ansible - example of calling a handler when a fact is set

0_play.yml

---
- hosts: localhost
  connection: local
  gather_facts: False
  tasks:
    - debug: msg="call handler"
      changed_when: True
      notify: do something
      
  handlers:
    - name: do something
      debug: msg="I'm doing something"
      when: foo|default(False)|bool
      
- hosts: localhost
  connection: local
  gather_facts: False
  tasks:
    - set_fact: 
        foo: True
      
    - debug: msg="call handler"
      changed_when: True
      notify: do something
      
  handlers:
    - name: do something
      debug: msg="I'm doing something"
      when: foo|default(False)|bool

1_output

$ ansible-playbook ~/tmp.yml -i '127.0.0.1,'

PLAY [localhost] ***************************************************************

TASK [debug] *******************************************************************
ok: [127.0.0.1] => {
    "msg": "call handler"
}

RUNNING HANDLER [do something] *************************************************
skipping: [127.0.0.1]

PLAY [localhost] ***************************************************************

TASK [set_fact] ****************************************************************
ok: [127.0.0.1]

TASK [debug] *******************************************************************
ok: [127.0.0.1] => {
    "msg": "call handler"
}

RUNNING HANDLER [do something] *************************************************
ok: [127.0.0.1] => {
    "msg": "I'm doing something"
}

PLAY RECAP *********************************************************************
127.0.0.1                  : ok=4    changed=2    unreachable=0    failed=0

Nice and helpful