//: Playground - noun: a place where people can playimport UIKitvar str = "Hello, playground"// nil的聚合运算可以说是为了可选值而出的 // 它的体现是"??"// eg:// a ?? b  ->  a != nil ? a! : bvar userNickname : String?userNickname = "Bobo"// 最原始:if userNickname != nil {    print("hello, \(userNickname!)")} else {    print("hello, guest")}// 简化成-->var outputName = userNickname != nil ? userNickname! : "guest"// 简化成-->// 简单理解为:如果userNickname为空就取"guest", 反之则取userNickname解包后的值var name = userNickname ?? "guest"