<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def register_class(self, cls, app_state=None):
 """Register a model class."""

        state = app_state if app_state is not None else self.app_state

        for Adapter in self.adapters_cls:
            if Adapter.can_adapt(cls):
                break
        else:
            return

        cls_fqcn = fqcn(cls)
        self.adapted[cls_fqcn] = Adapter(cls, self.schemas["default"])
        state.indexed_classes.add(cls)
        state.indexed_fqcn.add(cls_fqcn)