如何声明一个符合协议的特定类的属性？

Question

Suppose I want to create a property which is a subclass of UIViewController and also conformant to the protocol MyDelegateProtocol. In Objective-C I would write something like:

@property (strong, nonatomic) UIViewController<MyDelegateProtocol> *delegate;

However, I'm not sure how to write this in Swift. I know how to declare a property which is protocol-conformant, or a property which is of a particular type:

let delegate : MyDelegateProtocol?
let delegate : UIViewController?

But I can't quite figure out how to make it do both. If I try something like:

let delegate : UIViewController<MyDelegateProtocol> ?

Then I get a compiler error about Cannot specialize non-generic type 'UIViewController' . Probably because I'm wandering into the land of generics now. I've tried looking through the Swift book on protocols and other Stack Overflow questions regarding protocols, but I haven't found quite what I'm looking for.

Answer 1

First of all, I think this is a code smell. If you want a delegate to be multiple things, there is most likely a separation of concerns problem.

With that said, if you still want to do this, there isn't a way in Swift. You have a few options though:

1. Add the required UIViewController methods to your delegate protocol ( my favorite option )
2. Create a superclass for your view controller that is a subclass of UIViewController and implements MyDelegateProtocol. Declare your variable as that type
3. Store two references to your "delegate". One is of the MyDelegateProtocol and the other is of UIViewController
4. Make your class generic, but this will not be accessible from Objective-C

Answer 2

3年后回答我自己的问题，Swift 4支持组合类和协议类型：

let delegate: UIViewController & MyDelegateProtocol

Answer 3

一年半之后回答我自己的问题，将未来的任何人都指向本文 ，它基本上回答了Swift 2中的问题。这不是严格可能的，但是使用协议扩展可以非常接近。

Answer 4

One option would be to parameterize your whole class; unfortunately, template classes can't be used from Obj-C.

class MyClass<Delegate: protocol<UIViewController, MyDelegateProtocol>> {
    ...
    let delegate: Delegate?
    ...
}