public Optional<WindowPane<K, Object>> onMessage(TriggerKey<K> triggerKey, M message,
        MessageCollector collector, TaskCoordinator coordinator) {
      if (!isCancelled) {
        LOG.trace("Forwarding callbacks for {}", message);
        impl.onMessage(message, triggerScheduler);

        if (impl.shouldFire()) {
          // repeating trigger can trigger multiple times, So, clear the state to allow future triggerings.
          if (impl instanceof RepeatingTriggerImpl) {
            ((RepeatingTriggerImpl<M, K>) impl).clear();
          }
          return onTriggerFired(triggerKey, collector, coordinator);
        }
      }
      return Optional.empty();
    }
State corresponding to a created {@link TriggerImpl} instance.
samza-core/src/main/java/org/apache/samza/operators/impl/WindowOperatorImpl.java