26 原子属性和非原子属性之间有什么区别? 什么atomic和nonatomic财产申报是什么意思? @property(nonatomic, retain) UITextField *userName; @property(atomic, retain) UITextField *userName; @property(retain) UITextField *userName; 这三个之间的操作区别是什么? 1851 ios objective-c properties atomic nonatomic