turn POPO into callable to allow more flexibility during transitions

src/Fsm.php

@@ -74,9 +74,10 @@ public function getCurrentState() : string
      * Process action and return new state after transition.
      *
      * @param  string $action
+     * @param  mixed $payload
      * @return string
      */
-    public function process(string $action) : string
+    public function process(string $action, $payload = null) : string
     {
         if (!$this->isActionValid($action)) {
             throw new InvalidActionException(
@@ -86,7 +87,11 @@ public function process(string $action) : string
 
         $transition = $this->transitions[$this->state][$action];
 
-        $this->stateful_object->setState($transition->to_state);
+        if (is_callable($transition)) {
+            $transition($this->stateful_object, $payload);
+        } else {
+            $this->stateful_object->setState($transition->to_state);
+        }
 
         return $this->state = $transition->to_state;
     }

src/Transition.php

@@ -3,7 +3,7 @@
 namespace Sofa\StateMachine;
 
 /**
- * simple POPO to handle State Machine transitions
+ * Simple POPO to handle State Machine transitions
  */
 class Transition
 {
@@ -25,4 +25,13 @@ public static function make(string $from_state, string $action, string $to_state
     {
         return new static($from_state, $action, $to_state);
     }
+
+    // Extend this class and implement __invoke method to have more flexibility in your transition.
+    //
+    // public function __invoke(StateMachineInterface $stateful_object, $payload)
+    // {
+    //     // do more here if necessary
+    //
+    //     $stateful_object->setState($this->to_state);
+    // }
 }