Form tutorial back button discards step 2 values.

Thanks for your response Randy. When you say "we throw away the data", are you referring to the example form or to drupal core? I don't see where it would be happening in the example form and I don't see how Drupal knows the difference between the back/next/submit buttons.

FWIW, I played around with lots of variations based on your advice but was no successful.

I note that the values are in there in $form_state['input']. I've been trying to google for documentation about 'limit', but with no luck. I am considering just copying over these values into $form_state['page_values'][2] to make my own form work but I assume this is somewhat of a hack.

Ok, I worked it out.

The reason I was confused is that in form_example_tutorial_8_next_submit() the page 2 values are stored as if they will be retained. But storing these values is completely redundant for the reasons you say. Maybe add some comments like this?

<?php
function form_example_tutorial_8_next_submit($form, &$form_state) {
...
  // This won't actually work for any fields that have #limit_validation_errors set
  // because Drupal has destroyed the $form_state values that haven't been validated.
  // To make page 2 remember "color" setting after going Back, remove #limit_validation_errors
  // and remove '#required' => TRUE, on the color element.
  if (!empty($form_state['page_values'][2])) {
    $form_state['values'] = $form_state['page_values'][2];
  }
...
}
?>

For other people reading this, the related Drupal core code is in form.inc drupal_validate_form():

<?php
  // If validation errors are limited then remove any non validated form values,
  // so that only values that passed validation are left for submit callbacks.
  if (isset($form_state['triggering_element']['#limit_validation_errors']) && $form_state['triggering_element']['#limit_validation_errors'] !== FALSE) {
  ...
?>

Hi rfay, I think those changes would be great, and would help avoid confusion for future trekkers.

(Sorry, I didn't know about the "needs review" being for patches only.)

Regarding the Needs Review, it's a bit confusing to be honest. There is no other way to suggest a maintainer should consider a solution! Needs Review is used for non-patch solutions in the, for example, the Webmaster queue all the time.