Magento 2 update invoice state through REST API giving error

As by default megento issue, So we can not make changes in core file, So for this I have make one alternative solution...

I have created one custom invoice interface in my custom module with same API end point : {{magento_api_url}}/V1/invoices/ in webapi.xml file and defined the our custom model with preference in di.xml file and update the invoice state successfully.

Below are the code snippet

Custom Interface

interface InvoiceCustomInterface

{ /** * save api * @param \Magento\Sales\Api\Data\InvoiceInterface $entity Invoice interface * @return \Magento\Sales\Api\Data\InvoiceInterface Invoice interface */

public function save($entity);

}

Webapi.xml

<route url="/V1/invoices/" method="POST">
    <service class="Vendor\Module_Nmae\Api\InvoiceCustomInterface" method="save"/>
    <resources>
        <resource ref="Vendor_Module_Nmae::Module_Nmae_invoice" />
    </resources>
</route>

di.xml

<preference for="Vendor\Module_Nmae\Api\InvoiceCustomInterface" type="Vendor\Module_Nmae\Model\Api\Invoice"/>

Model file

class Invoice implements InvoiceCustomInterface

{ protected $logger;

/**
 * @var InvoiceRepositoryInterface
 */
private $invoiceRepository;

public function __construct(
    LoggerInterface $logger,
    InvoiceRepositoryInterface $invoiceRepository
)
{
    $this->invoiceRepository = $invoiceRepository;
    $this->logger = $logger;
}

/**
 * @inheritdoc
 * @param $entity
 */

public function save($entity)
{
    try {
        $invoiceRepo = $this->invoiceRepository->get($entity->getEntityId());
        $invoiceRepo->setState($entity->getState());
        $this->invoiceRepository->save($invoiceRepo);
    } catch (\Exception $e) {
        $this->logger->info($e->getMessage());
    }
    return $invoiceRepo;
}

}

So this solution will resolved the issue.