How ajax framework in D7 forms work?

Question

I am trying to understand and get the example working that was discussed here http://api.drupal.org/api/drupal/includes--ajax.inc/group/ajax

I understand the form building function.

function ajax_example_simplest($form, &$form_state) {
    $form = array();
    $form['changethis'] = array(
    '#type' => 'select',
    '#options' => array(
      'one' => 'one',
      'two' => 'two',
      'three' => 'three',
    ),
    '#ajax' => array(
      'callback' => 'ajax_example_simplest_callback',
      'wrapper' => 'replace_textfield_div',
    ),
    );

    // This entire form element will be replaced with an updated value.
    $form['replace_textfield'] = array(
    '#type' => 'textfield',
    '#title' => t("The default value will be changed"),
    '#description' => t("Say something about why you chose") . "'" .
    (!empty($form_state['values']['changethis'])
    ? $form_state['values']['changethis'] : t("Not changed yet")) . "'",
    '#prefix' => '<div id="replace_textfield_div">',
    '#suffix' => '</div>',
    );
    return $form;
}

What I don't understand is what is happening in ajax callback function.

function ajax_example_simplest_callback($form, $form_state) {
    // The form has already been submitted and updated. We can return the replaced
    // item as it is.
    return $form['replace_textfield'];
}

The same form field is returned. I thought we can change the value or something for the form item $form['replace_textfield'] through ajax callback function. Can some one pls help me to explain this part?

Answer 1

All ajax logic placed in construct form function (ajax_example_simplest), ajax callback just return part of form (replace_textfield) or ajax commands. Its Drupal architecture. This form can be validate and altering.

Answer 2

Please spend some time with the AJAX Example in the Examples project. This is also available on api.drupal.org, but it's best just to get the code and experiment with it. It tries to explain exactly what's going on and best practices with it. Also, http://randyfay.com/ajax.