iOS关联对象示例详解

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：

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

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对小牛知识库的支持。