Common: Add PyPubSubIslandEventQueue

monkey/common/event_queue/pypubsub_island_event_queue.py

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+import logging
+from typing import Any, Callable
+
+from pubsub.core import Publisher
+
+from . import IIslandEventQueue, IslandEventTopics
+
+logger = logging.getLogger(__name__)
+
+
+class PyPubSubIslandEventQueue(IIslandEventQueue):
+    def __init__(self, pypubsub_publisher: Publisher):
+        self._pypubsub_publisher = pypubsub_publisher
+        self._refs = []
+
+    def subscribe(self, topic: IslandEventTopics, subscriber: Callable[..., None]):
+        topic_value = topic.value  # needs to be a string for pypubsub
+        try:
+            subscriber_name = subscriber.__name__
+        except AttributeError:
+            subscriber_name = subscriber.__class__.__name__
+
+        logging.debug(f"Subscriber {subscriber_name} subscribed to {topic_value}")
+
+        # NOTE: The subscriber's signature needs to match the MDS (message data specification) of
+        #       the topic, otherwise, errors will arise. The MDS of a topic is set when the topic
+        #       is created, which in our case is when a subscriber subscribes to a topic which
+        #       is new (hasn't been subscribed to before). If the topic is being subscribed to by
+        #       a subscriber for the first time, the topic's MDS will automatically be set
+        #       according to that subscriber's signature.
+        self._pypubsub_publisher.subscribe(topicName=topic_value, listener=subscriber)
+        self._keep_subscriber_strongref(subscriber)
+
+    def _keep_subscriber_strongref(self, subscriber: Callable[..., None]):
+        # NOTE: PyPubSub stores subscribers by weak reference. From the documentation:
+        #           > PyPubSub holds listeners by weak reference so that the lifetime of the
+        #           > callable is not affected by PyPubSub: once the application no longer
+        #           > references the callable, it can be garbage collected and PyPubSub can clean up
+        #           > so it is no longer registered (this happens thanks to the weakref module).
+        #           > Without this, it would be imperative to remember to unsubscribe certain
+        #           > listeners, which is error prone; they would end up living until the
+        #           > application exited.
+        #
+        #           https://pypubsub.readthedocs.io/en/v4.0.3/usage/usage_basic_tasks.html?#callable
+        #
+        #       In our case, we're OK with subscribers living until the application exits. We don't
+        #       provide an unsubscribe method (at this time) as subscriptions are expected to last
+        #       for the life of the process.
+        #
+        #       Specifically, if an instance object of a callable class is created and subscribed,
+        #       we don't want that subscription to end if the callable instance object goes out of
+        #       scope. Adding subscribers to self._refs prevents them from ever going out of scope.
+        self._refs.append(subscriber)
+
+    def publish(self, topic: IslandEventTopics, event_data: Any = None):
+        topic_value = topic.value  # needs to be a string for pypubsub
+
+        logger.debug(f"Publishing {topic_value} event")
+
+        # NOTE: `event_data` needs to match the MDS (message data specification) of the topic,
+        #       otherwise, errors will arise. The MDS of a topic is set when the topic is created,
+        #       which in our case is when a subscriber subscribes to a topic (in `subscribe()`)
+        #       which is new (hasn't been subscribed to before).
+        self._pypubsub_publisher.sendMessage(topicName=topic_value, event=event_data)