Custom cache context sample

cache-context-block.php

<?php
namespace Drupal\rohit\Plugin\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Cache\Cache;
use Drupal\user\Entity\User;
use Symfony\Component\DependencyInjection\ContainerInterface;
/**
 * Provides a block to display 'Site branding' elements.
 *
 * @Block(
 *   id = "user_favorite_color_block",
 *   admin_label = @Translation("User favorite color")
 * )
 */
class UserFavoriteColorBlock extends BlockBase {
  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = array();
    $current_user_id = \Drupal::currentUser()->id();
    // Load user object.
    $user = User::load($current_user_id);
    $favorite_color = $user->get('field_favorite_color')->getValue()[0]['value'];
    $build['rohit'] = array(
      '#markup' => 'User favorite color is ' . $favorite_color,
    );
    return $build;
  }
  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    return Cache::mergeContexts(
      parent::getCacheContexts(),
      ['user_favorite_color']
      );
  }
}

cache-context-rohit.php

<?php
namespace Drupal\rohit\CacheContext;
use Drupal\Core\Cache\Context\CacheContextInterface;
use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Session\AccountProxyInterface;
use Drupal\user\Entity\User;
class UserFavoriteColorCacheContext implements CacheContextInterface {
  /**
   * @var \Drupal\Core\Session\AccountProxyInterface
   */
  protected $currentUser;
  public function __construct(AccountProxyInterface $current_user) {
      $this->currentUser = $current_user;
  }
  /**
   * {@inheritdoc}
   */
  public static function getLabel() {
    return t('User favorite color cache context');
  }
  /**
  * {@inheritdoc}
  */
  public function getContext() {
    $current_user_id = $this->currentUser->getAccount()->id();
    // Load user object.
    $user = User::load($current_user_id);
    $favorite_color = $user->get('field_favorite_color')->getValue()[0]['value'];
    return $favorite_color;
  }
  /**
  * {@inheritdoc}
  */
  public function getCacheableMetadata() {
    return new CacheableMetadata();
  }
}

cache-context-service.yml

services:
  cache_context.user_favorite_color:
    class: Drupal\rohit\CacheContext\UserFavoriteColorCacheContext
    arguments: ['@current_user']
    tags:
      - { name: cache.context }
  cache_context.my_cache_context:
    class: Drupal\rohit\CacheContext\MyCacheContextClass
    arguments: []
    tags:
      - { name: cache_context }