时间：2021-04-14 09:02:35 | 栏目：iOS代码 | 点击：次
背景
在iOS开发中如果我们想给一个对象动态添加属性或者给category添加属性的时候，都是通过runtime的关联对象去实现，那我们添加的属性到底是如何存取的呢？是直接添加到了对象自身的内存中了去吗？带着这些疑问让我们看一runtime的源码，解开关联对象的神秘面纱。
关联对象源码
存值
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
我们调用此方法的时候，一共传递了四个参数：
| 参数名称 | 解释 |
|---|---|
| id object | 需要关联的对象 |
| void *key | 对应的key |
| id value | 对应的值 |
| objc_AssociationPolicy policy | 内存管理策略 |
内存管理策略:
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. */
};
对于四个参数理解完了之后让我们看看它真正的实现函数_object_set_associative_reference
void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
// retain the new value (if any) outside the lock.
ObjcAssociation old_association(0, nil);
id new_value = value ? acquireValue(value, policy) : nil;
{
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
disguised_ptr_t disguised_object = DISGUISE(object);//得到对象地址
if (new_value) {
// break any existing association.
AssociationsHashMap::iterator i = associations.find(disguised_object);//首先通过对象的地址获取对象的hashmap
if (i != associations.end()) {//判断是否已经存在，已经存在
// secondary table exists
ObjectAssociationMap *refs = i->second;//取值，对应的map
ObjectAssociationMap::iterator j = refs->find(key);//通过key查找
if (j != refs->end()) {//如果已经存在
old_association = j->second;//取到原来老的值,以便后边对其释放
j->second = ObjcAssociation(policy, new_value);//存储新的值
} else {//不存在
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {//如果不存在,创建一个
// create the new association (first time).
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
} else {//不存在则创建一个
// setting the association to nil breaks the association.
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
old_association = j->second;
refs->erase(j);
}
}
}
}
// release the old value (outside of the lock).
if (old_association.hasValue()) ReleaseValue()(old_association);
}
通过以上代码我们可以看出其实关联对象在存储的时候在，生成了一个AssociationsManager单例对象，所以应用中所有的管理对象都存储于此AssociationsManager中。
具体存储的实现是借助了C++的关联容器unordered_map实现的。具体可以参看代码中我加的注释。
整个过程就是通过object对象的地址存储了一个类似hashmap的东西;取到此hashmap，然后通过键值对的方式将我们需要存储的值存储到此hashmap中，这个过程中如果有旧值，则最后会将旧值就行释放
取值
取值的过程其实就比较简单了，就相当于从一个hashmap中取值
id objc_getAssociatedObject(id object, const void *key) {
return _object_get_associative_reference(object, (void *)key);
}
id _object_get_associative_reference(id object, void *key) {
id value = nil;
uintptr_t policy = OBJC_ASSOCIATION_ASSIGN;
{
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
disguised_ptr_t disguised_object = DISGUISE(object);
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
ObjcAssociation &entry = j->second;
value = entry.value();
policy = entry.policy();
if (policy & OBJC_ASSOCIATION_GETTER_RETAIN) {
objc_retain(value);
}
}
}
}
if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
objc_autorelease(value);
}
return value;
}
总结