RunTime的一些知识点之对象一

RunTime的一些知识点之对象一

先来看一个问题：category能不能创建一个实例化对象？不能？怎么才能创建？
看下runtime是如何创建对象的，过会来回答这个问题

/** 
 * Adds a new instance variable to a class.
 * 
 * @return YES if the instance variable was added successfully, otherwise NO 
 *         (for example, the class already contains an instance variable with that name).
 *
 * @note This function may only be called after objc_allocateClassPair and before objc_registerClassPair. 
 *       Adding an instance variable to an existing class is not supported.
 * @note The class must not be a metaclass. Adding an instance variable to a metaclass is not supported.
 * @note The instance variable's minimum alignment in bytes is 1<<align. The minimum alignment of an instance 
 *       variable depends on the ivar's type and the machine architecture. 
 *       For variables of any pointer type, pass log2(sizeof(pointer_type)).
 */
OBJC_EXPORT BOOL class_addIvar(Class cls, const char *name, size_t size, 
                               uint8_t alignment, const char *types) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0);

有以下注意点：

• objc_allocateClassPair 与 objc_registerClassPair 之间调用
• 不能用于已经存在的类
• 不能操作元类（metaclass）

由于category是在运行期间决议的，所以就不能添加实例变量

再来看下如何进行创建？可以使用关联对象，下面截取了关联对象API

/* Associative References */

/**
 * Policies related to associative references.
 * These are options to objc_setAssociatedObject()
 */
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. */
};

/** 
 * Sets an associated value for a given object using a given key and association policy.
 *  通过 key 和 policy 给一个对象设置关联值
 * @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);

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);

所以就可以通过设置关联对象这种方式来处理 category 的"实例化对象",其中主要涉及的方法是： objc_setAssociatedObject 和 objc_getAssociatedObject

property 为什么不可以？

由于 category 是运行期决议的，所以不能对已有类添加 ivar， 那么property 也就不能生成 set 和 get 方法。

如何解决？

利用 property 手动实现 set 和 get 和 Associated ，来实现 category 的"实例对象"的创建.

- (void)setName:(NSString *)name
{
    objc_setAssociatedObject(self, @"name", name, OBJC_ASSOCIATION_COPY);
}

- (NSString *)name
{
    return objc_getAssociatedObject(self, @"name");
}

使用时需要注意，一定要使用 self.name 去设置和获取。