feat(attr,ipub): ElementChildren to provide a universal unmarshalling of child elements

ipub/element/element.go

@@ -4,6 +4,9 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"io"
+	"reflect"
+	"slices"
 	"strings"
 
 	"forge.capytal.company/capytalcode/project-comicverse/ipub/element/attr"
@@ -13,8 +16,55 @@ type Element interface {
 	Kind() ElementKind
 }
 
+type ElementChildren []Element
+
+func (ec *ElementChildren) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
+	elErr := fmt.Errorf("unable to unsmarshal element %q", attr.FmtXMLName(start.Name))
+
+	i := slices.IndexFunc(start.Attr, func(a xml.Attr) bool {
+		return a.Name == elementKindAttrName
+	})
+	if i == -1 {
+		return errors.Join(elErr, fmt.Errorf("element kind not specified"))
+	}
+
+	var k ElementKind
+	if err := k.UnmarshalXMLAttr(start.Attr[i]); err != nil {
+		return err
+	}
+
+	ks := elementKindList[k]
+
+	// Get a pointer of a new instance of the underlying implementation so we can
+	// change it without manipulating the value inside the elementKindList.
+	ep := reflect.New(reflect.TypeOf(ks))
+	if ep.Elem().Kind() == reflect.Pointer {
+		// If the implementation is a pointer, we need the underlying value so we can
+		// manipulate it.
+		ep = reflect.New(reflect.TypeOf(ks).Elem())
+	}
+
+	if err := d.DecodeElement(ep.Interface(), &start); err != nil && err != io.EOF {
+		return errors.Join(elErr, err)
+	}
+
+	if ec == nil {
+		c := ElementChildren{}
+		ec = &c
+	}
+
+	s := *ec
+	s = append(s, ep.Interface().(Element))
+	*ec = s
+
+	return nil
+}
+
 type ElementKind string
 
+// NewElementKind registers a new Element implementation to a private list which is
+// consumed bu [ElementChildren] to properly find what underlying type is a children
+// of another element struct.
 func NewElementKind(n string, s Element) ElementKind {
 	k := ElementKind(n)
 

ipub/element/sections.go

@@ -16,6 +16,8 @@ func (Section) Kind() ElementKind {
 type Body struct {
 	XMLName xml.Name `xml:"body"`
 	Test    string   `xml:"test,attr"`
+
+	Children ElementChildren `xml:",any"`
 }
 
 var KindBody = NewElementKind("body", Body{})