Property and Its Attributes in Objective-C

All the attributes of a property defines how the compiler generate the getter and setter accessors.

• atomic / nonatomic
• strong / weak: used for ARC.
• assign / retain / copy: go to here for reference. These properties define how the compiler generate the setter accessor.
• readonly / readwrite: if a property is declared as readonly, the compiler will only declare the getter accessor, so that you can not call setter accessor.

Apart from above, @synthesize define the instance variable used for accessors. @synthesize can only be used to property, not iVar. For more info, see Property and Its Attributes in Objective-C, and Property vs Instance Variable (iVar) in objective-c-in-Objective-C.html) is deprecated.

Here has a detail description. And the useful part:

• 什么时候用assign、什么时候用retain和copy呢？就看你想让编译器怎么实现该Property的setter函数了。推荐做法是NSString用copy,delegate用assign（且一定要用assign，不要问为什么，只管去用就是了，以后你会明白的。如果用retain，容易造成retain cycle。），非objc数据类型，比如int，float等基本数据类型用assign（默认就是assign），而其它objc类型，比如NSArray，NSDate用retain。
• 如果你自己实现了getter和setter的话，atomic/nonatomic/retain/assign/copy这些只是给编译的建议，编译会首先会到你的代码里面去找，如果你定义了相应的getter和setter的话，那么好，用你的。如果没有，编译器就会根据atomic/nonatomic/retain/assign/copy这其中你指定的某几个规则去生成相应的getter和setter。