in this tuto, I'll sho you how to alter an existing form in drupal 8.
add classes and prefix, suffix to Form tag
// add prefix to <form> tag
$form['#prefix'] = '<div class="row my-class">';
// add suffix to </form> tag
$form['#suffix'] = '</div>';
//add a class in form tag <form class="col-10 ..."
$form['#attributes']['class'][] = 'col-10';
alter form element :
delete description, add class to input, add class to label, add class to wrapper and alter title display option.
// delete description from name element
unset($form['name']['#description']);
// add class to input
$form['name']['#attributes']['class'][] = 'form-control';
// change placeholder
$form['name']['#attributes']['placeholder'] = t("Title");
// add class to wrapper
$form['name']['#wrapper_attributes']['class'][] = 'col-12 form-group';
// add class to label
$form['name']['#label_attributes']['class'][] = 'my-form-label';
// invisible label
// $form['name']['#title_display'] = 'invisible';
alter submit buttons :
// change submit button value
$form['actions']['submit']['#value'] = 'Send';
// add class to submit buttons
$form['actions']['submit']['#attributes']['class'][] = 'my-form-submit';
// add prefix to submit buttons
$form['actions']['submit']['#prefix'] = '<div class="col-12"><div class="form-group">';
// add suffix to submit buttons
$form['actions']['submit']['#suffix'] = '</div></div>';
add new element form and delete existing element :
// Add a new element checkbox to form.
$form['terms'] = array(
'#type' => 'checkbox',
'#title' => t("I accept the terms."),
'#required' => TRUE,
);
// example how to delete element in form
//unset($form['field_example']);
Full code :
use Drupal\Core\Form\FormStateInterface;
/**
* @param $form
* @param FormStateInterface $form_state
* @param $form_id
*/
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id)
{
switch ($form_id) {
case 'user_login_form':
// add prefix to <form> tag
$form['#prefix'] = '<div class="row my-class">';
// add suffix to </form> tag
$form['#suffix'] = '</div>';
//add a class in form tag <form class="col-10 ..."
$form['#attributes']['class'][] = 'col-10';
// delete description from name element
unset($form['name']['#description']);
// add class to input
$form['name']['#attributes']['class'][] = 'form-control';
// change placeholder
$form['name']['#attributes']['placeholder'] = t("Title");
// add class to wrapper
$form['name']['#wrapper_attributes']['class'][] = 'col-12 form-group';
// add class to label
$form['name']['#label_attributes']['class'][] = 'my-form-label';
// invisible label
// $form['name']['#title_display'] = 'invisible';
// change submit button value
$form['actions']['submit']['#value'] = 'Send';
// add class to submit buttons
$form['actions']['submit']['#attributes']['class'][] = 'my-form-submit';
// add prefix to submit buttons
$form['actions']['submit']['#prefix'] = '<div class="col-12"><div class="form-group">';
// add suffix to submit buttons
$form['actions']['submit']['#suffix'] = '</div></div>';
// Add a new element checkbox to form.
$form['terms'] = array(
'#type' => 'checkbox',
'#title' => t("I accept the terms."),
'#required' => TRUE,
);
// example how to delete element in form
//unset($form['field_example']);
break;
}
}
