unpopulatedFieldRanger.go

// unpopulatedFieldRanger wraps a protoreflect.Message and modifies its Range
// method to additionally iterate over unpopulated fields.
type unpopulatedFieldRanger struct{ pref.Message }

func (m unpopulatedFieldRanger) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
	fds := m.Descriptor().Fields()
	for i := 0; i < fds.Len(); i++ {
		fd := fds.Get(i)
		if m.Has(fd) || fd.ContainingOneof() != nil {
			continue // ignore populated fields and fields within a oneofs
		}
    
		v := m.Get(fd)
		isProto2Scalar := fd.Syntax() == pref.Proto2 && fd.Default().IsValid()
		isSingularMessage := fd.Cardinality() != pref.Repeated && fd.Message() != nil
		if isProto2Scalar || isSingularMessage {
			v = pref.Value{} // use invalid value to emit null
		}
		if !f(fd, v) {
			return
		}
	}
	m.Message.Range(f)
}