Swift generic type conforming to two protocols

问题

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 :]

回答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) {}

回答2:

Things have been changed in Swift 4: func foo<T: UIViewController> (viewController: T) where T:UIPickerViewDelegate {}

回答3:

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

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

来源：https://stackoverflow.com/questions/27633872/swift-generic-type-conforming-to-two-protocols