Drupal 7 Module Development

Creating New Fields
Since measurements of length do not really make sense without a unit, we will also
record what unit the dimensions are in, such as inches or meters. To keep things
simple we will only save integers, although in practice we would want to support
float values. Also note that the whole function is wrapped in an if() statement to
check for the field type. If we were defining multiple field types in this module, they
would define their schema using the same function and we'd have to differentiate
between them based on the value of $field['type'].
Defining empty
The second magic callback we need is to determine if a given field has an empty
value. While that may seem like a simple question, it is actually dependent on our
particular application.
Consider this: Is a dimension field empty if it has no height but only has a width, or
only if both values are empty? Drupal doesn't know which we mean, so we need to
tell it.
function dimfield_field_is_empty($item, $field) {
if ($field['type'] == 'dimensions') {
if (empty($item['height']) && empty($item['width']) &&
($field['settings']['num_dimensions'] == 2 ||
empty($item['depth'])))
{
return TRUE;
}
}
return FALSE;
}
In the preceding snippet, we define empty to mean that all dimensions in use are an
empty value, which PHP defines to include an empty string or 0. Again note that we
are checking against the specific field type since we could add another field type to
this module later.
Field settings
Although not absolutely required, we also need a configuration form for the field
settings. Most fields will be configured through Drupal's web interface, so we need a
form to allow users to set the available options. That's another magic callback. Let's
look at an example:
function dimfield_field_settings_form($field, $instance, $has_data) {
if ($field['type'] == 'dimensions') {
$settings = $field['settings'];
$form['num_dimensions'] = array(
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