用Go的xml包编组DIDL-Lite

Question

Here's a sample DIDL-Lite XML document from the UPnP AV ContentDirectory v2 Service Template:

<?xml version="1.0" encoding="UTF-8"?>
<DIDL-Lite
 xmlns:dc="http://purl.org/dc/elements/1.1/"
 xmlns="urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/"
 xmlns:upnp="urn:schemas-upnp-org:metadata-1-0/upnp/"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="
   urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/
    http://www.upnp.org/schemas/av/didl-lite-v2-20060531.xsd
   urn:schemas-upnp-org:metadata-1-0/upnp/
    http://www.upnp.org/schemas/av/upnp-v2-20061231.xsd">
   <item id="18" parentID="13" restricted="0">
      ...
   </item>
</DIDL-Lite>

How would one go about marshalling to this with Go's xml package? More specifically:

1. How are namespace prefixes defined, such as xmlns:dc and xmlns:upnp?
2. How are multiple name spaces configured on an element?
3. How is the namespace set for attributes, such as the xsi prefix on the schemaLocation attribute?

As a base, I have something like this:

type DIDLLite struct {
    XMLName xml.Name `xml:"urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/ DIDL-Lite"`
    // ??? namespace prefixes dc, upnp, xsi
    Objects []Object
}

I've also found this possibly related bug.

Answer 1

The example you gave is marshalled. I assume you mean to ask, "how would one define Go data types that marshal with xml.Marshal into this?"

package main

import (
    "encoding/xml"
    "fmt"
)

type DIDLLite struct {
    XMLName xml.Name
    DC      string   `xml:"xmlns:dc,attr"`
    UPNP    string   `xml:"xmlns:upnp,attr"`
    XSI     string   `xml:"xmlns:xsi,attr"`
    XLOC    string   `xml:"xsi:schemaLocation,attr"`
    Objects []Object `xml:"item"`
}

type Object struct {
    ID         string `xml:"id,attr"`
    Parent     string `xml:"parentID,attr"`
    Restricted string `xml:"restricted,attr"`
}

func main() {
    d := DIDLLite{
        XMLName: xml.Name{
            Space: "urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/",
            Local: "DIDL-Lite",
        },
        DC:   "http://purl.org/dc/elements/1.1/",
        UPNP: "urn:schemas-upnp-org:metadata-1-0/upnp/",
        XSI:  "http://www.w3.org/2001/XMLSchema-instance",
        XLOC: `
   urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/
    http://www.upnp.org/schemas/av/didl-lite-v2-20060531.xsd
   urn:schemas-upnp-org:metadata-1-0/upnp/
    http://www.upnp.org/schemas/av/upnp-v2-20061231.xsd`,
        Objects: []Object{{ID: "18", Parent: "13", Restricted: "0"}},
    }
    b, err := xml.MarshalIndent(d, "", "    ")
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(b))
}

Output:

<DIDL-Lite xmlns="urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:upnp="urn:schemas-upnp-org:metadata-1-0/upnp/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
   urn:schemas-upnp-org:metadata-1-0/DIDL-Lite/
    http://www.upnp.org/schemas/av/didl-lite-v2-20060531.xsd
   urn:schemas-upnp-org:metadata-1-0/upnp/
    http://www.upnp.org/schemas/av/upnp-v2-20061231.xsd">
    <item id="18" parentID="13" restricted="0"></item>
</DIDL-Lite>

which can be pretty printed to match your example above. xml.MarshallIndent is a little primitive yet.