class AccountSwitcher

class AccountSwitcher implements AccountSwitcherInterface {
    
    /**
     * A stack of previous overridden accounts.
     *
     * @var \Drupal\Core\Session\AccountInterface[]
     */
    protected $accountStack = [];
    
    /**
     * The current user service.
     *
     * @var \Drupal\Core\Session\AccountProxyInterface
     */
    protected $currentUser = [];
    
    /**
     * The write-safe session handler.
     *
     * @var \Drupal\Core\Session\WriteSafeSessionHandlerInterface
     */
    protected $writeSafeHandler;
    
    /**
     * The original state of session saving prior to account switching.
     *
     * @var bool
     */
    protected $originalSessionSaving;
    
    /**
     * Constructs a new AccountSwitcher.
     *
     * @param \Drupal\Core\Session\AccountProxyInterface $current_user
     *   The current user service.
     * @param \Drupal\Core\Session\WriteSafeSessionHandlerInterface $write_safe_handler
     *   The write-safe session handler.
     */
    public function __construct(AccountProxyInterface $current_user, WriteSafeSessionHandlerInterface $write_safe_handler) {
        $this->currentUser = $current_user;
        $this->writeSafeHandler = $write_safe_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public function switchTo(AccountInterface $account) {
        // Prevent session information from being saved and push previous account.
        if (!isset($this->originalSessionSaving)) {
            // Ensure that only the first session saving status is saved.
            $this->originalSessionSaving = $this->writeSafeHandler
                ->isSessionWritable();
        }
        $this->writeSafeHandler
            ->setSessionWritable(FALSE);
        array_push($this->accountStack, $this->currentUser
            ->getAccount());
        $this->currentUser
            ->setAccount($account);
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function switchBack() {
        // Restore the previous account from the stack.
        if (!empty($this->accountStack)) {
            $this->currentUser
                ->setAccount(array_pop($this->accountStack));
        }
        else {
            throw new \RuntimeException('No more accounts to revert to.');
        }
        // Restore original session saving status if all account switches are
        // reverted.
        if (empty($this->accountStack)) {
            if ($this->originalSessionSaving) {
                $this->writeSafeHandler
                    ->setSessionWritable(TRUE);
            }
        }
        return $this;
    }

}