Add ability to trigger JsonResponseHandler for AJAX only

src/Damnit/Handler/Handler.php

@@ -19,8 +19,10 @@ abstract class Handler implements HandlerInterface
      * Return constants that can be returned from Handler::handle
      * to message the handler walker.
      */
-    const LAST_HANDLER = 0x10;
-    const QUIT         = 0x20;
+    const DONE         = 0x10; // returning this is optional, only exists for
+                               // semantic purposes
+    const LAST_HANDLER = 0x20;
+    const QUIT         = 0x30;
 
     /**
      * @var Damnit\Run

src/Damnit/Handler/JsonResponseHandler.php

@@ -19,6 +19,11 @@ class JsonResponseHandler extends Handler
      */
     private $returnFrames = false;
 
+    /**
+     * @var bool
+     */
+    private $onlyForAjaxRequests = false;
+
     /**
      * @param  bool|null $returnFrames
      * @return null|bool
@@ -32,11 +37,40 @@ class JsonResponseHandler extends Handler
         $this->returnFrames = (bool) $returnFrames;
     }
 
+    /**
+     * @param  bool|null $onlyForAjaxRequests
+     * @return null|bool
+     */
+    public function onlyForAjaxRequests($onlyForAjaxRequests = null)
+    {
+        if(func_num_args() == 0) {
+            return $this->onlyForAjaxRequests;
+        }
+
+        $this->onlyForAjaxRequests = (bool) $onlyForAjaxRequests;
+    }
+
+    /**
+     * Check, if possible, that this execution was triggered by an AJAX request.
+     * @param bool
+     */
+    private function isAjaxRequest()
+    {
+        return (
+            !empty($_SERVER['HTTP_X_REQUESTED_WITH'])
+            && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest')
+        ;
+    }
+
     /**
      * @return int
      */
     public function handle()
     {
+        if($this->onlyForAjaxRequests() && !$this->isAjaxRequest()) {
+            return Handler::DONE;
+        }
+
         $exception = $this->getException();
 
         $response = array(

src/Damnit/Handler/PrettyPageHandler.php

@@ -33,7 +33,7 @@ class PrettyPageHandler extends Handler
         // Check conditions for outputting HTML:
         // @todo: make this more robust
         if(php_sapi_name() === 'cli') {
-            return;
+            return Handler::DONE;
         }
 
         // Get the 'pretty-template.php' template file