bidirectional awareness update support

src/ypywidgets/comm.py

@@ -8,6 +8,7 @@
     TransactionEvent,
     YMessageType,
     YSyncMessageType,
+    create_awareness_message,
     create_sync_message,
     create_update_message,
     handle_sync_message,
@@ -51,6 +52,7 @@ def __init__(
         self._ydoc = ydoc
         self._comm = comm
         self._awareness = Awareness(ydoc)
+        self._awareness_sub_id = self._awareness.observe(self._send_awareness)
         msg = create_sync_message(ydoc)
         self._comm.send(buffers=[msg])
         self._comm.on_msg(self._receive)
@@ -59,6 +61,12 @@ def __init__(
     def awareness(self) -> Awareness:
         return self._awareness
 
+    # send awareness to the frontend
+    def _send_awareness(self, topic, data):
+        update = self._awareness.encode_awareness_update([self._awareness.client_id])
+        message = create_awareness_message(update)
+        self._comm.send(buffers=[message])
+
     def _receive(self, msg):
         message = bytes(msg["buffers"][0])
         match message[0]:

tests/test_comm_awareness.py

@@ -42,3 +42,17 @@ async def test_comm_widget_awareness_observe_and_unobserve():
     events.clear()
     widget.awareness.set_local_state({"ping": 2})
     assert events == []
+
+
+async def test_comm_provider_sends_awareness_to_frontend(synced_widgets, context):
+    async with context:
+        local_widget = await synced_widgets.get_local_widget()
+
+        sent_messages = []
+        local_widget._comm_provider._comm.send = lambda buffers: sent_messages.append(
+            buffers[0]
+        )
+
+        local_widget.awareness.set_local_state({"role": "python-test"})
+
+        assert any(msg[0] == YMessageType.AWARENESS for msg in sent_messages)