Swift で Class-Only Protocol を定義する ref: https://qiita.com/lovee/items/a686e11f00b31323e683

after

protocol SomeProtocol: class {
    var aVariable: Int { get set }
    func increase()
}

extension SomeProtocol {
    func increase() {
        self.aVariable += 1
    }
}

class SomeClass: SomeProtocol {
    var aVariable: Int
    init() {
        self.aVariable = 0
    }
}

let some = SomeClass()
some.increase()

before

protocol SomeProtocol {
    var aVariable: Int { get set }
    mutating func increase()
}

extension SomeProtocol {
    mutating func increase() {
        self.aVariable += 1
    }
}

class SomeClass: SomeProtocol {
    var aVariable: Int
    init() {
        self.aVariable = 0
    }
}

let some = SomeClass()
some.increase()

file1.swift

protocol ADelegate: class {

}

file5.swift

protocol SomeProtocol: AnyObject {
    // ...
}