feat: writer support sort attr

etree.go

@@ -128,6 +128,9 @@ type WriteSettings struct {
 	//
 	// Deprecated: UseCRLF is deprecated. Use IndentSettings.UseCRLF instead.
 	UseCRLF bool
+
+	// SortAttrs to sort attr
+	SortAttrs bool
 }
 
 // dup creates a duplicate of the WriteSettings object.
@@ -1330,6 +1333,10 @@ func (e *Element) Index() int {
 func (e *Element) WriteTo(w Writer, s *WriteSettings) {
 	w.WriteByte('<')
 	w.WriteString(e.FullTag())
+
+	if s.SortAttrs {
+		e.SortAttrs()
+	}
 	for _, a := range e.Attr {
 		w.WriteByte(' ')
 		a.WriteTo(w, s)

etree_test.go

@@ -770,6 +770,16 @@ func TestSortAttrs(t *testing.T) {
 	checkStrEq(t, out, `<el AAA="1" Foo="2" a01="3" aaa="4" foo="5" z="6" สวัสดี="7" a:AAA="8" a:ZZZ="9"/>`+"\n")
 }
 
+func TestWriteSortAttrs(t *testing.T) {
+	doc := NewDocument()
+	doc.WriteSettings.SortAttrs = true
+	s := `<el foo='5' Foo='2' aaa='4' สวัสดี='7' AAA='1' a01='3' z='6' a:ZZZ='9' a:AAA='8'/>`
+	doc.ReadFromString(s)
+	doc.Indent(2)
+	out, _ := doc.WriteToString()
+	checkStrEq(t, out, `<el AAA="1" Foo="2" a01="3" aaa="4" foo="5" z="6" สวัสดี="7" a:AAA="8" a:ZZZ="9"/>`+"\n")
+}
+
 func TestCharsetReaderDefaultSetting(t *testing.T) {
 	// Test encodings where the default pass-through charset conversion
 	// should work for common single-byte character encodings.