使用objc_getAssociatedObject, objc_setAssociatedObject動態擴展屬性

category與associative作為objective-c的擴展機制的兩個特性，category即類型，可以通過它來擴展方法；associative，可以通過它來擴展屬性；在iOS開發中，可能category比較常見，相對的associative，就用的比較少，要用它必須使用<objc/runtime.h>的頭文件，然後就可以自由使用objc_getAssociatedObject以及objc_setAssociatedObject，我們來看下這兩個方法：

/** 
 * Sets an associated value for a given object using a given key and association policy.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * @param value The value to associate with the key key for object. Pass nil to clear an existing association.
 * @param policy The policy for the association. For possible values, see “Associative Object Behaviors.”
 * 
 * @see objc_setAssociatedObject
 * @see objc_removeAssociatedObjects
 */
OBJC_EXPORT void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

/** 
 * Returns the value associated with a given object for a given key.
 * 
 * @param object The source object for the association.
 * @param key The key for the association.
 * 
 * @return The value associated with the key \e key for \e object.
 * 
 * @see objc_setAssociatedObject
 */
OBJC_EXPORT id objc_getAssociatedObject(id object, const void *key)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

另外還有一個方法：

/** 
 * Removes all associations for a given object.
 * 
 * @param object An object that maintains associated objects.
 * 
 * @note The main purpose of this function is to make it easy to return an object 
 *  to a "pristine state”. You should not use this function for general removal of
 *  associations from objects, since it also removes associations that other clients
 *  may have added to the object. Typically you should use \c objc_setAssociatedObject 
 *  with a nil value to clear an association.
 * 
 * @see objc_setAssociatedObject
 * @see objc_getAssociatedObject
 */
OBJC_EXPORT void objc_removeAssociatedObjects(id object)
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_1);

objc_getAssociatedObject、objc_setAssociatedObject、objc_removeAssociatedObjects都是Obj-c中的外聯方法，object 參數作為待擴展的對象實例，key作為該對象實例的屬性的鍵，而value就是對象實例的屬性的值，policy作為關聯的策略，它的枚舉包括：

enum {
OBJC_ASSOCIATION_ASSIGN = 0,
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
OBJC_ASSOCIATION_RETAIN = 01401,
OBJC_ASSOCIATION_COPY = 01403
};

另外，objc_removeAssociatedObjects可以刪除指定對象實例的所有擴展屬性。

可以用它來綁定數據到控制項上，並可以自由讀取。