ADD mail box for gear device

multi_tasking/gear_device.hpp

@@ -26,20 +26,29 @@
 
 #pragma once
 
+#include "common/constants.hpp"
 #include "mbed.h"
 
 namespace multi_tasking {
 
 class GearDevice {
    public:
-    GearDevice(Callback<void()> cbOnUp,
-               Callback<void()> cbOnDown);  // NOLINT(runtime/references)
+    explicit GearDevice(Mail<uint8_t, 16>* mailBox);  // NOLINT(runtime/references)
 
     // make the class non copyable
     GearDevice(GearDevice&)            = delete;
     GearDevice& operator=(GearDevice&) = delete;
 
     // method called for updating the bike system
+    void onUp();
+    void onDown();
+
+    void sendMail(uint32_t data);
+
+   private:
+    // data members
+    volatile uint8_t _currentGear = bike_computer::kMinGear;
+    Mail<uint8_t, 16>* _mailBox;
 };
 
 }  // namespace multi_tasking