Created middleware used with short codes creation actions to handle content negotiation

module/Rest/config/dependencies.config.php

@@ -36,6 +36,7 @@ return [
             Middleware\CrossDomainMiddleware::class => InvokableFactory::class,
             Middleware\PathVersionMiddleware::class => InvokableFactory::class,
             Middleware\CheckAuthenticationMiddleware::class => ConfigAbstractFactory::class,
+            Middleware\ShortCode\CreateShortCodeContentNegotiationMiddleware::class => InvokableFactory::class,
         ],
     ],
 

module/Rest/config/routes.config.php

@@ -1,6 +1,8 @@
 <?php
 declare(strict_types=1);
 
+namespace Shlinkio\Shlink\Rest;
+
 use Shlinkio\Shlink\Rest\Action;
 
 return [
@@ -9,8 +11,12 @@ return [
         Action\AuthenticateAction::getRouteDef(),
 
         // Short codes
-        Action\ShortCode\CreateShortCodeAction::getRouteDef(),
+        Action\ShortCode\CreateShortCodeAction::getRouteDef([
-        Action\ShortCode\SingleStepCreateShortCodeAction::getRouteDef(),
+            Middleware\ShortCode\CreateShortCodeContentNegotiationMiddleware::class,
+        ]),
+        Action\ShortCode\SingleStepCreateShortCodeAction::getRouteDef([
+            Middleware\ShortCode\CreateShortCodeContentNegotiationMiddleware::class,
+        ]),
         Action\ShortCode\EditShortCodeAction::getRouteDef(),
         Action\ShortCode\ResolveUrlAction::getRouteDef(),
         Action\ShortCode\ListShortCodesAction::getRouteDef(),

module/Rest/src/Action/ShortCode/AbstractCreateShortCodeAction.php

@@ -77,7 +77,6 @@ abstract class AbstractCreateShortCodeAction extends AbstractRestAction
                                    ->withScheme($this->domainConfig['schema'])
                                    ->withHost($this->domainConfig['hostname']);
 
-            // TODO Make response to be generated based on Accept header
             return new JsonResponse([
                 'longUrl' => (string) $longUrl,
                 'shortUrl' => (string) $shortUrl,

module/Rest/src/Middleware/ShortCode/CreateShortCodeContentNegotiationMiddleware.php

@@ -0,0 +1,47 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Rest\Middleware\ShortCode;
+
+use Psr\Http\Message\ResponseInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Server\MiddlewareInterface;
+use Psr\Http\Server\RequestHandlerInterface;
+use Zend\Diactoros\Response;
+use Zend\Diactoros\Response\JsonResponse;
+
+class CreateShortCodeContentNegotiationMiddleware implements MiddlewareInterface
+{
+    private const PLAIN_TEXT = 'text';
+    private const JSON = 'json';
+
+    /**
+     * Process an incoming server request and return a response, optionally delegating
+     * response creation to a handler.
+     * @throws \RuntimeException
+     * @throws \InvalidArgumentException
+     */
+    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
+    {
+        /** @var JsonResponse $response */
+        $response = $handler->handle($request);
+        $acceptedType = $this->determineAcceptedType($request);
+        if ($acceptedType === self::JSON) {
+            return $response;
+        }
+
+        // If requested, return a plain text response containing the short URL only
+        $resp = (new Response())->withHeader('Content-Type', 'text/plain');
+        $body = $resp->getBody();
+        $body->write($response->getPayload()['shortUrl'] ?? '');
+        $body->rewind();
+        return $resp;
+    }
+
+    private function determineAcceptedType(ServerRequestInterface $request): string
+    {
+        $accepts = \explode(',', $request->getHeaderLine('Accept'));
+        $accept = \strtolower(\array_shift($accepts));
+        return \strpos($accept, 'text/plain') !== false ? self::PLAIN_TEXT : self::JSON;
+    }
+}