Does Swift have a way of mixing in traits, a la Scala? The section of the Swift book on using extensions to add protocols to existing classes comes tantalizingly close. However, since protocols can't contain an implementation, this can't be used to mix code into a class. Is there another way?
As of Swift 2.0, yes!
Providing Default Implementations
You can use protocol extensions to provide a default implementation to any method or property requirement of that protocol. If a conforming type provides its own implementation of a required method or property, that implementation will be used instead of the one provided by the extension.