add callback hook for building mailable data

src/Illuminate/Mail/Mailable.php

@@ -132,6 +132,13 @@ class Mailable implements MailableContract, Renderable
      */
     public $callbacks = [];
 
+    /**
+     * The callback that should be invoked while building the view data.
+     *
+     * @var callable
+     */
+    public static $viewDataCallback;
+
     /**
      * Send the message using the given mailer.
      *
@@ -263,6 +270,10 @@ public function buildViewData()
     {
         $data = $this->viewData;
 
+        if (static::$viewDataCallback) {
+            $data = array_merge($data, call_user_func(static::$viewDataCallback, $this));
+        }
+
         foreach ((new ReflectionClass($this))->getProperties(ReflectionProperty::IS_PUBLIC) as $property) {
             if ($property->getDeclaringClass()->getName() !== self::class) {
                 $data[$property->getName()] = $property->getValue($this);
@@ -793,6 +804,17 @@ public function withSwiftMessage($callback)
         return $this;
     }
 
+    /**
+     * Register a callback to be called while building the view data.
+     *
+     * @param  callable  $callback
+     * @return void
+     */
+    public static function buildViewDataUsing(callable $callback)
+    {
+        static::$viewDataCallback = $callback;
+    }
+
     /**
      * Dynamically bind parameters to the message.
      *