我可以声明一个符合Swift协议的特定类的类型吗？

Question

If I have the following code:

class MyClass {
    ...
}

protocol MyProtocol {
    ...
}

Is it possible to declare a type that accepts a class or subclass of MyClass conforming to MyProtocol ?

eg in pseudo code:

var thing: MyClass & MyProtocol = ...

Answer 1

The naive approach works (ie specify the type first and then use it in the variable declaration):

class MCImplementingMP: MyClass, MyProtocol {   
}

var thing: MCImplementingMP = ...

Answer 2

Nope, it was possible in Objective-C but not possible in Swift. All solutions that I know looks like hacks and require lot of runtime type checking. So I came out with my own - declare a wrapper type which can act like needed class or protocol depending on circumstances:

class MyClass {}
protocol MyProtocol: class {}

class Wrapper {
    var instance: AnyObject
    init?(instance: MyClass) {
        guard instance is MyProtocol else { return nil }
        self.instance = instance
    }

    init?(instance: MyProtocol) {
        guard instance is MyClass else { return nil }
        self.instance = instance
    }

    var instanceAsMyClass: MyClass {
        return instance as! MyClass
    }

    var instanceAsMyProtocol: MyProtocol {
        return instance as! MyProtocol
    }
}

You may want to change property names but idea is clear.