Explain how a prefilled field can react to another field before the user touches it

[KamilSztandur]

Problem

A field the user has never edited never validates on its own. That is deliberate: a form filled from the server should not open covered in errors. The side effect is that subscribeToFields does nothing for such a field. Example: the instructor is prefilled from the server, the user switches the aircraft, and the instructor is no longer rated for it. Nothing happens, because nobody touched the instructor field.

Someone porting two real forms took this for a missing feature and ended up writing the error by hand with setError, next to a validator that already knew the rule. We are keeping the rule. What was missing is the documented way to get around it in the cases that need it.

Solution

Docs only, no code change. Call validate() on the dependent field from a listener on the watched field. validate() ignores both the mode and the "never edited" gate, so it reaches a prefilled field, and the sync result appears at once. The rule stays in the validator; the listener only decides when it runs.

var lastAircraft = aircraft.fieldValue;
aircraft.addListener(() {
  if (aircraft.fieldValue == lastAircraft) return;
  lastAircraft = aircraft.fieldValue;
  instructor.validate();
});

The write-up names the two traps I hit while checking this: a validate() that is already running is shared, so a second call in the same tick gets the first round's result, and validate() also runs the async validator, so a field with a server check will call the server on every change.

In this PR

• SKILL.md: a new bullet in the cross-field section, with the snippet and the traps.
• FAQ: a paragraph in "My subscribeToFields dependency does not re-validate".
• Cross-field docs: one sentence in "What it does, precisely".

The snippet was run against the package before writing it up.

🤖 Generated with Claude Code

[github-actions]

Docs preview: https://advanced-forms-pjxkmu9x6-leancode.vercel.app

Built from 2eb4f01; the landing page is at /, the docs under /docs.