GoF_Visitor.swift

import Foundation

protocol Alphabet {
    func accept(_ visitor: AlphabetVisitor)
}

class A {
    let aProperty = "A"
}

extension A: Alphabet {
    func accept(_ visitor: AlphabetVisitor) {
        visitor.visitA(self)
    }
}

class B {
    let bProperty = "B"
}

extension B: Alphabet {
    func accept(_ visitor: AlphabetVisitor) {
        visitor.visitB(self)
    }
}

class AlphabetSet {
    var alphabets: [Alphabet]
    init(_ alphabets: [Alphabet] = []) {
        self.alphabets = alphabets
    }
}

extension AlphabetSet: Alphabet {
    func accept(_ visitor: AlphabetVisitor) {
        alphabets.forEach {
            $0.accept(visitor)
        }
    }
}


class AlphabetVisitor {
    func visitA(_ object: A) {
        print(object.aProperty)
    }
    func visitB(_ object: B) {
        print(object.bProperty)
    }
}

AlphabetSet([A(), AlphabetSet([A(), B(), A(), A(), B()]), A()]).accept(AlphabetVisitor())