diff --git a/synapse/handlers/state_deltas.py b/synapse/handlers/state_deltas.py
new file mode 100644
index 0000000000..b268bbcb2c
--- /dev/null
+++ b/synapse/handlers/state_deltas.py
@@ -0,0 +1,70 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+from twisted.internet import defer
+
+logger = logging.getLogger(__name__)
+
+
+class StateDeltasHandler(object):
+
+ def __init__(self, hs):
+ self.store = hs.get_datastore()
+
+ @defer.inlineCallbacks
+ def _get_key_change(self, prev_event_id, event_id, key_name, public_value):
+ """Given two events check if the `key_name` field in content changed
+ from not matching `public_value` to doing so.
+
+ For example, check if `history_visibility` (`key_name`) changed from
+ `shared` to `world_readable` (`public_value`).
+
+ Returns:
+ None if the field in the events either both match `public_value`
+ or if neither do, i.e. there has been no change.
+ True if it didnt match `public_value` but now does
+ False if it did match `public_value` but now doesn't
+ """
+ prev_event = None
+ event = None
+ if prev_event_id:
+ prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
+
+ if event_id:
+ event = yield self.store.get_event(event_id, allow_none=True)
+
+ if not event and not prev_event:
+ logger.debug("Neither event exists: %r %r", prev_event_id, event_id)
+ defer.returnValue(None)
+
+ prev_value = None
+ value = None
+
+ if prev_event:
+ prev_value = prev_event.content.get(key_name)
+
+ if event:
+ value = event.content.get(key_name)
+
+ logger.debug("prev_value: %r -> value: %r", prev_value, value)
+
+ if value == public_value and prev_value != public_value:
+ defer.returnValue(True)
+ elif value != public_value and prev_value == public_value:
+ defer.returnValue(False)
+ else:
+ defer.returnValue(None)
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