How to make a promotion in Bitrix 3 products at a price of 2?

Question

1C-Bitrix

Bitrix has a discount module, but it is not suitable for any discount option.
If you have experience or can tell how to organize a discount of 3 products for the price of 2, or how to interrupt the resave cycle during the OnSaleBasketSaved event, thank you.

Answer 1

in the sense of the same product, if there are 3 pieces, then the price is for two?
1) translated into mathematical language: if the quantity is 3 pieces, then the discount on the product is 33.33%.
such a rule can be set by standard discounts
2) if there are still some other conditions, or if there are more than 3 products, and the behavior of the standard discount is not what you expect, then I would use
$eventManager->addEventHandler('sale', 'OnBasketUpdate ', function($ID, $arItem){})
inside this handler, we get the current user's cart, find the desired product in the cart, look at its quantity, and if necessary, put down a custom price, if not, then return the optimal price of the product for this group user.
I have only such an event worked as it should.
(we considered discounts for sets, if the basket contains, for example, a hat and a scarf and mittens, then everyone has a discount, if there are 2 scarves, then a discount is only for one of them, since the second is no longer a set. but the price goes to the basket position)

Answer 2

I will give the final solution for those who will search.
Some things will need to be adapted a bit, but in general here is the solution:

public static function OnBasketUpdateHandler($ID, $arItem)
    {
        global $USER;
  if(!is_object($USER)){
    $USER = new CUser();
  }
  $userId=$USER->GetID();
  $strGroups = $USER->GetUserGroupString();
  $arUserGroups = explode(',', $strGroups); 


    $productID = $arItem['PRODUCT_ID'];
    $quantity = $arItem['QUANTITY'];

    
  if ($userId=="81035" ){//Пока выводим только для меня
    \Bitrix\Main\Loader::IncludeModule('iblock');
    \Bitrix\Main\Loader::IncludeModule('catalog');
    \Bitrix\Main\Loader::IncludeModule('sale');

    $fuser=\Bitrix\Sale\Fuser::getId();
    $basket = \Bitrix\Sale\Basket::loadItemsForFUser( $fuser, \Bitrix\Main\Context::getCurrent()->getSite() );
    
    $res=[];
    $tovar=[];
    foreach ($basket as $basketItem) {
      if(!$basketItem->isDelay()){
        $mxResult=[];
        $aa = \CCatalogSku::GetProductInfo($basketItem->getProductId())['ID'];
        $mxResult[$aa] = $aa;
        \CIBlockElement::GetPropertyValuesArray($mxResult, 26, array(
          'ID' => $mxResult,
          'IBLOCK_ID' => 26,
        ));
        $raschet='';
        if(in_array($mxResult[$aa]['ROZNICHNAYA_SET']['VALUE_ENUM_ID'], [1753,1754]) && $mxResult[$aa]['RASPRODAZHA']['VALUE']=='true'){$raschet='raschet';}
                
        $basketPropertyCollection = $basketItem->getPropertyCollection(); 				
        $res[]=[$basketItem->getProductId(),$basketItem->getPrice(), $basketItem->getQuantity(), $basketItem->getFinalPrice(), $basketPropertyCollection->getPropertyValues()['BAR_CODE']['VALUE'], $raschet];
        $tovar[]=$basketPropertyCollection->getPropertyValues()['BAR_CODE']['VALUE'];
          
      }
    }
    
    $coli=[];
    $coli0=[];
    foreach($res as $vl){
      if($vl[5]=='raschet'){
      for ($x=1; $x<=$vl[2]; $x++){
        $coli[]=[$vl[0],$vl[1],$vl[5]];
      }
      }else{
        $coli0[]=[$vl[0],$vl[1],$vl[5]];
      }
    }
    $cont=count($coli);
    usort($coli, function($a, $b){
      return $a['1'] <=> $b['1'];
    });
    $skidka = floor($cont / 3);
    for ($x=0; $x<$skidka; $x++){
      $coli[$x][1]=0;
    }
    $coli = array_reverse($coli)+$coli0;
    $arFilter = Array( "IBLOCK_ID"=>27, "PROPERTY_CML2_BAR_CODE"=>$tovar );
    $res = \CIBlockElement::GetList(Array("SORT"=>"ASC",), $arFilter, false,false,array('ID',"PROPERTY_CML2_BAR_CODE",'CATALOG_QUANTITY','PROPERTY_CML2_LINK','IBLOCK_EXTERNAL_ID','XML_ID'));
    $productsId=[];
    $productsIdXml=[];
    
