常见的有这么几种方法

第一种最简单也是最常用的，这里的所有单例init方法一定要定义成private的，不然外部依然可以使用init方法初始化变量。

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

第二种

public extension DispatchQueue {
    private static var onceToken = [String]()
    public class func once(_ token: String,_ block:@escaping () -> 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("YouShaoduo") {
            Singleton.single = shared()
        }
        return Singleton.single
    }
    
    private override init() {}
}

第三种

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

第四种

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