I want to request a new hook that can be called when categorization changed.
Use case: I store all category names and number of childs in memcache.
Atm I have to call all kinds of hooks for that: edit, move and deletition hooks.
And onPageContentSaveComplete won't tell you if categorization changed so I flush my cache uselessly 90% of the time.
So a hook would be nice that is fired whenever the category table changes.
Update: MW 1.27 destroyed the usage of all these hooks since categorization is fired after onPageContentSaveComplete.
onPageContentSaveComplete does not like this :(
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Having the hooks run from within the web request that "makes" the change is mostly not possible because of cascading updates via templates where the template edit request cannot be allowed to also re-parse all the pages that use the template. There is also an unattended way that things change, which is that conditional templates may change based on the current date, which means after they expire, they are categorised differently.
In both cases, the change is managed by LinksUpdate and queues CategoryMembershipChangeJob. This job makes a public interface via Recent changes, that external parties can listen to via RCFeed (e.g. EventStreams in case of Wikipedia).
On the server-side, there is also hook for these. RecentChange::newForCategorization()->save() triggers Hooks::run('RecentChange_save'). The RecentChange_save hook can be used to do additional logic in your extension to synchronise data to the Memcached use case that was described. This is also how the CheckUser extension works, for example.