關于類目（分類）能不能或者說應不應該添加屬性，本文不做討論。只是介紹利如何用運行時為類添加屬性。

/** 
 * 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)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);

/** 
 * 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)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);

/** 
 * 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)
    OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0);

以上三個為<objc/runtime.h>里提供的在運行時處理屬性的方法。
·第一個是添加關聯(lián)策略
·第二個是獲取關聯(lián)策略
·第三個是移除所有關聯(lián)策略（慎用?。。。?br> ·如果需要移除某個關聯(lián)策略，那么可以使用第一個方法，傳入參數(shù)為nil即可。

網上已經有大量的相關的方法。但是大多是對象類型，缺少基本數(shù)據(jù)類型的添加。
下面就是以BOOL類型為例子，進行相關的處理。

const void *theKey = @"theKey";
- (void)setYourProperty:(BOOL)yourPropertyName {
    objc_setAssociatedObject(self, theKey, @(isXXX), OBJC_ASSOCIATION_ASSIGN);
}

- (BOOL)yourProperty {
    return [objc_getAssociatedObject(self, theKey) boolValue];
}

上面是利用運行時方法為某個類添加一個BOOL類型的屬性。下面介紹常規(guī)的BOOL屬性賦初值的方法。利用懶加載給一個BOOL屬性賦一個初值。（ARC環(huán)境）

初值為NO的情況：

- (BOOL)isXXX {
    if (!_isXXX) {
        _isXXX = NO;
    }
    return _isXXX;
}

初值為YES的情況：
因為布爾屬性在沒有初值的情況下值并不明確，類似于NO的情況，那么可以利用dispatch_once_t函數(shù)進行處理。雖然官方API里面解釋的是"Use lazily initialized globals instead"，但是布爾類型基本數(shù)據(jù)類型，使用的是assign來修飾。不會產生內存問題。

- (BOOL)isXXX {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _isXXX = YES;
    });
    return _isXXX;
}