Improve loop with fps control

include/app/app.h

@@ -252,26 +252,35 @@ struct App {
     current_scene_id = scene_pool.acquire(loading_scene);
   }
 
-  void loop() {
-    const uint64_t TARGET_NS = 1e9 / 60;
+  void set_fps(float fps) {
+    this->fps = fps;
+    target_frame_time = 1e9 / fps;
+  }
 
-    uint64_t time_a = window.get_time();
-    uint64_t time_b = window.get_time();
+  uint64_t get_fps() const {
+    return fps;
+  }
+
+  void loop() {
+    uint64_t frame_start = window.get_time();
+    uint64_t frame_end = window.get_time();
+    uint64_t previous_frame_end = 0;
 
     while (running) {
-      time_a = window.get_time();
-      uint64_t frame_time = time_a - time_b;
+      frame_start = window.get_time();
+      uint64_t frame_time = frame_start - frame_end;
 
-      if (frame_time <= TARGET_NS) {
+      if (frame_time <= target_frame_time) {
         struct timespec req;
         req.tv_sec = 0;
-        req.tv_nsec = TARGET_NS - frame_time;
+        req.tv_nsec = target_frame_time - frame_time;
         nanosleep(&req, nullptr);
       }
 
-      time_b = window.get_time();
+      previous_frame_end = frame_end;
+      frame_end = window.get_time();
 
-      uint64_t dt = window.delta_time();
+      float dt = frame_end - previous_frame_end;
 
       Scene *scene = scene_pool[current_scene_id];
 
@@ -329,6 +338,10 @@ struct App {
       window.clear();
     }
   }
+
+private:
+  uint64_t fps = 60;
+  uint64_t target_frame_time = 1e9 / fps;
 };
 } // namespace app