by Primsi: basic module structure.

commerce_datatrans.info.yml

+name: Commerce Datatrans
+type: module
+description: 'Provides payment gateway for Datatrans.'
+package: Commerce
+core: 8.x
+dependencies:
+  - commerce:commerce_payment

commerce_datatrans.routing.yml

+commerce_datatrans.dummy_redirect_post:
+  path: 'commerce_datatrans/dummy_redirect_post'
+  defaults:
+    _controller: '\Drupal\commerce_datatrans\Controller\DummyRedirectController::post'
+  options:
+    no_cache: TRUE
+  requirements:
+    _access: 'TRUE'
+commerce_datatrans.dummy_redirect_302:
+  path: 'commerce_datatrans/dummy_redirect_302'
+  defaults:
+    _controller: '\Drupal\commerce_datatrans\Controller\DummyRedirectController::on302'
+  options:
+    no_cache: TRUE
+  requirements:
+    _access: 'TRUE'

config/schema/commerce_datatrans.schema.yml

+commerce_payment.commerce_payment_gateway.plugin.datatrans:
+  type: commerce_payment_gateway_configuration
+  mapping:
+    redirect_method:
+      type: string
+      label: 'Redirect method'

src/Controller/DummyRedirectController.php

+<?php
+
+namespace Drupal\commerce_datatrans\Controller;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Routing\TrustedRedirectResponse;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * This is a dummy controller for mocking an off-site gateway.
+ */
+class DummyRedirectController implements ContainerInjectionInterface {
+
+  /**
+   * The current request.
+   *
+   * @var \Symfony\Component\HttpFoundation\Request
+   */
+  protected $currentRequest;
+
+  /**
+   * Constructs a new DummyRedirectController object.
+   *
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
+   */
+  public function __construct(RequestStack $request_stack) {
+    $this->currentRequest = $request_stack->getCurrentRequest();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('request_stack')
+    );
+  }
+
+  /**
+   * Callback method which accepts POST.
+   *
+   * @throws \Drupal\commerce\Response\NeedsRedirectException
+   */
+  public function post() {
+    $cancel = $this->currentRequest->request->get('cancel');
+    $return = $this->currentRequest->request->get('return');
+    $total = $this->currentRequest->request->get('total');
+
+    if ($total > 20) {
+      return new TrustedRedirectResponse($return);
+    }
+
+    return new TrustedRedirectResponse($cancel);
+  }
+
+  /**
+   * Callback method which reacts to GET from a 302 redirect.
+   *
+   * @throws \Drupal\commerce\Response\NeedsRedirectException
+   */
+  public function on302() {
+    $cancel = $this->currentRequest->query->get('cancel');
+    $return = $this->currentRequest->query->get('return');
+    $total = $this->currentRequest->query->get('total');
+
+    if ($total > 20) {
+      return new TrustedRedirectResponse($return);
+    }
+
+    return new TrustedRedirectResponse($cancel);
+  }
+
+}

src/Plugin/Commerce/PaymentGateway/Datatrans.php

+<?php
+
+namespace Drupal\commerce_datatrans\Plugin\Commerce\PaymentGateway;
+
+use Drupal\commerce_order\Entity\OrderInterface;
+use Drupal\commerce_payment\Plugin\Commerce\PaymentGateway\OffsitePaymentGatewayBase;
+use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Provides the Datatrans payment gateway.
+ *
+ * @CommercePaymentGateway(
+ *   id = "datatrans",
+ *   label = "Datatrans",
+ *   display_label = "Datatrans",
+ *    forms = {
+ *     "offsite-payment" = "Drupal\commerce_datatrans\PluginForm\Datatrans\DatatransForm",
+ *   },
+ *   payment_method_types = {"credit_card"},
+ *   credit_card_types = {
+ *     "amex", "dinersclub", "discover", "jcb", "maestro", "mastercard", "visa",
+ *   },
+ * )
+ */
+class Datatrans extends OffsitePaymentGatewayBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'redirect_method' => 'post',
+    ] + parent::defaultConfiguration();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $form = parent::buildConfigurationForm($form, $form_state);
+
+    // A real gateway would always know which redirect method should be used,
+    // it's made configurable here for test purposes.
+    $form['redirect_method'] = [
+      '#type' => 'radios',
+      '#title' => $this->t('Redirect method'),
+      '#options' => [
+        'get' => $this->t('Redirect via GET (302 header)'),
+        'post' => $this->t('Redirect via POST'),
+      ],
+      '#default_value' => $this->configuration['redirect_method'],
+    ];
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    parent::submitConfigurationForm($form, $form_state);
+    if (!$form_state->getErrors()) {
+      $values = $form_state->getValue($form['#parents']);
+      $this->configuration['redirect_method'] = $values['redirect_method'];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onReturn(OrderInterface $order, Request $request) {
+    // @todo Add examples of request validation.
+    $payment_storage = $this->entityTypeManager->getStorage('commerce_payment');
+    $payment = $payment_storage->create([
+      'state' => 'authorization',
+      'amount' => $order->getTotalPrice(),
+      'payment_gateway' => $this->entityId,
+      'order_id' => $order->id(),
+      'test' => $this->getMode() == 'test',
+      'remote_id' => $request->query->get('txn_id'),
+      'remote_state' => $request->query->get('payment_status'),
+      'authorized' => REQUEST_TIME,
+    ]);
+    $payment->save();
+    drupal_set_message('Payment was processed');
+  }
+
+}

src/PluginForm/OffsiteRedirect/DatatransForm.php

+<?php
+
+namespace Drupal\commerce_datatrans\PluginForm\OffsiteRedirect;
+
+use Drupal\commerce_payment\PluginForm\PaymentOffsiteForm as BasePaymentOffsiteForm;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+
+class PaymentOffsiteForm extends BasePaymentOffsiteForm {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    $form = parent::buildConfigurationForm($form, $form_state);
+
+    /** @var \Drupal\commerce_payment\Entity\PaymentInterface $payment */
+    $payment = $this->entity;
+    /** @var \Drupal\commerce_payment\Plugin\Commerce\PaymentGateway\OffsitePaymentGatewayInterface $payment_gateway_plugin */
+    $payment_gateway_plugin = $payment->getPaymentGateway()->getPlugin();
+    $redirect_method = $payment_gateway_plugin->getConfiguration()['redirect_method'];
+    if ($redirect_method == 'post') {
+      $redirect_url = Url::fromRoute('commerce_datatrans.dummy_redirect_post')->toString();
+    }
+    else {
+      $order = $payment->getOrder();
+      // Gateways that use the GET redirect method usually perform an API call
+      // that prepares the remote payment and provides the actual url to
+      // redirect to. Any params received from that API call that need to be
+      // persisted until later payment creation can be saved in $order->data.
+      // Example: $order->setData('my_gateway', ['test' => '123']), followed
+      // by an $order->save().
+      $redirect_url = Url::fromRoute('commerce_datatrans.dummy_redirect_302', [], ['absolute' => TRUE])->toString();
+    }
+    $data = [
+      'return' => $form['#return_url'],
+      'cancel' => $form['#cancel_url'],
+      'total' => $payment->getAmount()->getNumber(),
+    ];
+
+    return $this->buildRedirectForm($form, $form_state, $redirect_url, $data, $redirect_method);
+  }
+
+}