How do I manipulate core data in unit tests while not exposing private functions?

Question

Thanks in advance for any help.

I have a caching service I've implemented with a core data stack in Swift 3.1. The exposed interface conforms to this protocol:

protocol WeekForecastCacheService {
  func add(cacheModels: [DayForecastCacheModel])
  func get(cityId: Int, callback: @escaping ([DayForecastCacheModel]?) -> () )
}

Ideally I want the internals of my core data stack to stay private.

However I also want to be able to unit test the class. Specifically the exposed interface. Because the core data stack is persistent I want to be able to delete every entity (a reset if you will to start tests in a known state). How do I go about doing this while keeping the unit test implementation outside of my main target.

Ideally I would also like my tests to be independent of implementation...

I'm thinking along the lines of the following but could do with some advice:

Add a delete all function to the cache class
Use a class extension and implement the functionality there - it would mean a fair amount of copy paste
Change private functions / variables to be internal giving enough access to easily create a delete all function in a class extension
Stop worrying because only protocols are used by classes consuming the service so it doesn't matter if functions and properties within the class are not private

Tom Harrington · Accepted Answer · 2017-05-09 14:47:43Z

1

When unit testing Core Data, a typical approach is to use the in-memory store type to effectively remove the "persistent" part of Core Data. With an in-memory store you get all of Core Data's usual capabilities, but the persistent store isn't written to a file so it always starts off empty. That gets you to a known starting state. If necessary you can pre-load some other known state into the in-memory store before beginning the test.

The key for this is NSInMemoryStoreType. If you're setting explicitly adding the persistent store, that would be the type value when calling addPersistentStore(ofType:configurationName:at:options:). If you're using NSPersistentContainer, you'd include the store type in the persistentStoreDescriptions property.

answered May 9, 2017 at 14:47

Tom Harrington

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How do I manipulate core data in unit tests while not exposing private functions?

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