如何使使用 Callable (callAsFunction()) 的结构与协议一起工作？

Question

I'm trying to make the callAsFunction() works when working directly with protocol or dependency injection , but it seems like the protocol instance is not aware of the Callable feature. I tried adding the func callAsFunction() within the protocol decorating , but it did not work too. As you can see the sample code below, when creating an instance with the concrete struct, I'm able to work with the callAsFunction() , but not with the protocol instance.

protocol Decorating {
  //to implement
}

struct Decorate: Decorating {
 func callAsFunction() {
    print("Decorated!")
 }
}

let decorate: Decorate = Decorate()
decorate() //worked

let decorate2: Decorating = Decorate()
decorate2() //failed

Answer 1

You can extend your protocol:

protocol Decorating {

}

extension Decorating {
    func callAsFunction() -> {
        // Your code
    }
}

Example of use:

protocol Decorating {
    var foo: Foo
}

extension Decorating {
    func callAsFunction() -> {
        foo.foo()
    }
}

let decorate: Decorating = Decorate()
decorate()