use struct for timer and use generic callback

include/devices.h

@@ -137,8 +137,8 @@ namespace Devices {
         Display::brightness.registerCallback([]{
             brightnessMqtt->updateState(Display::brightness);
         });
-        Display::Timer::remainingCallback = [](int8 current){
+        Display::Timer::timer.registerCallback([](int8 current){
             timerRemainingMqtt->updateState(to_string(current).c_str());
-        };
+        });
     }
 }

include/display.h

@@ -101,13 +101,38 @@ namespace Display {
     } brightness;
 
     namespace Timer {
-        int8 timer = 0, current = 0;
+        typedef void (*remaining_callback_t)(int8);
-        void (*remainingCallback)(int8);
+        struct : public CallbackAware<remaining_callback_t> {
+
+            void operator=(int8 value) {
+                initial = remaining = value;
+            }
+
+            operator int8() {
+                return remaining;
+            }
+
+            void start() {
+                remaining = initial + 1;
+            }
+
+            void decrease() {
+                remaining--;
+                if (callback) callback(remaining);
+            }
+
+            bool atBeginning() {
+                return initial == remaining;
+            }
+        
+        private:
+            int8 initial = 0, remaining = 0;
+        } timer;
 
         Task tDisplayTimer(SECONDS(10), TASK_ONCE + 1, 
             []{
-                if (current >= 0) {
+                if (timer >= 0) {
-                    clockDisplay.print(current, DEC);
+                    clockDisplay.print(timer, DEC);
                     clockDisplay.writeDisplay();
                 }
             }, &ts, false,
@@ -123,18 +148,17 @@ namespace Display {
         Task tTimer(MINUTES(1), TASK_FOREVER, 
             []{
                 static constexpr uint8 threshold = 16;
-                current--;
+                timer.decrease();
-                if (current == timer) {
+                if (timer.atBeginning()) {
                     if (timer <= threshold) tDisplayTimer.setIterations(TASK_FOREVER);
                     tDisplayTimer.restart();
-                } else if (current == threshold) {
+                } else if (timer == threshold) {
                     tDisplayTimer.setIterations(TASK_FOREVER);
                     tDisplayTimer.restart();
                 }
-                if (remainingCallback) remainingCallback(current);
             }, &ts, false, 
             []{
-                current = timer+1;
+                timer.start();
                 return true;
             },
             []{