Ok the new generation of ui grid in magento 2 give you the ability to create or change grid  with xml file, this xml use external php file for getting all part needed .
Our example here is the grid action like edit in catalog product grid or delete preview in page cms grid ,
ok we want add custom action .

Ok the xml responsible for ui grid of  product list in backend is vendor/magento/module-catalog/view/adminhtml/ui_component/product_listing.xml , and for action you will inside this file :

 
<item name="fieldAction" xsi:type="array">
     <item name="provider" xsi:type="string">product_listing.product_listing.product_columns.actions</item>
             <item name="target" xsi:type="string">applyAction</item>
                   <item name="params" xsi:type="array">
                   <item name="0" xsi:type="string">edit</item>
              <item name="1" xsi:type="string">${ $.$data.rowIndex }</item>
     </item>
 </item>
 

it's tell to grid : the current row  get the url from edit action but the important part is in the last of file :

 
        <actionsColumn name="actions" class="Magento\Catalog\Ui\Component\Listing\Columns\ProductActions">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="indexField" xsi:type="string">entity_id</item>
                    <item name="sortOrder" xsi:type="number">200</item>
                </item>
            </argument>
        </actionsColumn>
 

the ui grid use the class  Magento\Catalog\Ui\Component\Listing\Columns\ProductActions for adding all action related to this ui grid we go to class in path vendor/magento/module-catalog/Ui/Component/Listing/Columns/ProductActions.php , we find:

 
    public function prepareDataSource(array $dataSource)
    {
        if (isset($dataSource['data']['items'])) {
            $storeId = $this->context->getFilterParam('store_id');
            foreach ($dataSource['data']['items'] as &$item) {
                $item[$this->getData('name')]['edit'] = [
                    'href' => $this->urlBuilder->getUrl(
                        'catalog/product/edit',
                        ['id' => $item['entity_id'], 'store' => $storeId]
                    ),
                    'label' => __('Edit'),
                    'hidden' => false,
                ];
            }
        }
      return $dataSource;
    }
 

Ok now we want add our action , it's simple you need grid plugin in your module and add event 'after' , and push your action , inside Ibnab/CustomAction/etc/adminhtml/di.xml (Ibnab = YouVendor , CustomAction= YourModule) :

 
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Catalog\Ui\Component\Listing\Columns\ProductActions">
        <plugin name="prepare_data_source_after" type="Ibnab\CustomAction\Plugin\Adminhtml\ProductActions"/>
    </type>
</config>
 

Ok now create your class in :
Ibnab/CustomAction/Plugin/Adminhtml/ProductActions .php and push :

 
<?php
namespace Ibnab\CustomAction\Plugin\Adminhtml;
use Magento\Framework\View\Element\UiComponent\ContextInterface;
use Magento\Framework\Url;
class ProductActions {
    protected $urlBuilder;
    protected $context;
    public function __construct(
    ContextInterface $context, Url $urlBuilder
    ) {
        $this->urlBuilder = $urlBuilder;
        $this->context = $context;
    }
    public function afterPrepareDataSource($productActions, $result) {
            if (isset($result['data']['items'])) {
                $storeId = $this->context->getFilterParam('store_id');
                 foreach ($result['data']['items'] as &$item) {
                $item[$productActions->getData('name')]['preview'] = [
                    'href' => $this->urlBuilder->getUrl('catalog/product/view', ['id' => $item['entity_id'], '_nosid' => true]),
                    'target' => '_blank',
                    'label' => __('ُPreview'),
                ];
                }
            }   
        return $result;
    }
}
 

thats is all .