在价差策略模组的UI模块下面，有一个SpreadDataMonitor Class。其中定义event_type为EVENT_SPREAD_DATA。但是在其中的register_event()函数中却注册监听了EVENT_SPREAD_POS。请问这是一个typo么？还是什么特别用途？ 代码如下：
class SpreadDataMonitor(BaseMonitor):
"""
Monitor for spread data.
"""

event_type = EVENT_SPREAD_DATA
data_key = "name"
sorting = False

headers = {
    "name": {"display": "名称", "cell": BaseCell, "update": False},
    "bid_volume": {"display": "买量", "cell": BidCell, "update": True},
    "bid_price": {"display": "买价", "cell": BidCell, "update": True},
    "ask_price": {"display": "卖价", "cell": AskCell, "update": True},
    "ask_volume": {"display": "卖量", "cell": AskCell, "update": True},
    "net_pos": {"display": "净仓", "cell": PnlCell, "update": True},
    "datetime": {"display": "时间", "cell": TimeCell, "update": True},
    "price_formula": {"display": "定价", "cell": BaseCell, "update": False},
    "trading_formula": {"display": "交易", "cell": BaseCell, "update": False},
}

def register_event(self):
    """
    Register event handler into event engine.
    """
    super().register_event()
    self.event_engine.register(EVENT_SPREAD_POS, self.signal.emit)