Perform a copy of the layers list in order to avoid the list changing
        without updating the index.

        Then update the index.

def layers(self, value: List['BaseLayer']):
        """
        Perform a copy of the layers list in order to avoid the list changing
        without updating the index.

        Then update the index.
        """
        self._layers = list(value)  # type: List[BaseLayer]
        self._index = self._make_index()
        self._transformed = {}