Created default delegate that returns a JSON response when accepted type is json

module/Core/config/dependencies.config.php

@@ -6,8 +6,10 @@ use Shlinkio\Shlink\Common\Service\PreviewGenerator;
 use Shlinkio\Shlink\Core\Action;
 use Shlinkio\Shlink\Core\Middleware;
 use Shlinkio\Shlink\Core\Options;
+use Shlinkio\Shlink\Core\Response\NotFoundDelegate;
 use Shlinkio\Shlink\Core\Service;
 use Zend\Expressive\Router\RouterInterface;
+use Zend\Expressive\Template\TemplateRendererInterface;
 use Zend\ServiceManager\AbstractFactory\ConfigAbstractFactory;
 
 return [
@@ -15,6 +17,7 @@ return [
     'dependencies' => [
         'factories' => [
             Options\AppOptions::class => Options\AppOptionsFactory::class,
+            NotFoundDelegate::class => ConfigAbstractFactory::class,
 
             // Services
             Service\UrlShortener::class => ConfigAbstractFactory::class,
@@ -29,9 +32,15 @@ return [
             Action\PreviewAction::class => ConfigAbstractFactory::class,
             Middleware\QrCodeCacheMiddleware::class => ConfigAbstractFactory::class,
         ],
+
+        'aliases' => [
+            'Zend\Expressive\Delegate\DefaultDelegate' => NotFoundDelegate::class,
+        ],
     ],
 
     ConfigAbstractFactory::class => [
+        NotFoundDelegate::class => [TemplateRendererInterface::class],
+
         // Services
         Service\UrlShortener::class => ['httpClient', 'em', Cache::class, 'config.url_shortener.shortcode_chars'],
         Service\VisitsTracker::class => ['em'],

module/Core/src/Response/NotFoundDelegate.php

@@ -0,0 +1,54 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Response;
+
+use Fig\Http\Message\StatusCodeInterface;
+use Interop\Http\ServerMiddleware\DelegateInterface;
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Zend\Diactoros\Response;
+use Zend\Expressive\Template\TemplateRendererInterface;
+
+class NotFoundDelegate implements DelegateInterface
+{
+    /**
+     * @var TemplateRendererInterface
+     */
+    private $renderer;
+    /**
+     * @var string
+     */
+    private $template;
+
+    public function __construct(TemplateRendererInterface $renderer, string $template = 'ShlinkCore::error/404')
+    {
+        $this->renderer = $renderer;
+        $this->template = $template;
+    }
+
+    /**
+     * Dispatch the next available middleware and return the response.
+     *
+     * @param ServerRequestInterface $request
+     *
+     * @return ResponseInterface
+     * @throws \InvalidArgumentException
+     */
+    public function process(ServerRequestInterface $request): ResponseInterface
+    {
+        $accepts = explode(',', $request->getHeaderLine('Accept'));
+        $accept = array_shift($accepts);
+        $status = StatusCodeInterface::STATUS_NOT_FOUND;
+
+        // If the first accepted type is json, return a json response
+        if (in_array($accept, ['application/json', 'text/json', 'application/x-json'], true)) {
+            return new Response\JsonResponse([
+                'error' => 'NOT_FOUND',
+                'message' => 'Not found',
+            ], $status);
+        }
+
+        return new Response\HtmlResponse($this->renderer->render($this->template, ['request' => $request]), $status);
+    }
+}

module/Rest/src/ErrorHandler/JsonErrorResponseGenerator.php

@@ -18,6 +18,7 @@ class JsonErrorResponseGenerator implements ErrorResponseGeneratorInterface, Sta
      * @param Request $request
      * @param Response $response
      * @return Response
+     * @throws \InvalidArgumentException
      */
     public function __invoke($e, Request $request, Response $response)
     {
@@ -31,11 +32,7 @@ class JsonErrorResponseGenerator implements ErrorResponseGeneratorInterface, Sta
         ], $status);
     }
 
-    /**
+    protected function responsePhraseToCode(string $responsePhrase): string
-     * @param string $responsePhrase
-     * @return string
-     */
-    protected function responsePhraseToCode($responsePhrase): string
     {
         return strtoupper(str_replace(' ', '_', $responsePhrase));
     }