Choosing Between Structures and Classes

In addition to the Apple guidelines, we generally prefer to use struct for:

Value types like the Networking Models
Stateless helpers

But consider using class instead if:

You need to manage mutable states. Especially if there are more than a few mutating functions, the struct becomes harder to reason about.
You have to set a struct property declaration as var because it has a mutating function. In this case, a constant (let) class property may be easier to reason about.