如何在协议扩展中存储提交到计算属性的数据？

In the app that I'm currently working on, I try to take advantage of the new protocol extension feature in Swift. The idea is that I have a lot of classes implementing the same protocol. Since all these classes should have the same computed properties, and since the properties should behave identically in de different classes, I thought it would be nice to add the functionality only once.In the app that I'm currently working on, I try