana"/>
</obj>

where FieldA and FieldB are mandatory, and FieldC is optional. How do I specify the struct tags so as to get the field's value from the "value" attribute? This:

FieldA string `xml:"fieldA>value,attr"`
FieldB string `xml:"fieldB>value,attr"`
FieldC string `xml:"fieldC>value,attr,omitempty"`

generates "xml: fieldA>value chain not valid with attr flag" and this:

FieldA string `xml:"fieldA"`
FieldB string `xml:"fieldB"`
FieldC string `xml:"fieldC,omitempty"`

doesn't generate an error, but doesn't find the values of the fields.

Answer 1

To support both XML and JSON, you will have to define a simple type and implement the xml.Unmarshaler and xml.Marshaler interfaces on it, here's an example:

type Field string

func (f Field) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
    v := struct {
        Value string `xml:"value,attr"`
    }{string(f)}
    return e.EncodeElement(v, start)
}

func (f *Field) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
    var v struct {
        Value string `xml:"value,attr"`
    }
    err := d.DecodeElement(&v, &start)
    *f = Field(v.Value)
    return err
}

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