code2.cpp

int demo_modes() {
  static int counter = 0;
  auto increment_counter = [] {
    counter++;
    println("count: {}", counter);
  };

  // Timer ticks every 100ms and prints the latest count
  auto t = RepeatingTimer(Milliseconds(100), increment_counter);

  // Enter the selfish run loop and stop being selfish after 1 second
  CFRunLoopRunInMode(
      /* mode                        */ kSelfishMode,
      /* seconds to run in this mode */ Seconds(1).ToSecondsF(),
      /* returnAfterSourceHandled    */ false);

  return 0;
}

test_code.cpp

struct RepeatingTimer {
  using Callback = std::function<void()>;

  RepeatingTimer(TimeDelta interval, Callback callback)
      : callback_holder_(std::move(callback)), interval_(interval) {
    CFRunLoopTimerContext timerContext = {
        .version = 0,
        .info = &callback_holder_,
        .retain = nullptr,
        .release = nullptr,
        .copyDescription = nullptr,
    };

    timer_ref_ = AdoptCFRef(CFRunLoopTimerCreate(
        /* allocator */ kCFAllocatorDefault,
        /* fireDate  */ CFAbsoluteTimeGetCurrent(),
        /* interval  */ interval_.ToSecondsF(),
        /* flags     */ 0,
        /* order     */ 0,
        /* callout   */ &TimerCallbackTrampoline,
        /* context   */ &timerContext));

    CFRunLoopAddTimer(CFRunLoopGetMain(), timer_ref_, kCFRunLoopCommonModes);
  }

  ~RepeatingTimer() {
    // Detach the timer, the `ScopedCFTypeRef` will release the ref on the timer after.
    CFRunLoopRemoveTimer(CFRunLoopGetMain(), timer_ref_, kCFRunLoopCommonModes);
  }

  // Move only
  RepeatingTimer(RepeatingTimer&&) = default;
  RepeatingTimer& operator=(RepeatingTimer&&) = default;
  RepeatingTimer(const RepeatingTimer&) = delete;
  RepeatingTimer& operator=(const RepeatingTimer&) = delete;

private:
  struct CallbackHolder {
    void Run() { cb_(); }
    Callback cb_;
  };

  // Called every `interval_` seconds by the CFRunLoop
  static void TimerCallbackTrampoline(CFRunLoopTimerRef timer, void* info) {
    // Cast the opaque pointer back to our callback, and run.
    static_cast<CallbackHolder*>(info)->Run();
  }

  CallbackHolder callback_holder_;
  TimeDelta interval_;
  ScopedCFTypeRef<CFRunLoopTimerRef> timer_ref_;
};