Drupal ajax form, partial render - ajax

Question

Good day.
I created custom form in my module and defined submit button like this:
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Save'),
'#ajax' => array(
'callback' => 'fmg_addbanner_ajax_callback',
'method' => 'replace',
'wrapper' => 'banner_add_wrapper'
)
);
Then outputted my form like this:
$form = drupal_get_form('fmg_banner_add_form', $region_id);
print render($form);
But ajax requests don't work. I think because of there are no needed drupal js files. How can I solve this problem? Thanks.

Answer 1

The normal case will be to have #ajax attached to a common input, like checkbox, text box etc.
The behavior is that when user change the input value, it'll fire your callback through ajax and send entire form over, and then you can process this form behind the scene and adjust certain things before sending the form back and change the element to the wrapper you defined.
One of the question that I had for partial form rendering is when I need to do some ajax call or inject partial form element to certain area of the page while keeping the integrity of the form rendering. (I don't know exactly what form build did, i guess i don't want to know)
Since form rendering uses the renderable array, if you just want to extract certain element, you could just do
$form = drupal_get_form('application_form');
$body = render($form['body']);
The following is a real example that i took a form and split into header, footer and the rest of form elements.
// get a form for updating application info.
$form = drupal_get_form('pgh_awards_review_application_form', $app);
$elements = array();
$form_render = render($form);
$elements = array(
'qualify' => render($form['qualify']),
'threshold_met' => render($form['threshold_met']),
'submit' => render($form['submit']),
);
if (preg_match('/<form.+div>/', $form_render, $matches)) {
$elements['header'] = $matches[0];
}
if (preg_match('/<input type="hidden".+form>/s', $form_render, $matches)) {
$elements['footer'] = $matches[0];
}
$vars['form'] = $elements;
It's definitely not pretty approach, it just serves the needs at the moment.