How to unit test that private method is called in Swift

Question

I have a ViewController class that presents a series of two choice popup views. Each two choice popup view is different.

Popup1 - Choice1 -> Choice1Popup

Popup1 - Choice2 -> Choice2Popup

I intend the method to present Popup1 to be public, but I want the other methods that present Choice1Popup and Choice2Popup to be private.

If I decide I need to test Choice1Popup and Choice2Popup then I may have to make them internal instead of private, but they are unlikely to ever be used from any other place.

I want to write a unit test that tests when the button for Choice1 is touched that the method that presents Choice1Popup is called. I've used a protocol with method type variables to allow a Mock to inject the Mock versions of the popup presenters. I'm not feeling 100% comfortable about my approach so I wanted to get input as to whether or not there is a better way.

An aside I'm feeling conflicted about internal versus private. It would be nice to be able to test my private methods but I don't want them to be able to be called from anywhere but a unit test and making them internal exposes them.

Here is the code and a single Unit test is at the bottom:

// protocol to be used by both UserChoices class and UserChoicesMock for method injection
protocol UserChoicesPrivateUnitTesting {
    static var choice1Method:(UIViewController) -> Void { get set }
    static var choice2Method:(UIViewController) -> Void { get set }
}

// this popup that will be presented with a public method
public class ChoiceViewController:UIViewController {

    @IBOutlet weak var titleLabel: UILabel!
    @IBOutlet weak var subjectLabel: UILabel!
    @IBOutlet weak var choice1Button: UIButton!
    @IBOutlet weak var choice2Button: UIButton!

     var choice1Action:(() -> Void)?
     var choice2Action:(() -> Void)?

    //    ...
}

public class UserChoices: UIViewController, UserChoicesPrivateUnitTesting {
    static var choice1Method: (UIViewController) -> Void = choice1
    static var choice2Method: (UIViewController) -> Void = choice2

    private static func choice1(onTopViewController: UIViewController) {
    //present choice1Popup
    }

    private static func choice2(onTopViewController: UIViewController) {
    //present choice2Popup
    }

    public static func presentChoiceViewController(onTopViewController: UIViewController, ChoiceViewController: ChoiceViewController = ChoiceViewController.instantiateFromAppStoryBoard(appStoryBoard: .MenuStoryboard)) {
        let isCustomAnimated = true
    //        ChoiceViewController.transitioningDelegate = transitionDelegate

        ChoiceViewController.choice1Action = { [weak onTopViewController]() in
            guard let weakSelf = onTopViewController else {
                return
            }
            weakSelf.dismiss(animated: false, completion: nil)
            UserChoices.choice1Method(onTopViewController!)
        }

        ChoiceViewController.choice2Action = { [weak onTopViewController]() in
            guard let weakSelf = onTopViewController else {
                return
            }
            weakSelf.dismiss(animated: false, completion: nil)
            UserChoices.choice2Method(onTopViewController!)
        }
        onTopViewController.present(ChoiceViewController, animated: isCustomAnimated, completion: nil)
    }
}

import XCTest
@testable import ChoiceModule

public class UserChoicesMock:UserChoicesPrivateUnitTesting {
    static public var choice1Method: (UIViewController) -> Void = choice1
    static public var choice2Method: (UIViewController) -> Void = choice2
    static var choice1MethodCalled = false
    static var choice2MethodCalled = false

    static func choice1(onTopViewController: UIViewController) {
        choice1MethodCalled = true
    }

    static func choice2(onTopViewController: UIViewController) {
        choice2MethodCalled = true
    }
}

class UserChoicesTests: XCTestCase {

    func testChoice1CallsPrivateChoice1Method() {
        // This is an example of a functional test case.
        let vc = UIViewController()
        let choiceViewController = ChoiceViewController.instantiateFromAppStoryBoard(appStoryBoard: .MenuStoryboard)

        UserChoices.choice1Method = UserChoicesMock.choice1Method

        UserChoices.presentChoiceViewController(onTopViewController: vc, ChoiceViewController: choiceViewController)

        choiceViewController.choice1Button.sendActions(for: .touchUpInside)

        if UserChoicesMock.choice1MethodCalled == false {
            XCTFail("choice1Method not called")
        }
    }
}

Answer 1

Tests can't access anything declared private. They can access anything declared internal as long as the test code does @testable import.

When you get that queasy feeling, "But I shouldn't have to expose this," consider that your class actually has multiple interfaces. There's the "everything it does interface" and there's the "parts needed by production code interface." There are various things to consider about this:

• Is there another type that is trying to get out?
• Is there another protocol to express a subset of the interface? This could be used by the rest of production code.
• Or maybe it's like a home theater amplifier where "controls you don't need that often" are hidden behind a panel. Don't sweat it.