自定义 Drupal 7 Field 只保存第一个字符

I'm essentially trying to create my first field type module using the new Drupal 7 Field API. I've managed to get it to display correctly in the "edit" view.
However, when I try to save some data it only saves the first character.

这是模块:

<?php

function youtubefield_field_info() {
  return array(
    'youtubefield_video' => array(
      'label' => t('Youtube video'),
      'description' => t('This field stores a youtube video ID and displays the video associated with it'),
      'settings' => array(
        'max_length' => 11,
      ),
      'instance_settings' => array(
        'text_processing' => false,
      ),
      'default_widget' => 'youtubefield_video_widget',
      'default_formatter' => 'youtubefield_video_formatter',
    ),
  );
}

function youtubefield_field_widget_info() {
  return array(
    'youtubefield_video_widget' => array(
      'label' => t('Default'),
      'field types' => array('youtubefield_video'),
    ),
  );
}

function youtubefield_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, $items, $delta, $element) {
  $element['#type'] = 'textfield';
  $element['#default_value'] = isset($items[$delta]) ? $items[$delta] : '';
  return $element;
}

function youtubefield_field_is_empty($item, $field) {
  return !$item;
}

function youtubefield_field_formatter_info() {
  return array(
    'youtubefield_video_formatter' => array(
      'label' => t('Youtube video'),
      'field types' => array('youtubefield_video'),
    ),
  );
}

这是安装程序:

<?php

function youtubefield_field_schema($field) {
  return array(
    'columns' => array(
      'value' => array(
        'type' => 'varchar',
        'length' => 11,
        'not null' => true,
      ),
    ),
    'indexes' => array(),
  );
}

问题似乎与小部件有关，因为它在以编程方式获取数据时工作正常.

function youtubefield_field_widget_form(&$form, &$form_state, $field, $instance, $langcode, $items, $delta,
 $element) {
  $main_widget = array();
  switch ($instance['widget']['type']) {
  case 'youtubefield_video_widget':
    $main_widget = $element + array(
      '#type' => 'textfield',
      '#default_value' => isset($items[$delta]['value']) ? $items[$delta]['value'] : NULL,
      );
    break;
  }
  $element['value'] = $main_widget;
  return $element;
}