--- /dev/null
+<?php
+
+namespace Drupal\Core\EventSubscriber;
+
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;
+use Symfony\Component\HttpKernel\KernelEvents;
+
+/**
+ * Utility base class for exception subscribers.
+ *
+ * A subscriber may extend this class and implement getHandledFormats() to
+ * indicate which request formats it will respond to. Then implement an on*()
+ * method for any error code (HTTP response code) that should be handled. For
+ * example, to handle a specific error code like 404 Not Found messages add the
+ * method:
+ *
+ * @code
+ * public function on404(GetResponseForExceptionEvent $event) {}
+ * @endcode
+ *
+ * To implement a fallback for the entire 4xx class of codes, implement the
+ * method:
+ *
+ * @code
+ * public function on4xx(GetResponseForExceptionEvent $event) {}
+ * @endcode
+ *
+ * That method should then call $event->setResponse() to set the response object
+ * for the exception. Alternatively, it may opt not to do so and then other
+ * listeners will have the opportunity to handle the exception.
+ *
+ * Note: Core provides several important exception listeners by default. In most
+ * cases, setting the priority of a contrib listener to the default of 0 will
+ * do what you expect and handle the exceptions you'd expect it to handle.
+ * If a custom priority is set, be aware of the following core-registered
+ * listeners.
+ *
+ * - Fast404ExceptionHtmlSubscriber: 200. This subscriber will return a
+ * minimalist, high-performance 404 page for HTML requests. It is not
+ * recommended to have a priority higher than this one as it will only slow
+ * down that use case.
+ * - ExceptionLoggingSubscriber: 50. This subscriber logs all exceptions but
+ * does not handle them. Do not register a listener with a higher priority
+ * unless you want exceptions to not get logged, which makes debugging more
+ * difficult.
+ * - DefaultExceptionSubscriber: -256. The subscriber of last resort, this will
+ * provide generic handling for any exception. A listener with a lower
+ * priority will never get called.
+ *
+ * All other core-provided exception handlers have negative priorities so most
+ * module-provided listeners will naturally take precedence over them.
+ */
+abstract class HttpExceptionSubscriberBase implements EventSubscriberInterface {
+
+ /**
+ * Specifies the request formats this subscriber will respond to.
+ *
+ * @return array
+ * An indexed array of the format machine names that this subscriber will
+ * attempt to process, such as "html" or "json". Returning an empty array
+ * will apply to all formats.
+ *
+ * @see \Symfony\Component\HttpFoundation\Request
+ */
+ abstract protected function getHandledFormats();
+
+ /**
+ * Specifies the priority of all listeners in this class.
+ *
+ * The default priority is 1, which is very low. To have listeners that have
+ * a "first attempt" at handling exceptions return a higher priority.
+ *
+ * @return int
+ * The event priority of this subscriber.
+ */
+ protected static function getPriority() {
+ return 0;
+ }
+
+ /**
+ * Handles errors for this subscriber.
+ *
+ * @param \Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent $event
+ * The event to process.
+ */
+ public function onException(GetResponseForExceptionEvent $event) {
+ $exception = $event->getException();
+
+ // Make the exception available for example when rendering a block.
+ $request = $event->getRequest();
+ $request->attributes->set('exception', $exception);
+
+ $handled_formats = $this->getHandledFormats();
+
+ $format = $request->query->get(MainContentViewSubscriber::WRAPPER_FORMAT, $request->getRequestFormat());
+
+ if ($exception instanceof HttpExceptionInterface && (empty($handled_formats) || in_array($format, $handled_formats))) {
+ $method = 'on' . $exception->getStatusCode();
+ // Keep just the leading number of the status code to produce either a
+ // on400 or a 500 method callback.
+ $method_fallback = 'on' . substr($exception->getStatusCode(), 0, 1) . 'xx';
+ // We want to allow the method to be called and still not set a response
+ // if it has additional filtering logic to determine when it will apply.
+ // It is therefore the method's responsibility to set the response on the
+ // event if appropriate.
+ if (method_exists($this, $method)) {
+ $this->$method($event);
+ }
+ elseif (method_exists($this, $method_fallback)) {
+ $this->$method_fallback($event);
+ }
+ }
+ }
+
+ /**
+ * Registers the methods in this class that should be listeners.
+ *
+ * @return array
+ * An array of event listener definitions.
+ */
+ public static function getSubscribedEvents() {
+ $events[KernelEvents::EXCEPTION][] = ['onException', static::getPriority()];
+ return $events;
+ }
+
+}