试一下代码方式，给你一个类似的参考

function yourmodule_form_alter(&$form, &$form_state, $form_id) {

    if($form_id == 'user_register_form' || $form_id == 'user_profile_form'){

        $form['autoassignrole_user']['user_roles']['#ajax'] = array(
          // #ajax has two required keys: callback and wrapper.
          // 'callback' is a function that will be called when this element changes.
          'callback' => 'ajax_form_alter',
          // 'wrapper' is the HTML id of the page element that will be replaced.
          'wrapper' => 'replace_field_div',
          // Don't show any throbber...
          'progress' => array('type' => 'none'),              
         );

        # This profile field will be shown or hidden when the respective "user_roles" is selected
        $form['field_first_name']['und'][0]['value']['#type']       = 'hidden';
        $form['field_first_name']['und'][0]['value']['#required']   = FALSE;
        $form['field_first_name']['und'][0]['value']['#type']       = 'hidden';
        $form['field_first_name']['und'][0]['value']['#prefix']     = '<div id="replace_field_div">';
        $form['field_first_name']['und'][0]['value']['#suffix']     = '</div>';

        // An AJAX request calls the form builder function for every change.
        // We can change how we build the form based on $form_state.        
        if(isset($form_state['values']['autoassignrole_user']['user_roles']) && $form_state['values']['autoassignrole_user']['user_roles'] == {rid_here}){
            # when the respective user roles is selected, show this field and set required.
            $form['field_first_name']['und'][0]['value']['#type']       = 'textfield';
            $form['field_first_name']['und'][0]['value']['#required']   = TRUE;   
        }else{ 
            # when the respective user roles, hide this field and remove required.
            $form['field_first_name']['und'][0]['value']['#type']       = 'hidden';
            $form['field_first_name']['und'][0]['value']['#required']   = FALSE;           
        }
    }
}
/**
 * Callback for user_roles
 *
 * On an ajax submit, the form builder function is called again, then the $form
 * and $form_state are passed to this callback function so it can select which
 * portion of the form to send on to the client.
 *
 * @return renderable array (the textfield element)
 */     
function ajax_form_alter($form, $form_state){
    return $form['field_first_name']; 
}