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<?php

/**
 * @file
 * Contains Drupal\ajax_example\AjaxExampleForm
 */

namespace Drupal\ajax_example\Form;

use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\ChangedCommand;
use Drupal\Core\Ajax\CssCommand;
use Drupal\Core\Ajax\HtmlCommand;
use Drupal\Core\Ajax\InvokeCommand;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

class AjaxExampleForm extends FormBase {

  public function getFormId() {
    return 'ajax_example_form';
  }

  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['user_name'] = array(
      '#type' => 'textfield',
      '#title' => 'Username',
      '#description' => 'Please enter in a username',
      '#ajax' => array(
        // Function to call when event on form element triggered.
        'callback' => 'Drupal\ajax_example\Form\AjaxExampleForm::usernameValidateCallback',
        // Effect when replacing content. Options: 'none' (default), 'slide', 'fade'.
        'effect' => 'fade',
        // Javascript event to trigger Ajax. Currently for: 'onchange'.
        'event' => 'change',
        'progress' => array(
          // Graphic shown to indicate ajax. Options: 'throbber' (default), 'bar'.
          'type' => 'throbber',
          // Message to show along progress graphic. Default: 'Please wait...'.
          'message' => NULL,
        ),
      ),
    );
    
    $form['random_user'] = array(
      '#type' => 'button',
      '#value' => 'Random Username',
      '#ajax' => array(
        'callback' => 'Drupal\ajax_example\Form\AjaxExampleForm::randomUsernameCallback',
        'event' => 'click',
        'progress' => array(
          'type' => 'throbber',
          'message' => 'Getting Random Username',
        ),
        
      ),
    );
    return $form;
  }
  
  public function submitForm(array &$form, FormStateInterface $form_state) {
    drupal_set_message('Nothing Submitted. Just an Example.');
  }
  
  public function usernameValidateCallback(array &$form, FormStateInterface $form_state) {
    // Instantiate an AjaxResponse Object to return.
    $ajax_response = new AjaxResponse();
    
    // Check if Username exists and is not Anonymous User (''). 
    if (user_load_by_name($form_state->getValue('user_name')) && $form_state->getValue('user_name') != false) {
      $text = 'User Found';
      $color = 'green';
    } else {
      $text = 'No User Found';
      $color = 'red';
    }
    
    // Add a command to execute on form, jQuery .html() replaces content between tags.
    // In this case, we replace the desription with wheter the username was found or not.
    $ajax_response->addCommand(new HtmlCommand('#edit-user-name--description', $text));
    
    // CssCommand did not work.
    //$ajax_response->addCommand(new CssCommand('#edit-user-name--description', array('color', $color)));
    
    // Add a command, InvokeCommand, which allows for custom jQuery commands.
    // In this case, we alter the color of the description.
    $ajax_response->addCommand(new InvokeCommand('#edit-user-name--description', 'css', array('color', $color)));
    
    // Return the AjaxResponse Object.
    return $ajax_response;
  }

  public function randomUsernameCallback(array &$form, FormStateInterface $form_state) {
    // Get all User Entities.
    $all_users = entity_load_multiple('user');
    
    // Remove Anonymous User.
    array_shift($all_users);
    
    // Pick Random User.
    $random_user = $all_users[array_rand($all_users)];

    // Instantiate an AjaxResponse Object to return.
    $ajax_response = new AjaxResponse();
    
    // ValCommand does not exist, so we can use InvokeCommand.
    $ajax_response->addCommand(new InvokeCommand('#edit-user-name', 'val' , array($random_user->get('name')->getString())));
    
    // ChangedCommand did not work.
    //$ajax_response->addCommand(new ChangedCommand('#edit-user-name', '#edit-user-name'));
    
    // We can still invoke the change command on #edit-user-name so it triggers Ajax on that element to validate username.
    $ajax_response->addCommand(new InvokeCommand('#edit-user-name', 'change'));
    
    // Return the AjaxResponse Object.
    return $ajax_response;
  }
}

ajax_example.form:
    path: '/admin/ajax_example'
    defaults:
        _form: 'Drupal\ajax_example\Form\AjaxExampleForm'
        _title: 'Ajax Example'
    requirements:
        _permission: 'access administration pages'

name: 'Ajax Example'
description: 'Just an Ajax Example'
core: 8.x
type: module