取消订阅，并且增加订阅，哪里出错了？为何无法实现？

Vars Global Array<Integer> layers; Series<Numeric> Sub(1); Events OnInit() { layers[0] = SubscribeBar("IF888.CFFEX","1h",20210901); } OnBar(ArrayRef<Integer> indexs) { If(Sub == 1) { UnsubscribeBar(layers[0]); layers[0] = SubscribeBar("rb888.SHFE","1h",20220101); Sub = 0; } }