Reference

How contributions get updated.

A hook_civicrm_post is added via Symfony in civigiftaid_civigiftaid_civicrm_config(). Perhaps confusingly, a non-symfony version of this hook is also implemented by the function civigiftaid_civicrm_post() for a different purpose (see declarations below).

How individuals' declarations are updated.

hook_civicrm_postProcess is used to store submitted form declaration data on the session under uktaxpayer. (Note: it won't replace an existing session value, which could be a bug if someone goes back from confirmation page and changes it?)

Currently uses a hook_civicrm_post() that fires where the objectName is Contribution and the operation is either edit or create. This checks for a transaction and if one is active it schedules a callback using PHASE_POST_COMMIT, otherwise it calls it immediately.

The callback, civigiftaid_callback_civicrm_post_contribution calls CRM_Civigiftaid_Declaration::update($contributionID) but ensures this is only called once for the duration of the script.

That method looks for uktaxpayer on the session, and then proceeds to process it if found.