AnyAPIClientDelegate.swift

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

protocol APIClient {
    associatedtype GetDataType
    func get()
}

protocol APIClientDelegate: class {
    associatedtype GetDataType
    func getDataIsFinished(data: GetDataType)
}


class AnyAPIClientDelegate<GetType>: APIClientDelegate {
    private let _getDataIsFinished: ((GetType) -> Void)
    
    required init<U: APIClientDelegate>(_ delegate: U) where U.GetDataType == GetType {
        self._getDataIsFinished = delegate.getDataIsFinished
    }
    
    func getDataIsFinished(data: GetType) {
        _getDataIsFinished(data)
    }
    
}

class GetGoogleDataClient: APIClient {
    typealias GetDataType = String
    
    var delegate: AnyAPIClientDelegate<GetDataType>?
    
    private var dataTask: URLSessionDataTask?
    private let urlString: String = "https://google.com/"
    
    
    func get() {
        let dataTask = URLSession.shared.dataTask(with: URL(string: urlString)!) { [weak self] data, error, response in
            if let data = data, let stringData = String(data: data, encoding: .utf8) {
                self?.delegate?.getDataIsFinished(data: stringData)
            }
        }
        dataTask.resume()
        
    }
}



class Hoge {
    let dataClient = GetGoogleDataClient()
    
    init() {
        dataClient.delegate = .init(self)
    }
    
    func showSomeData() {
        dataClient.get()
    }
}

extension Hoge: APIClientDelegate {
    func getDataIsFinished(data: String) {
        print(data)
    }
}

let hoge = Hoge()
hoge.showSomeData()