如何在CustomTrigger中只在第一个元素上启动处理时间计时器？

public class FirstElementTrigger extends Trigger<Object, TimeWindow> {
    private static final long serialVersionUID = 1L;

    private final ValueStateDescriptor<Integer> elementPositionState =
            new ValueStateDescriptor<>("elementPosition", Integer.class);

    @Override
    public TriggerResult onElement(Object element, long timestamp, TimeWindow window, TriggerContext ctx) throws Exception {
        ValueState<Integer> elementPosition = ctx.getPartitionedState(elementPositionState);
        Integer currentPosition = elementPosition.value();

        if (currentPosition == null) {
            // 第一个元素，启动计时器
            ctx.registerProcessingTimeTimer(window.getEnd());
            elementPosition.update(1);
            return TriggerResult.CONTINUE;
        } else {
            // 非第一个元素，不进行任何操作
            return TriggerResult.CONTINUE;
        }
    }

    @Override
    public TriggerResult onProcessingTime(long time, TimeWindow window, TriggerContext ctx) throws Exception {
        // 计时器触发的逻辑
        // 可以根据需要执行一些操作，例如输出结果、更新状态等
        return TriggerResult.FIRE_AND_PURGE;
    }

    @Override
    public TriggerResult onEventTime(long time, TimeWindow window, TriggerContext ctx) throws Exception {
        // 不处理事件时间
        return TriggerResult.CONTINUE;
    }

    @Override
    public void clear(TimeWindow window, TriggerContext ctx) throws Exception {
        ValueState<Integer> elementPosition = ctx.getPartitionedState(elementPositionState);
        elementPosition.clear();
        ctx.deleteProcessingTimeTimer(window.getEnd());
    }
}