为何下面代码中要触发两次事件？可以在项目代码中找到处理EVENT_TRADE事件的代码，但是没有找到EVENT_TRADE + trade.vt_symbol的代码，这会不会导致该类事件越积越多，最后拖慢处理速度了？
def on_trade(self, trade: TradeData) -> None:
"""
Trade event push.
Trade event of a specific vt_symbol is also pushed.
"""
self.on_event(EVENT_TRADE, trade)
self.on_event(EVENT_TRADE + trade.vt_symbol, trade)
