<?php
namespace Genmato\Sample\Api;

interface DemoRepositoryInterface
{
  /**
  * Save demo list item.
  *
  * @api
  * @param \Genmato\Sample\Api\Data\DemoInterface $demo
  * @return \Magento\Customer\Api\Data\GroupInterface
  * @throws \Magento\Framework\Exception\InputException If there is a problem with the input
  * @throws \Magento\Framework\Exception\NoSuchEntityException If a group ID is sent but the group does not exist
  * @throws \Magento\Framework\Exception\State\InvalidTransitionException
  *If saving customer group with customer group code that is used by an existing customer group
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function save(\Genmato\Sample\Api\Data\DemoInterface $demo);

  /**
  * Get demo list item by ID.
  *
  * @api
  * @param int $id
  * @return \Genmato\Sample\Api\Data\DemoInterface
  * @throws \Magento\Framework\Exception\NoSuchEntityException If $groupId is not found
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function getById($id);

  /**
  * Retrieve demo list items.
  *
  * The list of demo items can be filtered
  *
  * @api
  * @param \Magento\Framework\Api\SearchCriteriaInterface $searchCriteria
  * @return \Genmato\Sample\Api\Data\DemoSearchResultsInterface
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function getList(\Magento\Framework\Api\SearchCriteriaInterface $searchCriteria);

  /**
  * Delete demo list item.
  *
  * @api
  * @param \Genmato\Sample\Api\Data\DemoInterface $demo
  * @return bool true on success
  * @throws \Magento\Framework\Exception\StateException If customer group cannot be deleted
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function delete(\Genmato\Sample\Api\Data\DemoInterface $demo);

  /**
  * Delete demolist by ID.
  *
  * @api
  * @param int $id
  * @return bool true on success
  * @throws \Magento\Framework\Exception\NoSuchEntityException
  * @throws \Magento\Framework\Exception\StateException If customer group cannot be deleted
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function deleteById($id);
}

<?php
namespace Genmato\Sample\Model\ResourceModel;

use Genmato\Sample\Model\DemoRegistry;
use Genmato\Sample\Model\DemoFactory;
use Genmato\Sample\Api\Data\DemoInterface;
use Genmato\Sample\Api\Data\DemoInterfaceFactory;
use Genmato\Sample\Api\Data\DemoExtensionInterface;
use Genmato\Sample\Api\Data\DemoSearchResultsInterfaceFactory;
use Genmato\Sample\Model\Demo;
use Genmato\Sample\Model\ResourceModel\Demo as DemoResource;
use Genmato\Sample\Model\ResourceModel\Demo\Collection;
use Genmato\Sample\Api\DemoRepositoryInterface;

use Magento\Framework\Exception\CouldNotDeleteException;
use Magento\Framework\Exception\CouldNotSaveException;
use Magento\Framework\Exception\NoSuchEntityException;
use Magento\Framework\Api\SearchCriteriaInterface;
use Magento\Framework\Reflection\DataObjectProcessor;
use Magento\Framework\Api\ExtensionAttribute\JoinProcessorInterface;

class DemoRepository implements DemoRepositoryInterface
{

  /** @var DemoRegistry  */
  private $demoRegistry;

  /** @var DemoFactory  */
  private $demoFactory;

  /** @var DemoInterfaceFactory  */
  private $demoDataFactory;

  /** @var  Demo */
  private $demoResourceModel;

  /**
  * @var DataObjectProcessor
  */
  private $dataObjectProcessor;

  /**
  * @var DemoSearchResultsInterfaceFactory
  */
  protected $searchResultsFactory;

  /**
  * @var JoinProcessorInterface
  */
  protected $extensionAttributesJoinProcessor;

  /**
  * @param DemoRegistry $demoRegistry
  * @param DemoFactory $demoFactory
  * @param DemoInterfaceFactory $demoDataFactory
  * @param DemoResource $demoResourceModel
  * @param DataObjectProcessor $dataObjectProcessor
  * @param DemoSearchResultsInterfaceFactory $searchResultsFactory
  * @param JoinProcessorInterface $extensionAttributesJoinProcessor
  */
  public function __construct(
    DemoRegistry $demoRegistry,
    DemoFactory $demoFactory,
    DemoInterfaceFactory $demoDataFactory,
    DemoResource $demoResourceModel,
    DataObjectProcessor $dataObjectProcessor,
    DemoSearchResultsInterfaceFactory $searchResultsFactory,
    JoinProcessorInterface $extensionAttributesJoinProcessor
  ) {
    $this->demoRegistry = $demoRegistry;
    $this->demoFactory = $demoFactory;
    $this->demoDataFactory = $demoDataFactory;
    $this->demoResourceModel = $demoResourceModel;
    $this->dataObjectProcessor = $dataObjectProcessor;
    $this->searchResultsFactory = $searchResultsFactory;
    $this->extensionAttributesJoinProcessor = $extensionAttributesJoinProcessor;
  }
  /**
  * {@inheritdoc}
  */
  public function save(DemoInterface $demo)
  {
    /** @var Demo $demoModel */
    $demoModel = $this->demoFactory->create();

