##oc在類別裡給類加屬性以及原理，

##oc在類別裡給類加屬性以及原理，
oc在類別裡給類加屬性以及原理

如何在類別裡給類加屬性呢,有鴨子模樣的我們就認為他是鴨子了.

@interface NSObject (XY)@property (nonatomic, strong) id                tempObject;@end@implementation NSObject (XY)@dynamic tempObject;- (id)tempObject- {    id object = objc_getAssociatedObject(self, NSObject_key_tempObject);    return object;}- (void)setTempObject:(id)tempObject- {    [self willChangeValueForKey:@"tempObject"];    objc_setAssociatedObject(self, NSObject_key_tempObject, tempObject, OBJC_ASSOCIATION_RETAIN_NONATOMIC);    [self didChangeValueForKey:@"tempObject"];}@end

屬性取變數是直接位移地址得到,關聯是取了2次map.

下面來看一下set get 還有dealloc時候發生的事情吧

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {#if SUPPORT_GC    if (UseGC) {        if ((policy & OBJC_ASSOCIATION_COPY_NONATOMIC) == OBJC_ASSOCIATION_COPY_NONATOMIC) {            value = objc_msgSend(value, SEL_copy);        }        auto_zone_set_associative_ref(gc_zone, object, (void *)key, value);    } else #endif    {        // Note, creates a retained reference in non-GC.        _object_set_associative_reference(object, (void *)key, value, policy);    }}PRIVATE_EXTERN void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {    // retain the new value (if any) outside the lock.    uintptr_t old_policy = 0; // NOTE:  old_policy is always assigned to when old_value is non-nil.    id new_value = value ? acquireValue(value, policy) : nil, old_value = nil;    {        // 有個管理關聯的單利        AssociationsManager manager;        AssociationsHashMap &associations(manager.associations());        if (new_value) {            // 如果有值            // break any existing association.            AssociationsHashMap::iterator i = associations.find(object);            if (i != associations.end()) {                // 如果在map裡有object的話                // secondary table exists                ObjectAssociationMap *refs = i->second;                ObjectAssociationMap::iterator j = refs->find(key);                if (j != refs->end()) {                    // 如果在ObjectAssociationMap裡沒找到key對應的值就new一個,塞進去                    ObjcAssociation &old_entry = j->second;                    old_policy = old_entry.policy;                    old_value = old_entry.value;                    old_entry.policy = policy;                    old_entry.value = new_value;                } else {                    // 找到了就直接賦值                    (*refs)[key] = ObjcAssociation(policy, new_value);                }            } else {                // 沒有object就建立                // create the new association (first time).                ObjectAssociationMap *refs = new ObjectAssociationMap;                associations[object] = refs;                (*refs)[key] = ObjcAssociation(policy, new_value);                _class_setInstancesHaveAssociatedObjects(_object_getClass(object));            }        } else {            // 沒值就清空            // setting the association to nil breaks the association.            AssociationsHashMap::iterator i = associations.find(object);            if (i !=  associations.end()) {                ObjectAssociationMap *refs = i->second;                ObjectAssociationMap::iterator j = refs->find(key);                if (j != refs->end()) {                    ObjcAssociation &old_entry = j->second;                    old_policy = old_entry.policy;                    old_value = (id) old_entry.value;                    refs->erase(j);                }            }        }    }    // release the old value (outside of the lock).    if (old_value) releaseValue(old_value, old_policy);}id objc_getAssociatedObject(id object, const void *key) {#if SUPPORT_GC    if (UseGC) {        return auto_zone_get_associative_ref(gc_zone, object, (void *)key);    } else #endif    {        return _object_get_associative_reference(object, (void *)key);    }}PRIVATE_EXTERN id _object_get_associative_reference(id object, void *key) {    id value = nil;    uintptr_t policy = OBJC_ASSOCIATION_ASSIGN;    {        AssociationsManager manager;        AssociationsHashMap &associations(manager.associations());        AssociationsHashMap::iterator i = associations.find(object);        if (i != associations.end()) {            // 取值的時候就在AssociationsHashMap裡面先找到object            ObjectAssociationMap *refs = i->second;            ObjectAssociationMap::iterator j = refs->find(key);            if (j != refs->end()) {                // 然後找到的key對應的值                 ObjcAssociation &entry = j->second;                value = (id)entry.value;                policy = entry.policy;                if (policy & OBJC_ASSOCIATION_GETTER_RETAIN) objc_msgSend(value, SEL_retain);            }        }    }    if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {        objc_msgSend(value, SEL_autorelease);    }    return value;}void *objc_destructInstance(id obj) {    if (obj) {        Class isa = _object_getClass(obj);        if (_class_hasCxxStructors(isa)) {            object_cxxDestruct(obj);        }        // 在釋放的代碼裡,釋放關聯的屬性        if (_class_instancesHaveAssociatedObjects(isa)) {            _object_remove_assocations(obj);        }        if (!UseGC) objc_clear_deallocating(obj);    }    return obj;}