Introduce initial payload.

Author: Andrey Zelenchuk

channels_graphql_ws/graphql_ws_consumer.py

@@ -176,6 +176,8 @@ async def on_operation(self, op_id, payload):
 
     # ------------------------------------------------------------------- IMPLEMENTATION
 
+    SKIP = object()
+
     # A prefix of Channel groups with subscription notifications.
     group_name_prefix: str = "GQLWS"
 
@@ -914,6 +916,10 @@ async def _on_gql_start__initialize_subscription_stream(
         # `notification_queue_lock` as a guard while reading or writing
         # to the queue.
         notification_queue: asyncio.Queue = asyncio.Queue(maxsize=queue_size)
+        # Enqueue the initial payload.
+        initial_payload = subscription_class.initial_payload
+        if initial_payload is not self.SKIP:
+            notification_queue.put_nowait(Serializer.serialize(initial_payload))
         # Lock to ensure that `notification_queue` operations are
         # thread safe.
         notification_queue_lock = threading.RLock()

channels_graphql_ws/subscription.py

@@ -162,6 +162,11 @@ class Subscription(graphene.ObjectType):
     # Useful to skip intermediate notifications, e.g. progress reports.
     notification_queue_limit: Optional[int] = None
 
+    # Initial payload. Set it to send an "initial" response with this
+    # payload to a client as soon as it is subscribed (before any call
+    # to `Subscription.broadcast`).
+    initial_payload = GraphqlWsConsumer.SKIP
+
     @classmethod
     def broadcast(cls, *, group=None, payload=None):
         """Call this method to notify all subscriptions in the group.