Drupal 7 Module Development

The callback function hello_world_simple_form_callback() passes the
$form and $form_state variables after they have gone through hello_world_
simple_form_example(). In this case we are returning the form element that is
being replaced.
Drupal knows this is a renderable array and renders it to the appropriate value.
Drupal sends the updated HTML back to the page where the Drupal AJAX
handlers retrieve the changes and replace the wrapper.
Chapter 10
AJAX automatically applied
AJAX can be automatically applied to elements on a page. This is done by applying
the use-ajax class to an element on a page. A typical use would be to apply the
use-ajax class to a link within a page to trigger an AJAX action. Links are
commonly used because the page the link points to might be for the cases when
JavaScript is disabled as a fallback.
In the following example we are going to provide a link that, when clicked, will add
"Hello World" to a div within the page. To start, we have two menu callbacks that
we add to hello_world_menu(). One item for the page we are generating and the
other is the callback URL used for AJAX or when JavaScript is disabled.
$items['hello_world/link'] = array(
'title' => 'Hello World: Link',
'page callback' => 'hello_world_link',
'access callback' => 'user_access',
'access arguments' => array('access content'),
);
$items['hello_world_link_callback'] = array(
'page callback' => 'hello_world_link_response',
'access callback' => 'user_access',
'access arguments' => array('access content'),
);
The first menu item is to our page where the link is located and where the AJAX will
add content. The second menu item is the callback that will handle the AJAX request
or the page request, when JavaScript is unavailable.
function hello_world_link() {
drupal_add_js('misc/ajax.js');
$link = l(t('Say Hello'), 'hello_world_link_callback/nojs/',
array('attributes' => array('class' => array('use-ajax'))));
return '' . $link . '';
}
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