Golang将XML属性编组为接口

Question

I'm trying to unmarshal some XML into a structure with an interface{} type. However whenever I try to run it, the code doesn't pick up anything at all. All of the other elements appear to work fine, and if I set the type to string or []byte it will work, however I need it to be more flexible than that.

The element I am interested in is on line 32 - FloorRefID

https://play.golang.org/p/Ehr8qx1aWf

<?xml version="1.0" encoding="UTF-8"?>
    <Locations totalPages="1" currentPage="1" pageSize="25">
       <WirelessClientLocation macAddress="00:00:00:00:00:00">
          <MapInfo mapHierarchyString="Head office&gt;Ground floor&gt;Store" floorRefId="-1122334455667789">
             <Image imageName="floorPlan1.png" />
          </MapInfo>
          <MapCoordinate x="2850" y="3000" unit="FEET" />
       </WirelessClientLocation>
       <WirelessClientLocation macAddress="11:11:11:11:11:11">
          <MapInfo mapHierarchyString="Head office&gt;Ground floor&gt;Store" floorRefId="-1122334455667789">
            <Image imageName="floorPlan1.png" />
          </MapInfo>
          <MapCoordinate x="10.72" y="76.49" unit="FEET" />
       </WirelessClientLocation>
       <WirelessClientLocation macAddress="26:cd:96:46:0b:2b">
          <MapInfo floorRefId="0" />
          <MapCoordinate x="51.52" y="4.2" unit="FEET" />
       </WirelessClientLocation>
    </Locations>

To give some context; I am working on a project integrating with a vendor in which sometimes we receive the data as XML and sometimes as JSON. I wanted to build something that could unmarshal the structure for both, rather than duplicating the structure set. It has many substructures which means that its a lot more work to keep 2 structures which are almost identical except for this one attribute.

When we receive the JSON data, the field can be given as a string or a number.

I have read that you cannot unmarshal into an interface, but does anyone know of a way around this issue for my scenario?

Answer 1

Its important to check the returned error always.

if err := xml.Unmarshal([]byte(xmlRawData), &xmlData); err != nil {
    fmt.Println(err)
}

The error you're getting is

cannot unmarshal into interface {}

Empty interface cannot be unmarshalled since the empty interface doesn't have any exported fields to map the xml keys/values to.

However there is way to get around. Implementing xml.Unmarshaler interface on your VendorMapInfo struct.

Example: Your updated code

type VendorMapInfo struct {
    MapHierarchyString string      `xml:"mapHierarchyString,attr"`
    FloorRefID         interface{} `xml:"floorRefId,attr"`
    Image              Image       `xml:"Image"`
    FloorDimension     VendorFloorDimension
}

type Image struct {
    Name string `xml:"imageName,attr"`
}

func (mf *VendorMapInfo) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
    // Attributes
    for _, attr := range start.Attr {
        switch attr.Name.Local {
        case "mapHierarchyString":
            mf.MapHierarchyString = attr.Value
        case "floorRefId":
            mf.FloorRefID = findFloorRefIDType(attr.Value)
        }
    }

    for {
        token, err := d.Token()
        if err != nil {
            return err
        }

        switch el := token.(type) {
        case xml.StartElement:
            if el.Name.Local == "Image" {
                item := new(Image)
                if err = d.DecodeElement(item, &el); err != nil {
                    return err
                }
                mf.Image = *item
            }
        case xml.EndElement:
            if el == start.End() {
                return nil
            }
        }
    }

    return nil
}

Complete code, play link: https://play.golang.org/p/wZQOsQv0Nq

Output:

{Locations:{Space: Local:} WirelessClientLocation:[{MacAddress:00:00:00:00:00:00 MapInfo:{MapHierarchyString:Head office>Ground floor>Store FloorRefID:-1122334455667789 Image:{Name:floorPlan1.png} FloorDimension:{Length:0 Width:0 Height:0 OffsetX:0 OffsetY:0 Unit:}}} {MacAddress:11:11:11:11:11:11 MapInfo:{MapHierarchyString:Head office>Ground floor>Store FloorRefID:-1122334455667789 Image:{Name:floorPlan1.png} FloorDimension:{Length:0 Width:0 Height:0 OffsetX:0 OffsetY:0 Unit:}}} {MacAddress:26:cd:96:46:0b:2b MapInfo:{MapHierarchyString: FloorRefID:0 Image:{Name:} FloorDimension:{Length:0 Width:0 Height:0 OffsetX:0 OffsetY:0 Unit:}}}]}