property's synthesized getter follows Cocoa naming convention for returning 'owned' objects
YouownanyobjectyoucreateYoucreateanobjectusingamethodwhosenamebeginswith“alloc”,“new”,“copy”,or“mutableCopy”(forexample, alloc, newObject,or mutableCopy).一定要注意命名规范,不能以alloc,new,copy,mutableCopy作为开头命名,