model: Add support for handling delete_message events.

This adds a new event action in model that looks for delete_message
events, potentially handling it by removing the message completely
from the index, and then updates the rendered_view to handle the
event dynamically without needing to switch narrows. It also looks
out for messages being unread before being deleted and updates the
count appropriately. The tests are still missing which will be added
in the next iteration of this commit.

Fixes one check-box of zulip#993.

tests/model/test_model.py

@@ -179,6 +179,7 @@ def test_register_initial_desired_events(self, mocker, initial_data):
         event_types = [
             "message",
             "update_message",
+            "delete_message",
             "reaction",
             "subscription",
             "typing",

zulipterminal/model.py

@@ -121,6 +121,7 @@ def __init__(self, controller: Any) -> None:
             [
                 ("message", self._handle_message_event),
                 ("update_message", self._handle_update_message_event),
+                ("delete_message", self._handle_delete_message_event),
                 ("reaction", self._handle_reaction_event),
                 ("subscription", self._handle_subscription_event),
                 ("typing", self._handle_typing_event),
@@ -1160,6 +1161,58 @@ def _update_topic_index(self, stream_id: int, topic_name: str) -> None:
         # Update the index.
         self.index["topics"][stream_id] = topic_list
 
+    def _handle_delete_message_event(self, event: Event) -> None:
+        """
+        Handles message delete event.
+        TODO: Handle bulk_message_deletion when we support that
+        """
+        assert event["type"] == "delete_message"
+
+        message_id = event["message_id"]
+        indexed_message = self.index["messages"].get(message_id, None)
+
+        if indexed_message:
+            # Update unread_count if message was unread before being deleted
+            # We need to do this before removing the message from index.
+            # FIXME: Does it feel hacky to mark the message as read instead
+            # of using something like unset_count?
+            if "read" not in indexed_message["flags"]:
+                set_count([message_id], self.controller, -1)
+            if "starred" in indexed_message["flags"]:
+                self.index["starred_msg_ids"].discard(message_id)
+                self.controller.view.starred_button.update_count(
+                    self.controller.view.starred_button.count - 1
+                )
+
+            # Remove all traces of the message from index if present and
+            # update the rendered view.
+            # FIXME?: Do we need to archive the message instead of completely
+            # erasing from index?
+            self.index["messages"].pop(message_id, None)
+            self.index["all_msg_ids"].discard(message_id)
+            self.index["mentioned_msg_ids"].discard(message_id)
+            self.index["edited_messages"].discard(message_id)
+
+            if event["message_type"] == "private":
+                self.index["private_msg_ids"].discard(message_id)
+                sender_id = event["sender_id"]
+                private_msg_set = self.index["private_msg_ids_by_user_ids"]
+                for user_id_set, msg_id in private_msg_set.items():
+                    if sender_id in user_id_set:
+                        private_msg_set[user_id_set].discard(msg_id)
+            else:
+                stream_id, topic = event["stream_id"], event["topic"]
+                stream_msg_ids = self.index["stream_msg_ids_by_stream_id"].get(
+                    stream_id, None
+                )
+                if stream_msg_ids:
+                    stream_msg_ids.discard(message_id)
+                topic_msg_ids = self.index["topic_msg_ids"][stream_id].get(topic, None)
+                if topic_msg_ids:
+                    topic_msg_ids.discard(message_id)
+
+            self._update_rendered_view(message_id)
+
     def _handle_update_message_event(self, event: Event) -> None:
         """
         Handle updated (edited) messages (changed content/subject)
@@ -1327,6 +1380,11 @@ def _update_rendered_view(self, msg_id: int) -> None:
         for msg_w in view.message_view.log:
             msg_box = msg_w.original_widget
             if msg_box.message["id"] == msg_id:
+                # Remove the message if deleted
+                if msg_id not in self.index["messages"]:
+                    view.message_view.log.remove(msg_w)
+                    self.controller.update_screen()
+                    return
                 # Remove the message if it no longer belongs in the current
                 # narrow.
                 if (