How to unit tests a private or internal function in Swift?

Question

So I created a custom an abstract class who inherit from UIViewController (Inherited by RebloodViewController) class named MainViewController. In this class I write a reusable nib registration function

class MainViewController: RebloodViewController {

    typealias Cell = RebloodViewController.Constants.Cell

    internal func registerNib(_ cellNib: Cell.Nib, target: UICollectionView) {

        let nib = UINib(nibName: cellNib.rawValue, bundle: nil)

        do {
            let identifier = try getCellIdentifierByNib(cellNib)
            target.register(nib, forCellWithReuseIdentifier: identifier)
        } catch {
            fatalError("Cell identifier not found from given nib")
        }
    }

    private func getCellIdentifierByNib(_ nib: Cell.Nib) throws -> String {

        var identifier: String? = nil

        switch nib {
        case .articles:
            identifier = Cell.Identifier.articles.rawValue
        case .events:
            identifier = Cell.Identifier.events.rawValue
        }

        guard let CellIdentifier = identifier else {
            throw MainError.cellIdentifierNotFound
        }

        return CellIdentifier
    }

}

What is the best way to unit test these private and internal function? Because I can't access the functions from the test files.

Answer 1

You will not be able to test the private functions. But you can test internal ones. In your test, where you import your framework, eg import MyFramework, change it to:

@testable import MyFramework

Answer 2

I found the answer is we don't test private methods. We put a method private for a reason and it should be used by a public method. I assume whether we test private method, it's better to test the public method that uses the private method.

Another extra explanation I found it here: https://cocoacasts.com/how-to-unit-test-private-methods-in-swift/