This corrects two issues:
1. We were double serializing topic tracking state (as_json calls were not cached)
2. We were inefficiently serializing items by instantiating extra objects
What is the problem?
We have a hidden site setting `show_category_definitions_in_topic_lists`
which is set to false by default. What this means is that category
definition topics are not shown in the topic list by default. Only the
category definition topic for the category being viewed will be shown.
However, we have a bug where we would show that a category has new
topics when a new child category along with its category definition
topic is created even though the topic list does not list the child
category's category definition topic.
What is the fix here?
This commit fixes the problem by shipping down an additional
`is_category_topic` attribute in `TopicTrackingStateItemSerializer` when
the `show_category_definitions_in_topic_lists` site setting has been set
to false. With the new attribute, we can then exclude counting child
categories' category definition topics when counting new and unread
counts for a category.
## Why do we need this change?
When loading the ember app, [MessageBus does not start polling immediately](f31f0b70f8/app/assets/javascripts/discourse/app/initializers/message-bus.js (L71-L81)) and instead waits for `document.readyState` to be `complete`. What this means is that if there are new messages being created while we have yet to start polling, those messages will not be received by the client.
With sidebar being the default navigation menu, the counts derived from `topic-tracking-state.js` on the client side is prominently displayed on every page. Therefore, we want to ensure that we are not dropping any messages on the channels that `topic-tracking-state.js` subscribes to.
## What does this change do?
This includes the `MessageBus.last_id`s for the MessageBus channels which `topic-tracking-state.js` subscribes to as part of the preloaded data when loading a page. The last ids are then used when we subscribe the MessageBus channels so that messages which are published before MessageBus starts polling will not be missed.
## Review Notes
1. See https://github.com/discourse/message_bus#client-support for documentation about subscribing from a given message id.
Sam
Alan Guo Xiang Tan