    if ($demo->getId()) {
      $demoModel->load($demo->getId());
    }
    $demoModel
    ->setTitle($demo->getTitle())
    ->setIsVisible($demo->getIsVisible())
    ->setIsActive($demo->getIsActive());

    try {
      $demoModel->save();
    } catch (\Exception $exception) {
      throw new CouldNotSaveException(__($exception->getMessage()));
    }
    return $demoModel->getData();
  }

  /**
  * {@inheritdoc}
  */
  public function getById($id)
  {
    $demoModel = $this->demoRegistry->retrieve($id);
    $demoDataObject = $this->demoDataFactory->create()
      ->setId($demoModel->getId())
      ->setTitle($demoModel->getTitle())
      ->setCreationTime($demoModel->getCreationTime())
      ->setUpdateTime($demoModel->getUpdateTime())
      ->setIsVisible($demoModel->getIsVisible())
      ->setIsActive($demoModel->getIsActive());
    return $demoDataObject;
  }

  /**
  * {@inheritdoc}
  */
  public function getList(SearchCriteriaInterface $searchCriteria)
  {
    $searchResults = $this->searchResultsFactory->create();
    $searchResults->setSearchCriteria($searchCriteria);

    /** @var Collection $collection */
    $collection = $this->demoFactory->create()->getCollection();
    foreach ($searchCriteria->getFilterGroups() as $filterGroup) {
      foreach ($filterGroup->getFilters() as $filter) {
        $condition = $filter->getConditionType() ?: 'eq';
        $collection->addFieldToFilter($filter->getField(), [$condition => $filter->getValue()]);
      }
    }
    $searchResults->setTotalCount($collection->getSize());
    $sortOrders = $searchCriteria->getSortOrders();
    if ($sortOrders) {
      /** @var SortOrder $sortOrder */
      foreach ($sortOrders as $sortOrder) {
        $collection->addOrder(
          $sortOrder->getField(),
          ($sortOrder->getDirection() == SortOrder::SORT_ASC) ? 'ASC' : 'DESC'
        );
      }
    }
    $collection->setCurPage($searchCriteria->getCurrentPage());
    $collection->setPageSize($searchCriteria->getPageSize());
    /** @var DemoInterface[] $demos */
    $demos = [];
    /** @var Demo $demo */
    foreach ($collection as $demo) {
      /** @var DemoInterface $demoDataObject */
      $demoDataObject = $this->demoDataFactory->create()
        ->setId($demo->getId())
        ->setTitle($demo->getTitle())
        ->setCreationTime($demo->getCreationTime())
        ->setUpdateTime($demo->getUpdateTime())
        ->setIsVisible($demo->getIsVisible())
        ->setIsActive($demo->getIsActive());

      $demos[] = $demoDataObject;
    }
    $searchResults->setTotalCount($collection->getSize());
    return $searchResults->setItems($demos);
  }

  /**
  * Delete demo list item.
  *
  * @param DemoInterface $demo
  * @return bool true on success
  * @throws \Magento\Framework\Exception\StateException If customer group cannot be deleted
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function delete(DemoInterface $demo)
  {
    return $this->deleteById($demo->getId());
  }

  /**
  * Delete demo list item by ID.
  *
  * @param int $id
  * @return bool true on success
  * @throws \Magento\Framework\Exception\NoSuchEntityException
  * @throws \Magento\Framework\Exception\StateException If customer group cannot be deleted
  * @throws \Magento\Framework\Exception\LocalizedException
  */
  public function deleteById($id)
  {
    $demoModel = $this->demoRegistry->retrieve($id);

    if ($id <= 0) {
      throw new \Magento\Framework\Exception\StateException(__('Cannot delete demo item.'));
    }

    $demoModel->delete();
    $this->demoRegistry->remove($id);
    return true;
  }


}

<?php
/**
* Sample
*
* @package Genmato_Sample
* @author  Vladimir Kerkhoff <support@genmato.com>
* @created 2015-12-23
* @copyright Copyright (c) 2015 Genmato BV, https://genmato.com.
*/
namespace Genmato\Sample\Model;

use Genmato\Sample\Api\Data\DemoInterface;
use Magento\Framework\Exception\NoSuchEntityException;

class DemoRegistry
{
  /**
  * @var array
  */
  protected $registry = [];

