如何在运行时动态地将id转换为具体类？

I have several dataSources I use for one UIViewController. My view controller uses KeyValue Observing in order to follow the state of certain properties at runtime. When I swap dataSources, I need to stop observing those properties. The problem is, I'm not sure of the class of the dataSource at runtime, therefor something like this is not valid:I have several dataSources I use for one UIView