Create a generic function that takes in a view controller type as a parameter and returns a custom view controller based on that type

Question

I'm trying to create a generic function that takes in a view controller type as a parameter and returns a custom view controller based on that type.

func setUpInstance<T>(_ viewControllerType: T, toName: String?) -> T where T: UIViewController {

        switch viewControllerType {

            case  is GetInstructionsModalViewController:
               let getInstructionsVC = getViewController(GetInstructionsModalViewController.self, "Instructions", identifier: "GetInstructionsVC")
                     getInstructionsVC.delegate = self
                     getInstructionsVC.getDirectionsDelegate = self
                     getInstructionsVC.modalPresentationStyle = .overCurrentContext
                     moveButtonsAccordingToViewHeight(getInstructionsVC.height)

                 return getInstructionsVC as! T

           case is TurnModalViewController:
                let turnVC = getViewController(TurnModalViewController.self, "Turns", identifier: "TurnVC")
                    turnVC.modalPresentationStyle = .overCurrentContext
                    turnVC.delegate = self
                    turnVC.toName = toName
                    bottomContainer.isHidden = true
                    moveButtonsAccordingToViewHeight(turnVC.height)
               return turnVC as! T

           default:
                return UIViewController.self as! T
            }

        }

when I try and instantiate the custom view controller:

let turnVC = setUpInstance(TurnModalViewController, toName: nil)

I get an error:

Cannot convert value of type 'TurnByTurnModalViewController.Type' to expected argument type 'UIViewController'

Any idea on how to make this function do its job properly?

Answer 1

Looks like you mixed up how to use generics. First of all, you need to change the signature of the function to

func setUpInstance<T>(_ viewControllerType: T.Type, toName: String?) -> T where T: UIViewController

since you'd like to check the input type and return the instance of that type

then call it with the right parameter

let turnVC = setUpInstance(TurnModalViewController.self, toName: nil)