iOS關聯對象示例詳解

這篇文章主要給大家介紹了關於iOS關聯對象的相關資料，文中通過示例代碼結束的非常詳細，對大家學習或者使用iOS具有一定的參考學習價值，需要的朋友們下面隨着小編來一起學習學習吧

背景

在iOS開發中如果我們想給一個對象動態添加屬性或者給category添加屬性的時候，都是通過runtime的關聯對象去實現，那我們添加的屬性到底是如何存取的呢？是直接添加到了對象自身的內存中了去嗎？帶着這些疑問讓我們看一runtime的源碼，解開關聯對象的神祕面紗。

關聯對象源碼

存值

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
 _object_set_associative_reference(object, (void *)key, value, policy);
}

我們調用此方法的時候，一共傳遞了四個參數：

參數名稱	解釋
id object	需要關聯的對象
void *key	對應的key
id value	對應的值
objc_AssociationPolicy policy	內存管理策略

內存管理策略:

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. */
};

對於四個參數理解完了之後讓我們看看它真正的實現函數_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);//得到對象地址
 if (new_value) {
  // break any existing association.
  AssociationsHashMap::iterator i = associations.find(disguised_object);//首先通過對象的地址獲取對象的hashmap
  if (i != associations.end()) {//判斷是否已經存在，已經存在
  // secondary table exists
  ObjectAssociationMap *refs = i->second;//取值，對應的map
  ObjectAssociationMap::iterator j = refs->find(key);//通過key查找
  if (j != refs->end()) {//如果已經存在
   old_association = j->second;//取到原來老的值,以便後邊對其釋放
   j->second = ObjcAssociation(policy, new_value);//存儲新的值
  } else {//不存在
   (*refs)[key] = ObjcAssociation(policy, new_value);
  }
  } else {//如果不存在,創建一個
  // create the new association (first time).
  ObjectAssociationMap *refs = new ObjectAssociationMap;
  associations[disguised_object] = refs;
  (*refs)[key] = ObjcAssociation(policy, new_value);
  object->setHasAssociatedObjects();
  }
 } else {//不存在則創建一個
  // 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);
}

通過以上代碼我們可以看出其實關聯對象在存儲的時候在，生成了一個AssociationsManager單例對象，所以應用中所有的管理對象都存儲於此AssociationsManager中。

具體存儲的實現是藉助了C++的關聯容器unordered_map實現的。具體可以參看代碼中我加的註釋。

整個過程就是通過object對象的地址存儲了一個類似hashmap的東西;取到此hashmap，然後通過鍵值對的方式將我們需要存儲的值存儲到此hashmap中，這個過程中如果有舊值，則最後會將舊值就行釋放

取值

取值的過程其實就比較簡單了，就相當於從一個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;
}

總結

以上就是這篇文章的全部內容了，希望本文的內容對大家的學習或者工作具有一定的參考學習價值，如果有疑問大家可以留言交流，謝謝大家對神馬文庫的支持。