Swift中的Singleton

Question

我一直在尝试实现一个单例，以用作我从网络上传到iOS应用程序的照片的缓存。我在下面的代码中附加了三个变体。我试图让变体2工作，但它导致一个编译器错误，我不明白，并希望得到关于我做错了什么的帮助。变体 1 执行缓存，但我不喜欢使用全局变量。变体3不做实际的缓存，我相信这是因为我在var ic = ....的分配中得到了一个副本，这是正确的吗？

任何反馈和见解将不胜感激。

谢谢，Zvi

import UIKit

private var imageCache: [String: UIImage?] = [String : UIImage?]()

class ImageCache {
    class var imageCache: [String : UIImage?] {
        struct Static {
            static var instance: [String : UIImage?]?
            static var token: dispatch_once_t = 0
        }

        dispatch_once(&Static.token) {
            Static.instance = [String : UIImage?]()
        }
        return Static.instance!
    }
}

class ViewController: UIViewController {

    @IBOutlet weak var imageView: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()

        imageView.image = UIImage(data: NSData(contentsOfURL: NSURL(string: "http://images.apple.com/v/iphone-5s/gallery/a/images/download/photo_1.jpg")!)!)

        //variant 1 - this code is working
        imageCache["photo_1"] = imageView.image
        NSLog(imageCache["photo_1"] == nil ? "no good" : "cached")

        //variant 2 - causing a compiler error on next line: '@lvalue $T7' is not identical to '(String, UIImage?)'
        //ImageCache.imageCache["photo_1"] = imageView.image
        //NSLog(ImageCache.imageCache["photo_1"] == nil ? "no good" : "cached")

        //variant 3 - not doing the caching
        //var ic = ImageCache.imageCache
        //ic["photo_1)"] = imageView.image
        //NSLog(ImageCache.imageCache["photo_1"] == nil ? "no good" : "cached")
    }
}

Answer 1

以下是在swift 2.0中创建单例类的两种不同方法

方法1)该方法是通过swift实施的目标C。

import UIKit

class SomeManager: NSObject {

       class var sharedInstance : SomeManager {

              struct managerStruct {

                   static var onceToken : dispatch_once_t = 0
                   static var sharedObject : SomeManager? = nil
              }

              dispatch_once(&managerStruct.onceToken) { () -> Void in
                   managerStruct.sharedObject = SomeManager()
              }
              return managerStruct.sharedObject!
       }

       func someMethod(){
              print("Some method call")
       }
 }

方法2)一行singleton，不要忘记实现私有init(限制只使用Singleton)

import UIKit

class SomeManager: NSObject {

       static let sharedInstance = SomeManager()

       private override init() {

       }

       func someMethod(){
            print("Some method call")
       }
  }

调用单例方法，如：

  SomeManager.sharedInstance.someMethod()

Answer 2

Swift 3:

class SomeClass
{
    static let sharedInstance = SomeClass() 

    fileprivate override init() {
        //This prevents others from using the default '()' initializer
        super.init()
    }

    func sayHello()
    { 
        print("Hello!")
    }   
}

调用一些方法：

SomeClass.sharedInstance.sayHello() //--> "Hello"

通过创建新的类实例来调用某个方法（失败）：

SomeClass().sayHello() //--> 'SomeClass' cannot be constructed it has no accessible initailizers

Answer 3

标准的单例模式是:

final class Manager {
    static let shared = Manager()

    private init() { ... }

    func foo() { ... }
}

你可以这样使用它：

Manager.shared.foo()

值得称赞的是，appzYourLife指出应该将它声明为< code>final以确保它不会意外地被子类化，并为初始值设定项使用< code>private访问修饰符，以确保不会意外地实例化另一个实例。参见https://stackoverflow.com/a/38793747/1271826.

因此，回到您的图像缓存问题，您将使用这种单例模式：

final class ImageCache {

    static let shared = ImageCache()

    /// Private image cache.

    private var cache = [String: UIImage]()

    // Note, this is `private` to avoid subclassing this; singletons shouldn't be subclassed.

    private init() { }

    /// Subscript operator to retrieve and update cache

    subscript(key: String) -> UIImage? {
        get {
            return cache[key]
        }

        set (newValue) {
            cache[key] = newValue
        }
    }
}

然后，您可以：

ImageCache.shared["photo1"] = image
let image2 = ImageCache.shared["photo2"])

或

let cache = ImageCache.shared
cache["photo1"] = image
let image2 = cache["photo2"]

在上面展示了一个简单的单例缓存实现之后，我们应该注意到您可能希望（a）通过使用NSCache使其线程安全;（b）响应内存压力。因此，实际实现类似于 Swift 3 中的以下内容：

final class ImageCache: NSCache<AnyObject, UIImage> {

    static let shared = ImageCache()

    /// Observer for `UIApplicationDidReceiveMemoryWarningNotification`.

    private var memoryWarningObserver: NSObjectProtocol!

    /// Note, this is `private` to avoid subclassing this; singletons shouldn't be subclassed.
    ///
    /// Add observer to purge cache upon memory pressure.

    private override init() {
        super.init()

        memoryWarningObserver = NotificationCenter.default.addObserver(forName: .UIApplicationDidReceiveMemoryWarning, object: nil, queue: nil) { [weak self] notification in
            self?.removeAllObjects()
        }
    }

    /// The singleton will never be deallocated, but as a matter of defensive programming (in case this is
    /// later refactored to not be a singleton), let's remove the observer if deallocated.

    deinit {
        NotificationCenter.default.removeObserver(memoryWarningObserver)
    }

    /// Subscript operation to retrieve and update

    subscript(key: String) -> UIImage? {
        get {
            return object(forKey: key as AnyObject)
        }

        set (newValue) {
            if let object = newValue {
                setObject(object, forKey: key as AnyObject)
            } else {
                removeObject(forKey: key as AnyObject)
            }
        }
    }

}

您可以如下使用它：

ImageCache.shared["foo"] = image

和

let image = ImageCache.shared["foo"]

有关Swift 2.3的示例，请参见本答案的先前版本。