刚开始一直没把这个问题搞明白，后来搜了下google，发现了答案http://stackoverflow.com/questions/15811751/whats-the-difference-between-property-and-fields-in-objective-c

里面写了这么一段话

With the second one you are only creating instance variables. @property automatically generates you these as their associated getter and setter methods.

大概意思就是说，如果是写在{}大括号的字段，那么就只是实例变量，通过property方式，会给你自动生成get,set方法；