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iOS關(guān)聯(lián)對(duì)象示例詳解

 更新時(shí)間:2018年12月30日 08:42:15   作者:FlyOceanFish  
這篇文章主要給大家介紹了關(guān)于iOS關(guān)聯(lián)對(duì)象的相關(guān)資料,文中通過(guò)示例代碼結(jié)束的非常詳細(xì),對(duì)大家學(xué)習(xí)或者使用iOS具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

背景

在iOS開(kāi)發(fā)中如果我們想給一個(gè)對(duì)象動(dòng)態(tài)添加屬性或者給category添加屬性的時(shí)候,都是通過(guò)runtime的關(guān)聯(lián)對(duì)象去實(shí)現(xiàn),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(duì)象自身的內(nèi)存中了去嗎?帶著這些疑問(wèn)讓我們看一runtime的源碼,解開(kāi)關(guān)聯(lián)對(duì)象的神秘面紗。

關(guān)聯(lián)對(duì)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(diào)用此方法的時(shí)候,一共傳遞了四個(gè)參數(shù):

參數(shù)名稱(chēng) 解釋
id object 需要關(guān)聯(lián)的對(duì)象
void *key 對(duì)應(yīng)的key
id value 對(duì)應(yīng)的值
objc_AssociationPolicy policy 內(nèi)存管理策略

內(nèi)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(duì)于四個(gè)參數(shù)理解完了之后讓我們看看它真正的實(shí)現(xiàn)函數(shù)_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(duì)象地址
 if (new_value) {
  // break any existing association.
  AssociationsHashMap::iterator i = associations.find(disguised_object);//首先通過(guò)對(duì)象的地址獲取對(duì)象的hashmap
  if (i != associations.end()) {//判斷是否已經(jīng)存在,已經(jīng)存在
  // secondary table exists
  ObjectAssociationMap *refs = i->second;//取值,對(duì)應(yīng)的map
  ObjectAssociationMap::iterator j = refs->find(key);//通過(guò)key查找
  if (j != refs->end()) {//如果已經(jīng)存在
   old_association = j->second;//取到原來(lái)老的值,以便后邊對(duì)其釋放
   j->second = ObjcAssociation(policy, new_value);//存儲(chǔ)新的值
  } else {//不存在
   (*refs)[key] = ObjcAssociation(policy, new_value);
  }
  } else {//如果不存在,創(chuàng)建一個(gè)
  // 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(chuàng)建一個(gè)
  // 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(guò)以上代碼我們可以看出其實(shí)關(guān)聯(lián)對(duì)象在存儲(chǔ)的時(shí)候在,生成了一個(gè)AssociationsManager單例對(duì)象,所以應(yīng)用中所有的管理對(duì)象都存儲(chǔ)于此AssociationsManager中。

具體存儲(chǔ)的實(shí)現(xiàn)是借助了C++的關(guān)聯(lián)容器unordered_map實(shí)現(xiàn)的。具體可以參看代碼中我加的注釋。

整個(gè)過(guò)程就是通過(guò)object對(duì)象的地址存儲(chǔ)了一個(gè)類(lèi)似hashmap的東西;取到此hashmap,然后通過(guò)鍵值對(duì)的方式將我們需要存儲(chǔ)的值存儲(chǔ)到此hashmap中,這個(gè)過(guò)程中如果有舊值,則最后會(huì)將舊值就行釋放

取值

取值的過(guò)程其實(shí)就比較簡(jiǎn)單了,就相當(dāng)于從一個(gè)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(jié)

以上就是這篇文章的全部?jī)?nèi)容了,希望本文的內(nèi)容對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,如果有疑問(wèn)大家可以留言交流,謝謝大家對(duì)腳本之家的支持。

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