How to use the nanopb.nanopb.generator.nanopb_generator.OneOf function in nanopb

def count_all_fields(self):
        count = 0
        for f in self.fields:
            if isinstance(f, OneOf):
                count += len(f.fields)
            else:
                count += 1
        return count

self.fields = []
        self.oneofs = {}
        no_unions = []

        if message_options.msgid:
            self.msgid = message_options.msgid

        if hasattr(desc, 'oneof_decl'):
            for i, f in enumerate(desc.oneof_decl):
                oneof_options = get_nanopb_suboptions(desc, message_options, self.name + f.name)
                if oneof_options.no_unions:
                    no_unions.append(i) # No union, but add fields normally
                elif oneof_options.type == nanopb_pb2.FT_IGNORE:
                    pass # No union and skip fields also
                else:
                    oneof = OneOf(self.name, f)
                    if oneof_options.anonymous_oneof:
                        oneof.anonymous = True
                    self.oneofs[i] = oneof
                    self.fields.append(oneof)

        for f in desc.field:
            field_options = get_nanopb_suboptions(f, message_options, self.name + f.name)
            if field_options.type == nanopb_pb2.FT_IGNORE:
                continue

            field = Field(self.name, f, field_options)
            if (hasattr(f, 'oneof_index') and
                f.HasField('oneof_index') and
                f.oneof_index not in no_unions):
                if f.oneof_index in self.oneofs:
                    self.oneofs[f.oneof_index].add_field(field)