'Iterates through all instantiated xBlocks that were added and
collects all xBlock fields that were requested.'

def _collect_requested_xblock_fields(self):

    for (xblock_usage_key, xblock) in self._xblock_map.iteritems():
        block_data = self._get_or_create_block(xblock_usage_key)
        for field_name in self._requested_xblock_fields:
            self._set_xblock_field(block_data, xblock, field_name)
