diff --git a/composer.json b/composer.json
index 507d5d2..46a93c1 100644
--- a/composer.json
+++ b/composer.json
@@ -9,11 +9,11 @@
         "ext-mbstring" : "*",
         "ext-ctype"    : "*",
         "sabre/event"  : ">=1.0.0,<4.0.0",
-        "sabre/uri"    : "~1.0"
+        "sabre/uri"    : "^1.0.1"
     },
     "require-dev" : {
         "phpunit/phpunit" : "~4.3",
-        "sabre/cs" : "~0.0.1"
+        "sabre/cs" : "~0.0.8"
     },
     "suggest" : {
         "ext-curl" : " to make http requests with the Client class"
diff --git a/lib/Message.php b/lib/Message.php
index 45bd183..cead5ee 100644
--- a/lib/Message.php
+++ b/lib/Message.php
@@ -16,9 +16,9 @@ abstract class Message implements MessageInterface {
     /**
      * Request body
      *
-     * This should be a stream resource
+     * This should be a stream resource, string or a callback writing the body to php://output
      *
-     * @var resource
+     * @var resource|string|callable
      */
     protected $body;
 
@@ -47,6 +47,9 @@ abstract class Message implements MessageInterface {
     function getBodyAsStream() {
 
         $body = $this->getBody();
+        if (is_callable($body)) {
+            $body = $this->captureCallbackOutput($body);
+        }
         if (is_string($body) || is_null($body)) {
             $stream = fopen('php://temp', 'r+');
             fwrite($stream, $body);
@@ -74,6 +77,9 @@ function getBodyAsString() {
         if (is_null($body)) {
             return '';
         }
+        if (is_callable($body)) {
+            return $this->captureCallbackOutput($body);
+        }
         $contentLength = $this->getHeader('Content-Length');
         if (is_int($contentLength) || ctype_digit($contentLength)) {
             return stream_get_contents($body, $contentLength);
@@ -85,9 +91,9 @@ function getBodyAsString() {
     /**
      * Returns the message body, as it's internal representation.
      *
-     * This could be either a string or a stream.
+     * This could be either a string, a stream or a callback writing the body to php://output.
      *
-     * @return resource|string
+     * @return resource|string|callable
      */
     function getBody() {
 
@@ -96,9 +102,9 @@ function getBody() {
     }
 
     /**
-     * Replaces the body resource with a new stream or string.
+     * Replaces the body resource with a new stream, string or a callback writing the body to php://output.
      *
-     * @param resource|string $body
+     * @param resource|string|callable $body
      */
     function setBody($body) {
 
@@ -311,4 +317,17 @@ function getHttpVersion() {
         return $this->httpVersion;
 
     }
+
+    /**
+     * Runs given callback and captures data sent to php://output stream.
+     *
+     * @param callable $callback
+     * @return string
+     */
+    private function captureCallbackOutput($callback)
+    {
+        ob_start();
+        $callback();
+        return ob_get_clean();
+    }
 }
diff --git a/lib/MessageInterface.php b/lib/MessageInterface.php
index df55beb..5c8877f 100644
--- a/lib/MessageInterface.php
+++ b/lib/MessageInterface.php
@@ -35,16 +35,16 @@ function getBodyAsString();
     /**
      * Returns the message body, as it's internal representation.
      *
-     * This could be either a string or a stream.
+     * This could be either a string, a stream or a callback writing the body to php://output
      *
-     * @return resource|string
+     * @return resource|string|callable
      */
     function getBody();
 
     /**
      * Updates the body resource with a new stream.
      *
-     * @param resource|string $body
+     * @param resource|string|callable $body
      * @return void
      */
     function setBody($body);
diff --git a/lib/Sapi.php b/lib/Sapi.php
index 054380e..f56cad3 100644
--- a/lib/Sapi.php
+++ b/lib/Sapi.php
@@ -71,6 +71,11 @@ static function sendResponse(ResponseInterface $response) {
         $body = $response->getBody();
         if (is_null($body)) return;
 
+        if (is_callable($body)) {
+            $body();
+            return;
+        }
+
         $contentLength = $response->getHeader('Content-Length');
         if ($contentLength !== null) {
             $output = fopen('php://output', 'wb');
diff --git a/tests/HTTP/MessageTest.php b/tests/HTTP/MessageTest.php
index cb5aadc..078c113 100644
--- a/tests/HTTP/MessageTest.php
+++ b/tests/HTTP/MessageTest.php
@@ -42,13 +42,50 @@ function testStringBody() {
 
     }
 
+    function testCallbackBodyAsString() {
+
+        $body = $this->createCallback('foo');
+
+        $message = new MessageMock();
+        $message->setBody($body);
+
+        $string = $message->getBodyAsString();
+
+        $this->assertSame('foo', $string);
+
+    }
+
+    function testCallbackBodyAsStream() {
+
+        $body = $this->createCallback('foo');
+
+        $message = new MessageMock();
+        $message->setBody($body);
+
+        $stream = $message->getBodyAsStream();
+
+        $this->assertSame('foo', stream_get_contents($stream));
+
+    }
+
+    function testGetBodyWhenCallback() {
+
+        $callback = $this->createCallback('foo');
+
+        $message = new MessageMock();
+        $message->setBody($callback);
+
+        $this->assertSame($callback, $message->getBody());
+
+    }
+
     /**
      * It's possible that streams contains more data than the Content-Length.
      *
      * The request object should make sure to never emit more than
      * Content-Length, if Content-Length is set.
      *
-     * This is in particular useful when respoding to range requests with
+     * This is in particular useful when responding to range requests with
      * streams that represent files on the filesystem, as it's possible to just
      * seek the stream to a certain point, set the content-length and let the
      * request object do the rest.
@@ -208,11 +245,11 @@ function testMultipleHeaders() {
         $message->addHeader('A', '2');
 
         $this->assertEquals(
-            "1,2",
+            '1,2',
             $message->getHeader('A')
         );
         $this->assertEquals(
-            "1,2",
+            '1,2',
             $message->getHeader('a')
         );
 
@@ -241,6 +278,17 @@ function testHasHeaders() {
 
     }
 
+    /**
+     * @param string $content
+     * @return \Closure Returns a callback printing $content to php://output stream
+     */
+    private function createCallback($content)
+    {
+        return function() use ($content) {
+            echo $content;
+        };
+    }
+
 }
 
 class MessageMock extends Message { }
diff --git a/tests/HTTP/SapiTest.php b/tests/HTTP/SapiTest.php
index 158ce21..935b8f6 100644
--- a/tests/HTTP/SapiTest.php
+++ b/tests/HTTP/SapiTest.php
@@ -164,4 +164,25 @@ function testSendLimitedByContentLengthStream() {
 
     }
 
+    /**
+     * @runInSeparateProcess
+     * @depends testSend
+     */
+    function testSendWorksWithCallbackAsBody() {
+        $response = new Response(200, [], function() {
+            $fd = fopen('php://output', 'r+');
+            fwrite($fd, 'foo');
+            fclose($fd);
+        });
+
+        ob_start();
+
+        Sapi::sendResponse($response);
+
+        $result = ob_get_clean();
+
+        $this->assertEquals('foo', $result);
+
+    }
+
 }