Age Validation with dob in Drupal 8

I was been given a situation where there was 2 fields one was cateogry and another was date-of-birth.

So the cateogry was dropdown consists of 4 options OBC ,General ,ST and SC.

The age should be between 35 to 45 for General and OBC and 35 to 48 for ST and SC.

So i have written some code in drupal 8 for age validation.

created a .module file and witten the below code which validates


use Drupal\Core\Form\FormStateInterface;

 * Implements hook_form_alter().
function modulename_form_alter(&$form, FormStateInterface $form_state, $form_id) {
 if($form_id == "your-form-id"){
     $form['#validate'][] = 'dob_form_validate';

function dob_form_validate($form, FormStateInterface &$form_state) {
 $dob = $form_state->getValue('field_date_of_birth');
   $birthDate = $dob[0]['value']->format('Y-m-d');
   $birthDate = new DateTime($birthDate);
   $to = new DateTime('2016-01-31');
   $age = $birthDate->diff($to);
   $years = $age->y;
   $months = $age->m;
   $days = $age->d;
 $cateogry = $form_state->getValue('field_ap_category');
  $cat = $cateogry[0]['value'];
  if($cat == "BC 1" || $cat == "BC 2" || $cat == "General"){
    if($years == 45 && ($months > 0 || $days>0))
      $form_state->setErrorByName('field_ap_date_of_birth', t('Age must be between 35 and 45 for General and BC'));
    else if($years < 35 || $years > 45 ){
      $form_state->setErrorByName('field_ap_date_of_birth', t('Age must be between 35 and 45 for General and BC'));
  else if($cat == "SC" || $cat=="ST"){
    if($years == 48 && ($months > 0 || $days>0))
      $form_state->setErrorByName('field_ap_date_of_birth', t('Age must be between 35 and 48 for SC and ST'));
    else if($years < 35 || $years > 48){
      $form_state->setErrorByName('field_ap_date_of_birth',t('Age must be between 35 and 48 for SC and ST'));


Custom Submit handler in drupal 8

If we want to create our  custom submit handler to submit a form then we can easily use the below code

use Drupal\Core\Form;
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id){
  if($form_id == 'node_newmodule_form'){
    foreach (array_keys($form['actions']) as $action) {
     if ($action != 'preview' && isset($form['actions'][$action]['#type']) && $form['actions'][$action]['#type'] === 'submit') {
       $form['actions'][$action]['#submit'][] = 'custom_form_submit';

public function custom_form_submit(&$form, FormStateInterface $form_state){
   echo "It will invoke";

So we can easily choose which form id to invoke this submit handler.
The custom function custom_form_submit will be envoke when we submit the following form id

Drupal 8 Hooks hook_element_info_alter()

There are many hooks in drupal 8

So i will list the most important hooks which i have used so far


Alter the element type information returned from modules.

A module may implement this hook in order to alter the element type defaults defined by a module.

example code:

function hook_element_info_alter(&$type) {
  // Decrease the default size of textfields.
  if (isset($type['textfield']['#size'])) {
    $type['textfield']['#size'] = 40;

function mymodule_element_info_alter(array &$types) {
  //call the custom function after build.
  foreach ($types as $type => $info) {
    $types[$type]['#after_build'][] = 'my_custom_function';

This hook is mainly used to alter the elements so $type contains all the elements.


Important Services in drupal 8

$contentTypes = \Drupal::service('entity.manager')->getStorage('node_type')->loadMultiple();

It will list all the content types of your current project.
It is very useful to call service(‘entity.manager’);

$type = $contentTypes->label();

This will get the Name of content type.

$type = $contentTypes->id();

This will get the slug of content type. Means the content type machine name

$form_modes = \Drupal::service('entity_display.repository')->getFormModes('node');

This will get the list of all content type form mode in current project

$form element with #type => ‘submit’ with custom submit and validate function

$form[‘copy_local’] = [
‘#type’ => ‘submit’,
‘#prefix’ => ‘<br>’,
‘#name’ => ‘public’,
‘#value’ => $this->t(‘Copy local public files to S3’),
‘#validate’ => [
[$this, ‘copyLocalValidateForm’],
‘#submit’ => [
[$this, ‘copyLocalSubmitForm’],

public function copyLocalValidateForm(){

//Code Here


When we click on this submit button then the #validate is called first and the #submit.It will check for the custom method in same class and then call that method.






Drupal 8 userful functions in users

$role_names = user_role_names();

It returns array of all the user roles like anonymous,authenticated,admin.

To check whether a username exists or email exists then drupal 8 provide the below functions to check

$username = “Mahtab”

$hasUsername = user_load_by_name($username);

the $hasUsername will return true or false, the same we can check for email by

$hasEmail = user_load_by_mail($email);

To get the current user id

$user = \Drupal::currentUser();
$uid = $user->id();

How to build Drupal 8 theme with bootstrap as base theme

Building theme in drupal 8 is easy and you can easily build the base theme of bootstrap. Just follow the instructions below

First download the bootstrap theme from

Then in bootstrap theme you will find one folder named as starterkits

Copy the less folder of starter kits to your theme directory where the bootstrap theme is placed

Rename the less folder as your themename lets suppose we change the name less as mahi

Rename all these files from the less directory

  • THEMENAME.libraries.yml to mahi.libraries.yml
  • THEMENAME.starterkit.yml to
  • THEMENAME.theme to mahi.theme
  • config/install/THEMENAME.settings.yml to config/install/mahi.settings.yml
  • config/schema/THEMENAME.schema.yml to config/schema/mahi.schema.yml

Now open and change the following

name: ‘Mahi





After this Go to the bootstrap official page and download the bootstrap source

Place it in your theme directory and rename as bootstrap

Next compile the less file which will create the style.css file.
lessc less/style.less > css/style.css from terminal going to your theme directory

Now go to the appearence tab in drupal and install and set as default to your theme

Enjoy it will start working. If any issue occurs comment below so i will solve the problems