プロキシーパターンをSwift5で実装する

public protocol RemoteImage: CustomStringConvertible {
    init(url: URL)
    var image: UIImage? {get}
    var url: URL {get}
    var hasContent: Bool {get}
}

extension RemoteImage {
    public var description: String {
        let description = self.hasContent ? "Image available. Retrieved from \(self.url.absoluteString)" : "No image available yet!"
        return description
    }
}

public class ImageProxy: RemoteImage {
    public required init(url: URL) {
        self.url = url
    }

    //lazy修飾詞をつける
    public lazy var image: UIImage? = { [unowned self] in
        var result: UIImage?
        if let img = try? UIImage(data:
            Data(contentsOf: self.url)
            ) {
            result = img
            self.hasContent = true
        }

        return result
    }()

    public let url: URL
    public var hasContent: Bool = false
}

guard let imageURL = URL(string: "https://developer.apple.com/swift/images/swift-og.png") else {
    fatalError("Could not create URL")
}

let imageProxy = ImageProxy(url: imageURL)

print(imageProxy)// No image available yet!

let image = imageProxy.image
print(imageProxy)//Image available. Retrieved from https://developer.apple.com/swift/images/swift-og.png

import Foundation

public protocol RemoteData {
    func data(url: URL, completionHandler: @escaping(Error?, Data?) -> Void) -> RemoteData

    func run()
}

public class RemoteDataProxy: RemoteData {
    fileprivate var callback: ((Error?, Data?) -> Void)?
    fileprivate var url: URL?
    public init() {}

    //URL,コンプリーションハンドラを渡す処理
    public func data(url: URL, completionHandler: @escaping(Error?, Data?) -> Void) -> RemoteData {
        self.url = url
        self.callback = completionHandler
        return self
    }

    //実際にネットワーク接続を行う処理
    public func run() {
        if let callback = self.callback,
            let url = self.url {
            URLSession.shared.dataTask(with: url) {(data, response, error) in
                guard let data = data, error == nil else {
                    print("Could not download data from URL \(url.absoluteString). Reason: \(error!.localizedDescription)")
                    callback(error, nil)
                    return
                }

                print("Data successfully fetched from URL \(url.absoluteString)")
                callback(nil, data)
            }.resume()

            print("Downloading data from URL \(url.absoluteString)")
        } else {
            print("run() called before invoking data(url: completionHandler:)")
        }
    }
}

import Foundation
import PlaygroundSupport

guard let dataURL = URL(string: "https://developer.apple.com/swift/images/swift-og.png") else {
    fatalError("Could not create URL")
}

//URL、クロージャなど接続処理に必要な情報を渡す。この時点では接続は行われない
let dataProxy = RemoteDataProxy().data(url: dataURL) {(error, data) in
    guard error == nil else {
        print("Could not retrieve data from URL \(dataURL.absoluteString)")
        return
    }

    print("\(data?.count ?? 0) bytes retrieved from URL \(dataURL.absoluteString)")
}

//Playgroundで非同期処理を許可する
PlaygroundPage.current.needsIndefiniteExecution = true

//延期されたネットワーク接続処理
dataProxy.run()

public protocol Authenticating {
    var isAuthenticated: Bool {get}
    func authenticate(user: String) -> Bool
}

public class Authenticator: Authenticating {
    static public let shared = Authenticator()

    //認証が行われたか否かを表すBool型変数
    public var isAuthenticated: Bool = false

    //接続が許可されているユーザー名の一覧
    fileprivate let userWhiteList = ["John", "Mary", "Steve"]

    fileprivate let syncQueue = DispatchQueue(label: "com.leakka.authQueue")

    fileprivate init() {}

    //許可されたユーザーか否かを確認するメソッド
    public func authenticate(user: String) -> Bool {
        var result = false
        self.syncQueue.sync {
            result = self.userWhiteList.contains(user) ? true : false

            if result {
                print("Authorized!")
                self.isAuthenticated = true
            } else {
                print("Error: Unauthorized!")
                self.isAuthenticated = false
            }
        }
        return result
    }
}

//private修飾詞に変更
private class ImageProxy: RemoteImage {
//中略
}

//認証用のプロクシクラスを追加
public class SecureImageProxy: RemoteImage {

    //認証が完了していれば画像を返す
    public var image: UIImage? {
        get {
            return Authenticator.shared.isAuthenticated ? self.imageProxy.image : nil
        }
    }

    public let url: URL
    public var hasContent: Bool = false

    //ImageProxyクラスをprivateで保持
    fileprivate lazy var imageProxy: ImageProxy = ImageProxy(url: self.url)

    public required init(url: URL) {
        self.url = url
    }
}

import Foundation
import PlaygroundSupport

guard let imageURL = URL(string: "https://developer.apple.com/swift/images/swift-og.png") else {
    fatalError("Could not create URL")
}

let secureImageProxy = SecureImageProxy(url: imageURL)

print(secureImageProxy)// No image available yet!

Authenticator.shared.authenticate(user: "Jim")//Error: Unauthorized!
if secureImageProxy.image != nil {
    print("Proxy has a valid image.")
}

Authenticator.shared.authenticate(user: "John")//Authorized!
if secureImageProxy.image != nil {
    print("Proxy has a valid image.")//Proxy has a valid image.
}

PlaygroundPage.current.needsIndefiniteExecution = true