Answer to iOS Question.

gistfile1.txt

Given we must unit test the function in ClassToTest, how would you change the code below to allow us to achieve this?

Notes:

These will be unit tests so must work offline (ie does not rely on the API)
No need to write the tests, we just how we could change the code to make it testable.
We can't use any third party mocking libraries
Feel free to write your answer as text or pseudocode
class API {
    func fetchItems(completion: (Result<[Item], Error>) -> Void) {
        // Gets items from some hosted API
    }
}

class ClassToTest {
    private(set) var stateList = [String]()

    func functionToTest() {
        stateList.append("Loading")

        API().fetchItems { result in
            switch result {
            case .success(_):
                stateList.append("Success")
            case .failure(_):
                stateList.append("Failed")
            }
        }
    }
}


-------

Answer:

Indeed, in unit test, we are merely testing our own codes without the dependencies, in as small unit as possible.
Therefore, we would want to mock our `API`. It's not always that I'm required to create my own highly testable 
Networking class for a project, but let me do my best to explain one idea to do this.

I think first off, we could add a init function in our `API` class. This will take a `URLSession` object that 
can make our class a little more flexibe. In Foundation framework, we have an interface called `(NS)URLProtocol` and 
a class called `(NS)URLSessionConfiguration`.
We could make utilize these to create our mocked session and request. We could also add more twist like adding 
delay to our stubbing.

From there we can now proceed to coding our test cases.