Magento 2.1:- Shipping method issue in Plugin

Question

I create one plugin Vendor\Test\Plugin\Shipping for model Magento\Shipping\Model\Shipping.php using di.xml

app/code/Vendor/Test/etc/di.xml

<type name="Magento\Shipping\Model\Shipping">
    <plugin name="shipping_test" type="Vendor\Test\Plugin\Shipping" sortOrder="1"/>
</type>

app/code/Vendor/Test/Plugin/Shipping.php

<?php

namespace Vendor\Test\Plugin;

use Magento\Framework\Exception\LocalizedException;
use Magento\Quote\Model\Quote\Address\RateRequest;
//use Magento\Shipping\Model\Rate\Result;
use Magento\Checkout\Model\Cart;
use Vendor\Test\Helper\Data;
use Magento\Quote\Model\Quote\Address\RateResult\Error;
use Magento\Shipping\Model\Shipment\Request;
use Magento\Sales\Model\Order\Shipment;
use Magento\Quote\Model\Quote\Address\RateCollectorInterface;

/**
 * @SuppressWarnings(PHPMD.CouplingBetweenObjects)
 */
class Shipping
{

    protected $_code='Shipping';

    protected $_rateResultFactory;

    protected $_rateMethodFactory;

    protected $_result;

    public function __construct(
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Quote\Model\Quote\Address\RateResult\ErrorFactory $rateErrorFactory,
        \Psr\Log\LoggerInterface $logger,
        \Magento\Shipping\Model\Rate\ResultFactory $rateResultFactory,
        \Magento\Quote\Model\Quote\Address\RateResult\MethodFactory $rateMethodFactory,
        \Magento\Checkout\Model\Cart $cart,
        \Magento\Framework\App\Filesystem\DirectoryList $directory_list,
        \Vendor\Test\Helper\Data $helper,
        \Magento\Framework\App\ResourceConnection $resourceConnection,
        array $data = []
    ) {
        $this->_rateResultFactory = $rateResultFactory;
        $this->_rateMethodFactory = $rateMethodFactory;
        $this->directory_list = $directory_list;
        $this->cart = $cart;
        $this->helper = $helper;
        $this->resourceConnection = $resourceConnection;
    }

    public function getAllowedMethods()
    {
        $group_info=$this->helper->getAllMethods();
        $carriers=array();
        foreach ($group_info as &$carrier) {
            $carriers[$group_info['label']]=$group_info['label'];
        }
        return $carriers;
    }

    public function afterCollectRates()
    {
        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $customerSession = $objectManager->create('Magento\Customer\Model\Session');

        if ($customerSession->isLoggedIn()) {
            $gid=$customerSession->getCustomerGroupId();
            $group_info=$this->helper->getGroupData($gid);
        }

        $err = null;
        $this->_result = $this->_rateResultFactory->create();
        $method = $this->_rateMethodFactory->create();
        $method->setCarrier('Shipping');
        $method->setCarrierTitle('sdtgd');
        $method->setMethod('Shipping');
        $method->setMethodTitle('dryfyu');
        $amount=0;

        return $this->getResult();
    }

    public function getResult()
    {
        return $this->_result;
    }

}

?>

But i got the below error. What is issue in this code ?

s:1150:"Fatal Error: 'Uncaught Error: Call to undefined method Magento\Shipping\Model\Rate\Result::getResult() in /var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/Quote/Address.php:995
Stack trace:
#0 /var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/Quote/Address.php(936): Magento\Quote\Model\Quote\Address->requestShippingRates()
#1 /var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/Quote/Address/Total/Shipping.php(161): Magento\Quote\Model\Quote\Address->collectShippingRates()
#2 /var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/Quote/TotalsCollector.php(265): Magento\Quote\Model\Quote\Address\Total\Shipping->collect(Object(Magento\Quote\Model\Quote), Object(Magento\Quote\Model\ShippingAssignment), Object(Magento\Quote\Model\Quote\Address\Total))
#3 /var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/ShippingMethodManagement.php(273): Magento\Quote\Model\Quote\TotalsCollector->collectAddressTotals(Objec' in '/var/www/html/magento/m2/mage2/vendor/magento/module-quote/Model/Quote/Address.php' on line 995";

Answer 1

The collectRates method should return the Magento\Quote\Model\Quote\Address\RateCollectorInterface:

interface RateCollectorInterface
{
    /**
     * @param RateRequest $request
     * @return $this
     */
    public function collectRates(RateRequest $request);
}

Check is your result implements this interface and it is the instance of Magento\Shipping\Model\Rate. I think that your plugin returns result, but it should return the rate.

Update:

I have tried to check your plug-in, and have found that original method result is instance of the Magento\Shipping\Model\Shipping with the result inside:

So your plugin should return the Shipping (I think this is a shipping method instance).