Downcast / subclass UIViewController for mock in unit test

manpreet Best Answer 3 years ago

I have a unit test and I want to create a subclass version of an UIViewController, for example Test1ViewController. Specifically, I want to override the present method for this class.

I have a view controller extension which instantiates a view controller, depending on its class name.

public class func instanceFromStoryboard<T>(storyboard: Storyboard) -> T {
    return UIStoryboard(name: storyboard.rawValue, bundle: nil).instantiateViewController(withIdentifier: String(describing: T.self)) as! T
}

And a Storyboard class.

public enum Storyboard: String {
    case main = "Main"
}

In my unit test, I created a subclass from Test1ViewController.

class Test2ViewController: Test1ViewController {
    var presented: Bool = false
    override func present(_ viewControllerToPresent: UIViewController, animated flag: Bool, completion: (() -> Void)? = nil) {
        presented = true
    }
}

How can I use my extension method to retrieve the view controller from the storyboard, and then downcast / subclass to Test2ViewController?

manpreet 3 years ago

Because objects in storyboards are actually encoded objects, they can't be decoded and cast to anything else. This is a drawback of using storyboards. What you put in a storyboard is what you get out.

If you can, use a XIB-based view controller instead of a storyboard-based one. With XIBs (and with programmatic view controllers), tests can instantiate subclasses.

If not, then you'll need to introduce a back door to your view controller. This would be unfortunate, as it means mixing test code into your production code.