    while($ar_fields = $res->GetNext())
    {
      $productsId[$ar_fields["PROPERTY_CML2_BAR_CODE_VALUE"]]=$ar_fields['ID'];
      $productsIdXml[$ar_fields["PROPERTY_CML2_BAR_CODE_VALUE"]]=['IBLOCK_EXTERNAL_ID'=>$ar_fields['IBLOCK_EXTERNAL_ID'],'XML_ID'=>$ar_fields['XML_ID']];
      $productsIdXml2[$ar_fields["ID"]]=['IBLOCK_EXTERNAL_ID'=>$ar_fields['IBLOCK_EXTERNAL_ID'],'XML_ID'=>$ar_fields['XML_ID']];
    }
    $itogTovar=[];
    
    foreach($coli as $val){
      $itogTovar[$val[0]][$val[1]]++;
    }
        
    $itogo=[];	
    foreach($itogTovar as $productID0=>$value){
      $itogo[$productID0]['price']=0;
      $itogo[$productID0]['quantity']=0;
      $itogo[$productID0]['custom']='N';
      foreach($value as $price0=>$qunt0){
        if($price0>0){
          $arPrice0 = \bh\catalog\tools::getPrc($productID0);
          $itogo[$productID0]['price']=$itogo[$productID0]['price']+$arPrice0*$qunt0;
          $itogo[$productID0]['quantity']=$itogo[$productID0]['quantity']+$qunt0;
        }else{
          $itogo[$productID0]['quantity']=$itogo[$productID0]['quantity']+$qunt0;
          $itogo[$productID0]['custom']='Y';
        }
      }
      $itogo[$productID0]['price']=floor($itogo[$productID0]['price']/$itogo[$productID0]['quantity']);
    }
    foreach ($basket as $basketItem) {
      $id=$basketItem->getField('PRODUCT_ID');
      if($itogo[$id]['custom']=='Y'){
        $basketItem->setFields(array(
            'CUSTOM_PRICE' => 'Y',
            'PRICE' => $itogo[$id]['price'],
            'CATALOG_XML_ID'=>$productsIdXml2[$id]['IBLOCK_EXTERNAL_ID'],
            'PRODUCT_XML_ID'=>$productsIdXml2[$id]['XML_ID']
          ));
        $basketItem->save();
        $basketPropertyCollection = $basketItem->getPropertyCollection(); 
        $basketPropertyCollection->setProperty(array(
          'ACTION'=>array(
             'NAME' => 'Акция',
             'CODE' => 'ACTION',
             'VALUE' => 'Товар по акции 2=3',
             'SORT' => 1000,
          ),
          'CATALOG_XML_ID'=>array(
              'NAME' => 'Catalog XML_ID',
              'CODE' => 'CATALOG.XML_ID',
              'VALUE' => $productsIdXml2[$id]['IBLOCK_EXTERNAL_ID']
            ),
          'PRODUCT_XML_ID'=>array(
              "NAME" => "Product XML_ID",
              "CODE" => "PRODUCT.XML_ID",
              "VALUE" => $productsIdXml2[$id]['XML_ID']
            )
        ));
        $basketPropertyCollection->save();
      }else{
        $basketItem->setFields(array(
            'CATALOG_XML_ID'=>$productsIdXml2[$id]['IBLOCK_EXTERNAL_ID'],
            'PRODUCT_XML_ID'=>$productsIdXml2[$id]['XML_ID']
          ));
        $basketItem->save();
        $basketPropertyCollection = $basketItem->getPropertyCollection(); 
        $basketPropertyCollection->setProperty(array(
          'CATALOG_XML_ID'=>array(
              'NAME' => 'Catalog XML_ID',
              'CODE' => 'CATALOG.XML_ID',
              'VALUE' => $productsIdXml2[$id]['IBLOCK_EXTERNAL_ID']
            ),
          'PRODUCT_XML_ID'=>array(
              "NAME" => "Product XML_ID",
              "CODE" => "PRODUCT.XML_ID",
              "VALUE" => $productsIdXml2[$id]['XML_ID']
            )
        ));
        $basketPropertyCollection->save();
      }
    }
    
  }
    
    }

Answer 3

I can’t tell you about the implementation of such a discount by standard means, I don’t know myself. You can try through the cart events. (As I understand it, you went the same way)
To remove recursion in an event:
- at the beginning of the handler, declare a global variable (or a static one in the class). and check for it.

global $isOnSaleBasketSavedHandler; 
$isOnSaleBasketSavedHandler = true;
if(!$isOnSaleBasketSavedHandler){
     //Ваш код
}

- then check this variable
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