关于类目（分类）能不能或者说应不应该添加属性，本文不做讨论。只是介绍利如何用运行时为类添加属性。
/**
* 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);
以上三个为<objc/runtime.h>里提供的在运行时处理属性的方法。
·第一个是添加关联策略
·第二个是获取关联策略
·第三个是移除所有关联策略（慎用！！！）
·如果需要移除某个关联策略，那么可以使用第一个方法，传入参数为nil即可。
网上已经有大量的相关的方法。但是大多是对象类型，缺少基本数据类型的添加。
下面就是以BOOL类型为例子，进行相关的处理。
const void *theKey = @"theKey";
- (void)setYourProperty:(BOOL)yourPropertyName {
objc_setAssociatedObject(self, theKey, @(isXXX), OBJC_ASSOCIATION_ASSIGN);
}
- (BOOL)yourProperty {
return [objc_getAssociatedObject(self, theKey) boolValue];
}
上面是利用运行时方法为某个类添加一个BOOL类型的属性。下面介绍常规的BOOL属性赋初值的方法。利用懒加载给一个BOOL属性赋一个初值。（ARC环境）
初值为NO的情况：
- (BOOL)isXXX {
if (!_isXXX) {
_isXXX = NO;
}
return _isXXX;
}
初值为YES的情况：
因为布尔属性在没有初值的情况下值并不明确，类似于NO的情况，那么可以利用dispatch_once_t函数进行处理。虽然官方API里面解释的是"Use lazily initialized globals instead"，但是布尔类型基本数据类型，使用的是assign来修饰。不会产生内存问题。
- (BOOL)isXXX {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
_isXXX = YES;
});
return _isXXX;
}