  /**
  * @var DemoFactory
  */
  protected $demoFactory;

  /**
  * @param DemoFactory $demoFactory
  */
  public function __construct(DemoFactory $demoFactory)
  {
    $this->demoFactory = $demoFactory;
  }

  /**
  * Get instance of the Demo Model identified by an id
  *
  * @param int $demoId
  * @return Demo
  * @throws NoSuchEntityException
  */
  public function retrieve($demoId)
  {
    if (isset($this->registry[$demoId])) {
      return $this->registry[$demoId];
    }
    $demo = $this->demoFactory->create();
    $demo->load($demoId);
    if ($demo->getId() === null || $demo->getId() != $demoId)
    {
      throw NoSuchEntityException::singleField(DemoInterface::ID, $demoId);
    }
    $this->registry[$demoId] = $demo;
    return $demo;
  }

  /**
  * Remove an instance of the Demo Model from the registry
  *
  * @param int $demoId
  * @return void
  */
  public function remove($demoId)
  {
    unset($this->registry[$demoId]);
  }
}

<?php
namespace Genmato\Sample\Api\Data;

use Genmato\Sample\Api\Data\DemoExtensionInterface;
use Magento\Framework\Api\ExtensibleDataInterface;

interface DemoInterface extends ExtensibleDataInterface
{

  const ID = 'id';
  const TITLE = 'title';
  const CREATION_TIME = 'creation_time';
  const UPDATE_TIME = 'update_time';
  const IS_ACTIVE = 'is_active';
  const IS_VISIBLE = 'is_visible';

  /**
  * Get id
  *
  * @api
  * @return int|null
  */
  public function getId();

  /**
  * Set id
  *
  * @api
  * @param int $id
  * @return $this
  */
  public function setId($id);

  /**
  * Get Title
  *
  * @api
  * @return string
  */
  public function getTitle();

  /**
  * Set Title
  *
  * @api
  * @param string $title
  * @return $this
  */
  public function setTitle($title);

  /**
  * Get Is Active
  *
  * @api
  * @return bool
  */
  public function getIsActive();

  /**
  * Set Is Active
  *
  * @api
  * @param bool $isActive
  * @return $this
  */
  public function setIsActive($isActive);

  /**
  * Get Is Visible
  *
  * @api
  * @return bool
  */
  public function getIsVisible();

  /**
  * Set Is Active
  *
  * @api
  * @param bool $isVisible
  * @return $this
  */
  public function setIsVisible($isVisible);

  /**
  * Get creation time
  *
  * @api
  * @return string
  */
  public function getCreationTime();

  /**
  * Set creation time
  *
  * @api
  * @param string $creationTime
  * @return $this
  */
  public function setCreationTime($creationTime);

  /**
  * Get update time
  *
  * @api
  * @return string
  */
  public function getUpdateTime();

  /**
  * Set update time
  *
  * @api
  * @param string $updateTime
  * @return $this
  */
  public function setUpdateTime($updateTime);

  /**
  * Retrieve existing extension attributes object or create a new one.
  *
  * @api
  * @return DemoExtensionInterface|null
  */
  public function getExtensionAttributes();

  /**
  * Set an extension attributes object.
  *
  * @api
  * @param DemoExtensionInterface $extensionAttributes
  * @return $this
  */
  public function setExtensionAttributes(DemoExtensionInterface $extensionAttributes);
}

<?php
namespace Genmato\Sample\Model\Data;

use Magento\Framework\Api\AbstractExtensibleObject;
use Genmato\Sample\Api\Data\DemoInterface;
use Genmato\Sample\Api\Data\DemoExtensionInterface;

class Demo extends AbstractExtensibleObject implements DemoInterface
{

  /**
  * Get id
  *
  * @return int|null
  */
  public function getId()
  {
    return $this->_get(self::ID);
  }

  /**
  * Set id
  *
  * @param int $id
  * @return $this
  */
  public function setId($id)
  {
    return $this->setData(self::ID, $id);
  }

  /**
  * Get code
  *
  * @return string
  */
  public function getTitle()
  {
    return $this->_get(self::TITLE);
  }

  /**
  * Set code
  *
  * @param string $title
  * @return $this
  */
  public function setTitle($title)
  {
    return $this->setData(self::TITLE, $title);
  }

  /**
  * Get Is Active
  *
  * @return bool
  */
  public function getIsActive()
  {
    return $this->_get(self::IS_ACTIVE);
  }

  /**
  * Set Is Active
  *
  * @param bool $isActive
  * @return $this
  */
  public function setIsActive($isActive)
  {
    return $this->setData(self::IS_ACTIVE, $isActive);
  }

