code: public PeriodicEvent runEvery(Runnable task, float delay, float period,
              KeepRunning callback) {
          validateDelay(delay);
          validatePeriod(period);
          return new Event(task, delay, period, callback);
      }
docs: Run a task periodically, with a callback.
  
  @param task
  Task to run.
  @param delay
  The first execution will happen in {@code delay} seconds.
  @param period
  Subsequent executions will happen every {@code period} seconds
  after the first.
  @param callback
  Callback that lets you cancel the task. {@code null} means run
  indefinitely.
  @return An interface that lets you query the status; cancel; or
  reschedule the event.