Adding some simple middleware helpers

src/Middleware.php

@@ -4,7 +4,6 @@ namespace GuzzleHttp;
 use GuzzleHttp\Cookie\CookieJarInterface;
 use GuzzleHttp\Exception\ClientException;
 use GuzzleHttp\Exception\ServerException;
-use GuzzleHttp\Promise\RejectedPromise;
 use GuzzleHttp\Psr7;
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
@@ -228,4 +227,38 @@ final class Middleware
             };
         };
     }
+
+    /**
+     * Middleware that applies a map function to the request before passing to
+     * the next handler.
+     *
+     * @param callable $fn Function that accepts a RequestInterface and returns
+     *                     a RequestInterface.
+     * @return callable
+     */
+    public static function mapRequest(callable $fn)
+    {
+        return function (callable $handler) use ($fn) {
+            return function ($request, array $options) use ($handler, $fn) {
+                return $handler($fn($request), $options);
+            };
+        };
+    }
+
+    /**
+     * Middleware that applies a map function to the resolved promise's
+     * response.
+     *
+     * @param callable $fn Function that accepts a ResponseInterface and
+     *                     returns a ResponseInterface.
+     * @return callable
+     */
+    public static function mapResponse(callable $fn)
+    {
+        return function (callable $handler) use ($fn) {
+            return function ($request, array $options) use ($handler, $fn) {
+                return $handler($request, $options)->then($fn);
+            };
+        };
+    }
 }

tests/MiddlewareTest.php

@@ -14,6 +14,7 @@ use GuzzleHttp\Psr7\FnStream;
 use GuzzleHttp\Psr7\Request;
 use GuzzleHttp\Psr7\Response;
 use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
 
 class MiddlewareTest extends \PHPUnit_Framework_TestCase
 {
@@ -252,4 +253,34 @@ class MiddlewareTest extends \PHPUnit_Framework_TestCase
         $response = $p->wait();
         $this->assertEquals(200, $response->getStatusCode());
     }
+
+    public function testMapsRequest()
+    {
+        $h = new MockHandler([
+            function (RequestInterface $request, array $options) {
+                $this->assertEquals('foo', $request->getHeader('Bar'));
+                return new Response(200);
+            }
+        ]);
+        $stack = new HandlerStack($h);
+        $stack->push(Middleware::mapRequest(function (RequestInterface $request) {
+            return $request->withHeader('Bar', 'foo');
+        }));
+        $comp = $stack->resolve();
+        $p = $comp(new Request('PUT', 'http://www.google.com'), []);
+        $this->assertInstanceOf('GuzzleHttp\Promise\FulfilledPromise', $p);
+    }
+
+    public function testMapsResponse()
+    {
+        $h = new MockHandler([new Response(200)]);
+        $stack = new HandlerStack($h);
+        $stack->push(Middleware::mapResponse(function (ResponseInterface $response) {
+            return $response->withHeader('Bar', 'foo');
+        }));
+        $comp = $stack->resolve();
+        $p = $comp(new Request('PUT', 'http://www.google.com'), []);
+        $this->assertInstanceOf('GuzzleHttp\Promise\FulfilledPromise', $p);
+        $this->assertEquals('foo', $p->wait()->getHeader('Bar'));
+    }
 }