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：

id object需要关联的对象void *key对应的keyid 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;}