值得注意的小细节

• 发现@”“和nil不是一个含义
• isKindOfClass和isMemberOfClass的区别

isKindOfClass：
Returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class.

isMemberOfClass：
Returns a Boolean value that indicates whether the receiver is an instance of a given class.

也就是isKindOfClass的范围更大