Swift generic type conforming to two protocols

Question

I have a generic method in one of my classes where I want to have a generic type conforming to UIViewController and UIPickerViewDelegate . How can I do that? I thought of doing this:

func foo<T: UIViewController, UIPickerViewDelegate> (#viewController: T) {}

But this code doesn't "recognize" the UIPickerViewDelegate . I also thought of using the pipe | instead of the comma but this is even worse, the compiler doesn't accept that. Is it possible to do this or do I have to do 2 parameters for the class and the protocol? Or is there a nicer workaround?

Thanks for your help and Merry Christmas :]

Answer 1

Your code:

func foo<T: UIViewController, UIPickerViewDelegate> (#viewController: T) {}

declares 2 generics parameters:

• T which is UIViewController . And it's used as viewController parameter type.
• UIPickerViewDelegate which is Any . And it's not used.

Instead, you should use "Where Clause" , like:

func foo<T: UIViewController where T:UIPickerViewDelegate> (#viewController: T) {}

Answer 2

在Swift 4中已经改变了一些东西： func foo<T: UIViewController> (viewController: T) where T:UIPickerViewDelegate {}

Answer 3

Since Swift 4 you can use the power of Protocol Composition . Here you go:

func foo<T: UIViewController & UIPickerViewDelegate> (viewController: T) {}