@property in objc

Property attributes are special keywords to tell compiler how to generate the getters and setters. Here you specify two property attributes: nonatomic, which tells the compiler not to worry about multithreading, and retain, which tells the compiler to retain the passed-in variable before setting the instance variable.

In other situations, you might want to use the “assign” property attribute instead of retain, which tells the compiler NOT to retain the passed-in variable. Or perhaps the “copy” property attribute, which makes a copy of the passed-in variable before setting.