Mock NSCache Generic Methods in Swift - objective-c

Question

I'm trying to mock NSCache by building a protocol that NSCache can conform to in order to use dependency injection. I can't seem to figure out how or if you can add the generic methods to your protocol. For example, NSCache has this method:
open func removeObject(forKey key: KeyType)
So if in my protocol I try to declare:
func removeObject<KeyType: AnyObject>(forKey key: KeyType)
Xcode says NSCache doesn't fulfill the protocol and when I try to click on the FixIt to add the stubs, it then adds:
func removeObject<KeyType>(forKey key: KeyType) where KeyType : AnyObject {
}
Then that line has an error message:
Extension of a generic Objective-C class cannot access the class's generic parameters at runtime.
How can I mock something like this?

Answer 1

Your attempt is doomed to fail, due to the way the Objective-C generics work, they aren't 100% compatible with the Swift generics. You'll need to encapsulate NSCache within a Swift class, and mock that one instead if you want to go this road.
But I want to argue the fact that you need a mock here. Your NSCache instance works like a database, so why not injecting it directly instead of a protocol and assert based on the contents of the cache after performing some operations?
For example
class MyClassThatNeedsACacheTests: XCTestCase {
let nsCache = NSCache<Key, Value>()
let sut = MyClassThatNeedsACache(cache: nsCache)
func testBehaviorWhenCacheIsEmpty() {
sut.doSomething()
// assert what is needed
}
func testBehaviorWithCachedData() {
nsCache.setObject(someValue, forKey: somekey)
sut.doSomethingElse()
// assert what is needed
}
}
As a general rule, I'd recommend to use mocking as the last resort. Mocking tightly couples the tests and the implementation they test, as you'll have to keep the tests in sync with the implementation. If you have the possibility, input->output tests are the most reliable and most robust tests.