Set a property when changing to specific TODO state

Question

I would like to set/unset a property when entering a certain TODO state.

Example: Set the property :COOKIE_DATA: todo recursive automatically when the state is set to PROJ. So all the TODOs in project headings would be counted recursively, and not only TODOs which are direct children of the current tree (default).

How could this be done?

I do not want to set org-hierarchical-todo-statistics to activate recursive counting for every TODO globally.

Answer 1

Table of Contents

1. The answer
2. A minimal working example

The answer

As mentioned by @NickD, you can accomplish by writing a hook.

(defun my/org-set-cookie-data-when-state-changes-to-proj ()
  (interactive)
  (when (equal (org-get-todo-state) "PROJ")
    (org-set-property "COOKIE_DATA" "todo recursive")))

(add-hook 'org-after-todo-state-change-hook 'my/org-set-cookie-data-when-state-changes-to-proj)

A minimal working example

Consider the following minimal working exmaple

#+TODO: TODO(t) | DONE(d)
#+TODO: NOTPROJ(n) | PROJ(p)

* This is a heading
* This is another heading

After changing the states of the headings with org-todo (by default, bound to C-c C-t) , we get the following

#+TODO: TODO(t) | DONE(d)
#+TODO: NOTPROJ(n) | PROJ(p)

* PROJ This is a heading
:PROPERTIES:
:COOKIE_DATA: todo recursive
:END:

* DONE This is another heading