iOS Unit Testing by Example

Extract a Protocol to Support Test Doubles

The userDefaults property currently has an implicit type of UserDefaults. If we kept that type, then any replacement would have to be UserDefaults or a subclass. Subclassing is useful for allowing existing behavior to flow through, capturing extra information. (See ​How to Spy on the Animated Flag​.) But subclassing isn’t useful when you want to stop some existing behavior. It also isn’t useful when the type is a final class or a struct, or the type prevents you from creating your own instance of it.

What we need is a way to substitute an arbitrary type that honors a contract. In Swift, we can do this using protocols. Protocols give us a way to replace a concrete type with a test double.

What’s ...