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？帶著這些疑問讓我們看一runtime的源碼，解開關聯對象的神秘面紗。
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. */};對于四個參數理解完了之后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,以便后邊對其釋放 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于此AssociationsManager中。
具體存儲的實現是借助了C++的關聯容器unordered_map實現的。具體可以參看代碼中我加的注釋。
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，然后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，則最后會將舊值就行釋放
取值
取值的過程其實就比較簡單了，就相當于從一個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
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