Custom serializer for plist inside XML

[adrientetar]

I want to parse an XML file with plist embedded in it, like the <dict> element below:

<?xml version="1.0" encoding="UTF-8"?>
<glyph name="space" format="2">
  <lib>
    <dict>
      <key>public.markColor</key>
      <string>0.97,1,0,1</string>
    </dict>
  </lib>
</glyph>

I would like to deserialize the dict using a separate library (dd-plist). I wrote a custom serializer to NSDictionary, however I can't figure out how to tell the Decoder to decode an xml tag to a string. (It seems like I can't cast the Decoder to XmlDecoder as that's an internal type?)

class NSDictionarySerializer : KSerializer<NSDictionary> {
    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("dict", PrimitiveKind.STRING)

    override fun deserialize(decoder: Decoder): NSDictionary =
        PropertyListParser.parse(
            // wrap the element in a proper doctype for parsing
            buildString {
                append("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
                    "<!DOCTYPE plist PUBLIC \"-//Apple//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">\n" +
                    "<plist version=\"1.0\">\n")
                append(decoder.decodeString())
                append("\n</plist>")
            }.byteInputStream()
        ).toDictionary()

    override fun serialize(encoder: Encoder, value: NSDictionary) {
        val outputStream = ByteArrayOutputStream()
        XMLPropertyListWriter.write(value, outputStream)
        val plist = outputStream.toString(Charset.forName("UTF-8"))
        val dictString = buildString {
            // remove doctype etc. to just have the element
            val content = plist.substringAfter("<dict>")
                .substringBeforeLast("</dict>")

            append("<dict>")
            if (content.isNotBlank()) {
                append(content)
            }
            append("</dict>")
        }
        encoder.encodeString(dictString)
    }
}

I would appreciate any pointer; thanks for this library!

[pdvrieze]

First of all, the types to use for specialisation in custom serializers are the interfaces XmlInput and XmlOutput respectively. Due to the way that xml parsers work it is not possible to directly get the content of a tag as text. However in this particular case the easiest way may be to realise that the CompactFragment is more or less what you need (unless you want to parse plist directly using serialization - which looking at plist still requires custom serializers).

Compactfragments (or the XmlReader.siblingsToFragment extension function) exactly capture the xml contents of a file into a fragment that holds both namespace declarations and Xml content. You can access the raw xml content as text (you may want to use CombiningReader to send everything into the custom PropertyListParser.

[pdvrieze]

An alternative that might work (depending on your parsing library) is to use the support for parsing/serializing DOM instead. It is also possible to get the underlying xml parser/serializer and use it directly (but be aware that this may break things)