标准的单例模式为：
final class Manager { static let shared = Manager() private init() { ... } func foo() { ... }}您将像这样使用它：
Manager.shared.foo()
感谢appzYourLife指出应该声明它
final以确保它不会被意外子类化，以及
private对初始值设定项使用access修饰符，以确保您不会意外地实例化另一个实例。
因此，回到图像缓存问题，您将使用以下单例模式：
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 } }}那么你就可以：
ImageCache.shared["photo1"] = imagelet image2 = ImageCache.shared["photo2"])
要么
let cache = ImageCache.sharedcache["photo1"] = imagelet image2 = cache["photo2"]
上面显示了简单的单例缓存实现后，我们应该注意，您可能想要（a）通过使用使其线程安全
NSCache；（b）应对记忆压力。因此，实际的实现类似于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) } } }}您将按以下方式使用它：
ImageCache.shared["foo"] = image
和
let image = ImageCache.shared["foo"]
