Extract a Protocol to Support Test Doubles

iOS Unit Testing by Example — by Jon Reid (120 / 223)

👈 Isolate UserDefaults with Depen dency Injection | TOC | Make a Fake Object 👉

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 a test double? It’s like a stunt double in filmmaking. We temporarily replace the real actor with someone that looks like that actor. Or in our case, something that looks like the original type as far as the calling code is concerned.

Start by defining a new protocol in production code. For lack of a better name, let’s call it UserDefaultsProtocol. It’ll be empty to start with:

Now add an explicit type to the userDefaults property, specifying the protocol.

Xcode will show a Swift error:

Since Swift allows us to extend existing types, we can declare an extension saying that UserDefaults does conform to our new protocol:

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