原作: http://www.jianshu.com/p/25a319aee33d
三种方法的选择
Runtime提供三种方式来将原来的方法实现代替掉，那该怎样选择它们呢？
- Method Resolution：由于Method Resolution不能像消息转发那样可以交给其他对象来处理，所以只适用于在原来的类中代替掉。
- Fast Forwarding：它可以将消息处理转发给其他对象，使用范围更广，不只是限于原来的对象。
- Normal Forwarding：它跟Fast Forwarding一样可以消息转发，但它能通过NSInvocation对象获取更多消息发送的信息，例如：target、selector、arguments和返回值等信息。
Associated Objects
当使用Category对某个类进行扩展时，有时需要存储属性，Category是不支持的，这时需要使用Associated Objects来给已存在的类Category添加自定义的属性。Associated Objects提供三个API来向对象添加、获取和删除关联值：
- void objc_setAssociatedObject (id object, const void *key, id value, objc_AssociationPolicy policy )
- id objc_getAssociatedObject (id object, const void *key )
- void objc_removeAssociatedObjects (id object )
其中objc_AssociationPolicy是个枚举类型，它可以指定Objc内存管理的引用计数机制。
- typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
- OBJC_ASSOCIATION_ASSIGN = 0, /**< Specifies a weak reference to the associated object. */
- OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1, /**< Specifies a strong reference to the associated object.
- * The association is not made atomically. */
- OBJC_ASSOCIATION_COPY_NONATOMIC = 3, /**< Specifies that the associated object is copied.
- * The association is not made atomically. */
- OBJC_ASSOCIATION_RETAIN = 01401, /**< Specifies a strong reference to the associated object.
- * The association is made atomically. */
- OBJC_ASSOCIATION_COPY = 01403 /**< Specifies that the associated object is copied.
- * The association is made atomically. */
- };
下面有个关于NSObject+AssociatedObject Category添加属性associatedO