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);}复制代码 我们调用此方法的时候，一共传递了四个参数：
| 参数名称 | 解释 |
|---|---|
| id object | 需要关联的对象 |
| void *key | 对应的key |
| id 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. */};复制代码 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);}复制代码 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);}复制代码 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;}复制代码