使用Go将XML解码为接口类型？

Question

Is it possible to decode XML into an interface type with Go 1.3?

For example, if the structs look something like this (simplified):

type Field interface { ... }

// DataField and ControlField satisfy Field interface
type DataField struct { ... } // <- One concrete type, with XML tags
type ControlField struct { ... } // <- Another concrete type, with XML tags


type Record struct {
    Fields []Field // <- Field is an interface
}

...
// we want to decode into a record, e.g.
var record Record
decoder.DecodeElement(&record, &se)
...

As far as I can see, it is possible to decode XML with the concrete types, eg:

type Record struct {
    ControlFields []ControlField // <- Using concrete type works
    DataFields []DataField // <- Using concrete type works
}

But the interface type fails, although the implementations are annotated with the correct XML tags.

For a runnable example, see http://play.golang.org/p/tPlw4Y74tt or as gist .

Answer 1

From looking at the code in the encoding/xml package, it seems, interface types are just skipped:

...

switch v := val; v.Kind() {

case reflect.Interface:
    // TODO: For now, simply ignore the field. In the near
    //       future we may choose to unmarshal the start
    //       element on it, if not nil.
    return p.Skip()

...

Go version: 1.3.

Answer 2

You need to initialize the values inside your interface arrays with some concrete type or xml will not be able to infer to which type are you referring to:

http://play.golang.org/p/6LVgK7rza9

            // InterfaceRecord fails
            record := InterfaceRecord{
                Fields: []Field{&DataField{}},
            }

            decoder.DecodeElement(&record, &se)
            output, err := xml.MarshalIndent(record, "  ", "    ")
            if err != nil {
                fmt.Printf("error: %v\n", err)
            }

            os.Stdout.Write(output)

Output:

  <record>
      <d tag=""></d>
  </record>