IOS Swift3 四种单例模式详解及实例

final class Single: NSObject {   static let shared = Single()   private override init() {} } 

public extension DispatchQueue {   private static var onceToken = [String]()   public class func once(_ token: String, _ block: () -> Void) {     objc_sync_enter(self)     defer {       objc_sync_exit(self)     }     if onceToken.contains(token) {       return     }     onceToken.append(token)     block()   } }  final class Single2: NSObject {   static func shared() -> Single2 {     struct Singleton {       static var single: Single2?     }     DispatchQueue.once(NSUUID().uuidString) {       Singleton.single = shared()     }     return Singleton.single!   } } 

private let single = Single3() final class Single3: NSObject {   static var shared: Single3 {     return single   } } 

final class Single4: NSObject {   static var shared: Single4 {     struct Static {       static let instance: Single4 = Single4()     }     return Static.instance   }   private override init() {} }