  /**
  * Get Is Visible
  *
  * @return bool
  */
  public function getIsVisible()
  {
    return $this->_get(self::IS_VISIBLE);
  }

  /**
  * Set Is Active
  *
  * @param bool $isVisible
  * @return $this
  */
  public function setIsVisible($isVisible)
  {
    return $this->setData(self::IS_VISIBLE, $isVisible);
  }

  /**
  * Get creation time
  *
  * @return string
  */
  public function getCreationTime()
  {
    return $this->_get(self::CREATION_TIME);
  }

  /**
  * Set creation time
  *
  * @param string $creationTime
  * @return $this
  */
  public function setCreationTime($creationTime)
  {
    return $this->setData(self::CREATION_TIME, $creationTime);
  }

  /**
  * Get update time
  *
  * @return string
  */
  public function getUpdateTime()
  {
    return $this->_get(self::UPDATE_TIME);
  }

  /**
  * Set update time
  *
  * @param string $updateTime
  * @return $this
  */
  public function setUpdateTime($updateTime)
  {
    return $this->setData(self::UPDATE_TIME, $updateTime);
  }

  /**
  * {@inheritdoc}
  *
  * @return DemoExtensionInterface|null
  */
  public function getExtensionAttributes()
  {
    return $this->_getExtensionAttributes();
  }

  /**
  * {@inheritdoc}
  *
  * @param DemoExtensionInterface $extensionAttributes
  * @return $this
  */
  public function setExtensionAttributes(DemoExtensionInterface $extensionAttributes)
  {
    return $this->_setExtensionAttributes($extensionAttributes);
  }
}

<?php
namespace Genmato\Sample\Api\Data;

use Genmato\Sample\Api\Data\DemoInterface;
use Magento\Framework\Api\SearchResultsInterface;

interface DemoSearchResultsInterface extends SearchResultsInterface
{
  /**
  * Get demo item list.
  *
  * @api
  * @return DemoInterface[]
  */
  public function getItems();

  /**
  * Set demo item list.
  *
  * @api
  * @param DemoInterface[] $items
  * @return $this
  */
  public function setItems(array $items);

}

  <preference for="Genmato\Sample\Api\DemoRepositoryInterface"
    type="Genmato\Sample\Model\ResourceModel\DemoRepository" />

  <preference for="Genmato\Sample\Api\Data\DemoInterface" type="Genmato\Sample\Model\Data\Demo" />
  <preference for="Genmato\Sample\Api\Data\DemoSearchResultsInterface"
    type="Magento\Framework\Api\SearchResults" />

<?php
namespace Genmato\Sample\Controller\Index;

use Magento\Framework\App\Action\Action;
use Magento\Framework\App\Action\Context;
use Magento\Framework\View\Result\PageFactory;
use Magento\Framework\Api\SearchCriteriaBuilder;
use Genmato\Sample\Api\DemoRepositoryInterface;
use Genmato\Sample\Model\Data\DemoFactory;
use Genmato\Sample\Api\Data\DemoInterface;

class Test extends Action
{
  /**
  * @var PageFactory
  */
  private $resultPageFactory;

  /** @var  DemoRepositoryInterface */
  private $demoRepository;

  /** @var DemoFactory  */
  private $demo;

  /**
  * @var SearchCriteriaBuilder
  */
  private $searchCriteriaBuilder;
  /**
  * @param Context $context
  * @param PageFactory $resultPageFactory
  * @param DemoRepositoryInterface $demoRepository
  * @param SearchCriteriaBuilder $searchCriteriaBuilder
  * @param DemoFactory $demoFactory
  */
  public function __construct(
    Context $context,
    PageFactory $resultPageFactory,
    DemoRepositoryInterface $demoRepository,
    SearchCriteriaBuilder $searchCriteriaBuilder,
    DemoFactory $demoFactory
  )
  {
    $this->demoRepository = $demoRepository;
    $this->searchCriteriaBuilder = $searchCriteriaBuilder;
    $this->demo = $demoFactory;
    parent::__construct($context);
  }

  /**
  * Renders Sample
  */
  public function execute()
  {
    echo '<pre>';

    // Create new record through service layer/contract

    /** @var DemoInterface $demoRecord */
    $demoRecord = $this->demo->create();
    $demoRecord->setIsActive(1)
      ->setIsVisible(1)
      ->setTitle('Test through Service Layer');

    $demo = $this->demoRepository->save($demoRecord);
    print_r($demo);

    // Get list of available records
    $searchCriteria = $this->searchCriteriaBuilder->create();
    $searchResult = $this->demoRepository->getList($searchCriteria);
    foreach ($searchResult->getItems() as $item) {
      echo $item->getId().' => '.$item->getTitle().'<br>';
    }
  }

}

bin/magento setup:upgrade