以下代码删除并更换指定图层合约，但不成功，应该如何修改？

Params Vars Global Integer Subp; Global Array<Integer> layers; //记录订阅BAR之后返回的图层号 Events OnInit() { Subp = 0; layers[0] = SubscribeBar("IF888.CFFEX","1h",20210501); layers[1] = SubscribeBar("IF888.CFFEX","30m",20210501); layers[2] = SubscribeBar("IF888.CFFEX","15m",20210501); } OnBar(ArrayRef<Integer> indexs) { Range[0:DataSourceSize() - 1] { If (Subp == 0) { UnsubscribeBar(layers[1]); UnsubscribeBar(layers[2]); layers[1] = SubscribeBar("IF888.CFFEX","1m",20210501); layers[2] = SubscribeBar("IF888.CFFEX","5m",20210501); Subp = 1; } } }