Quick method for setting a Payment Source on a Commerce Order

BaseController.php

<?php namespace modules\mymodule\controllers;

use Craft;
use craft\web\Controller;

use yii\web\ForbiddenHttpException;

/**
 * The Base Controller for all Module Controllers
 *
 * @author oof. Studio <hello@oof.studio>
 * @since 1.0
 */
abstract class BaseController extends Controller
{
    /**
     * Returns an error based on the content type that the client accepts.
     * 
     * @param string $message
     * @param array|null $params Route params to pass back to the template.
     */
    protected function _sendErrorResponse(string $message, array $params = [])
    {
        if (Craft::$app->getRequest()->getAcceptsJson()) {
            return $this->asJson([
                'success' => false,
                'message' => $message
            ]);
        }

        Craft::$app->getSession()->setError($message);

        if ($params) {
            Craft::$app->getUrlManager()->setRouteParams($params);
        }

        return null;
    }

    /**
     * Returns a successful response based on the content type that the client accepts.
     * 
     * @param string $message
     * @param mixed $redirectParams Data to make available when rendering the redirect string template. Can be an object or array.
     */
    protected function _sendSuccessResponse(string $message, $redirectParams = null)
    {
        if (Craft::$app->getRequest()->getAcceptsJson()) {
            return $this->asJson([
                'success' => true,
                'message' => $message,
                'data' => $redirectParams
            ]);
        }

        Craft::$app->getSession()->setNotice($message);

        return $this->redirectToPostedUrl($redirectParams);
    }
}

OrdersController.php

<?php namespace modules\mymodule\controllers;

use Craft;

use craft\commerce\Plugin as Commerce;

/**
 * Orders Controllers
 * 
 * Supplemental actions for modifying orders.
 *
 * @author oof. Studio <hello@oof.studio>
 * @since 2.0.0
 */
class OrdersController extends BaseController
{
    /**
     * Updates or un-sets the payment method for the current cart.
     *
     * @return mixed
     */
    public function actionUpdatePaymentSource()
    {
        $this->requirePostRequest();
        $this->requireLogin();

        $currentUser = Craft::$app->getUser()->getIdentity();
        $cart = Commerce::getInstance()->getCarts()->getCart();

        $paymentSourceId = Craft::$app->getRequest()->getBodyParam('paymentSourceId');

        // Cast to integer, if one was provided:
        if ($paymentSourceId) {
            $cart->paymentSourceId = (int)$paymentSourceId;
        } else {
            $cart->paymentSourceId = null;
        }

        if (!Craft::$app->getElements()->saveElement($cart)) {
            return $this->_sendErrorResponse(Craft::t('mymodule', 'Could not set payment source.'), compact('cart'));
        }

        // The message can be a little more useful when we've just *removed* a payment source:
        if ($cart->paymentSourceId) {
            return $this->_sendSuccessResponse(Craft::t('mymodule', 'Payment source selected.'), $cart);
        }

        return $this->_sendSuccessResponse(Craft::t('mymodule', 'Please provide details for your new payment method.'), $cart);
    }
}