如何在当前类型的Swift中使用泛型函数

Question

I have a protocol with a generic function which returns generic ViewController. It's forcing me to add type constraint check in default implementation.

protocol RouterViewController {
    func getViewController<T: UIViewController>(_ name: Storyboard) -> T
}

extension RouterViewController {
    func getViewController<T>(_ name: Storyboard) -> T where T: UIViewController {
        let storyBoard = UIStoryboard.init(storyboard: name)
        return storyBoard.instantiateViewController()
    }
}

The same example I can do without generics

    struct RouterViewController {
     static func  getViewController(_ name: Storyboard) -> UIViewController {
     let storyBoard = UIStoryboard.init(storyboard: name)
            return storyBoard.instantiateViewController()
 }
}

I have a set of questions

1. If I had to do type check then I will go for factory method? How to do same with generics?
2. If I want to use generics how to check with current type in this function return like func<SignIn>() -> T

Answer 1

If you make small changes in your protocol declaration/extension, you will get desired functionality:

protocol RouterViewController {
    func getViewController<T: UIViewController>(_ viewControllerType: T.Type, _ name: Storyboard) -> T 
}

extension RouterViewController {
    func getViewController<T>(_ viewControllerType: T.Type, _ name: Storyboard) -> T where T: UIViewController {
        let storyBoard = UIStoryboard.init(storyboard: name)
        let identifier = String(describing: T.self) // Let's say view controller's storyboard identifier is same as class name
        return storyBoard.instantiateViewController(withIdentifier: identifier) as! T
    }
}

Usage:

struct TestStruct: RouterViewController {

}

TestStruct().getViewController(TestViewController.self, #YourStoryboardObject)

For more about view controllers and storyboards check: Swift protocol extension