Filter Empty Updates from YDoc.observe_after_transaction

examples/drawing/client.py

@@ -25,10 +25,7 @@ def async_loop():
 
     def send_updates(self, txn_event: Y.AfterTransactionEvent):
         update = txn_event.get_update()
-        # Sometimes transactions don't write, which means updates are empty.
-        # We only care about updates with meaningful mutations.
-        if update != b'\x00\x00':
-            self.send_q.put_nowait(update)
+        self.send_q.put_nowait(update)
 
     def apply_updates(self, doc: Y.YDoc):
         while not self.recv_q.empty():

src/y_doc.rs

@@ -168,12 +168,14 @@ impl YDoc {
     pub fn observe_after_transaction(&mut self, callback: PyObject) -> SubscriptionId {
         self.0
             .observe_transaction_cleanup(move |txn, event| {
-                Python::with_gil(|py| {
-                    let event = AfterTransactionEvent::new(event, txn);
-                    if let Err(err) = callback.call1(py, (event,)) {
-                        err.restore(py)
-                    }
-                })
+                if event.before_state != event.after_state {
+                    Python::with_gil(|py| {
+                        let event = AfterTransactionEvent::new(event, txn);
+                        if let Err(err) = callback.call1(py, (event,)) {
+                            err.restore(py)
+                        }
+                    })
+                }
             })
             .into()
     }

tests/test_y_doc.py

@@ -113,7 +113,6 @@ def test_get_update():
     """
     d = Y.YDoc()
     m = d.get_map("foo")
-    r = d.get_map("foo")
     update: bytes = None
 
     def get_update(event: AfterTransactionEvent) -> None:
@@ -126,3 +125,35 @@ def get_update(event: AfterTransactionEvent) -> None:
         m.set(txn, "hi", "there")
 
     assert type(update) == bytes
+
+
+def test_empty_updates_observe_after_transaction():
+    """
+    Transactions should be observable only when they are used to update the CRDT state.
+    If a transaction does not differ between its before and after state, it should be ignored.
+    """
+    doc = Y.YDoc()
+    items = doc.get_array("items")
+
+    def handle_update(e: Y.AfterTransactionEvent):
+        update = e.get_update()
+        # Ensures observe_after_transaction is only called on contentful updates.
+        assert update != b'\x00\x00'
+
+    doc.observe_after_transaction(handle_update)
+    # Updating the document state should result in a non empty update.
+    with doc.begin_transaction() as txn:
+        items.append(txn, "This triggers an AfterTransactionEvent")
+
+    # The following methods create an empty transaction in order to perform their operation.
+    # Check that none of these yield an empty update.
+    Y.encode_state_vector(doc)
+    update = Y.encode_state_as_update(doc)
+    Y.apply_update(doc, update)
+    doc.get_array("array")
+    doc.get_map("map")
+    doc.get_text("text")
+    doc.get_xml_element("xml_el")
+    doc.get_xml_text("xml_text")
+
+