Swift 单例与懒加载

//懒加载就是在闭包中创建对象,并且只加载一次
    lazy var btn : UIButton = { //() -> UIButton in

        let button = UIButton(frame: CGRect(x: 0, y: 0, width: 20, height: 20))

        return button
    } ()
    //懒加载数据
    lazy var lable : UILabel = {

        let lable1  = UILabel()
        return lable1
    } ()

//单例
class NetTools: NSObject {
    var urlString : String?
    //简单的单例
    static let singNetTool : NetTools = NetTools()
    //复杂的单例 也是一个闭包
    var singsss: String?
    static let singNetTools : NetTools = { //() -> NetTools in

        let singa = NetTools()
        singa.urlString = "http://www.baidu.com"
        return singa
    } ()
}