关联对象的用途
在Category中为已经注册的类增加存储字段，模拟实例变量。
@implementation GPBChatGroupInfoForList (Extension)
-(NSNumber*)isStick{
return objc_getAssociatedObject(self, @selector(isStick));
}
-(void)setIsStick:(NSNumber*)isStick{
objc_setAssociatedObject(self, @selector(isStick),isStick, OBJC_ASSOCIATION_RETAIN);
}
@end
关联对象存储原理
所有的关联对象都由一个全局的AssociationsManager管理，AssociationsManager里面是由一个静态AssociationsHashMap来存储所有的关联对象的。这相当于把所有对象的关联对象都存在一个全局map里面。而map的的key是这个对象的指针地址（任意两个不同对象的指针地址一定是不同的），而这个map的value又是另外一个AssociationsHashMap，里面保存了关联对象的kv对。
id objc_getAssociatedObject(id object, const void *key) {
return _object_get_associative_reference(object, (void *)key);
}
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
void objc_removeAssociatedObjects(id object)
{
if (object && object->hasAssociatedObjects()) {
_object_remove_assocations(object);
}
}
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) ((id(*)(id, SEL))objc_msgSend)(value, SEL_retain);
}
}
}
if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
((id(*)(id, SEL))objc_msgSend)(value, SEL_autorelease);
}
return value;
}
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);
if (new_value) {
// break any existing association.
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
// secondary table exists
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
old_association = j->second;
j->second = ObjcAssociation(policy, new_value);
} else {
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {
// create the new association (first time).
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
} else {
// 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);
}
void _object_remove_assocations(id object) {
vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;
{
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
if (associations.size() == 0) return;
disguised_ptr_t disguised_object = DISGUISE(object);
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
// copy all of the associations that need to be removed.
ObjectAssociationMap *refs = i->second;
for (ObjectAssociationMap::iterator j = refs->begin(), end = refs->end(); j != end; ++j) {
elements.push_back(j->second);
}
// remove the secondary table.
delete refs;
associations.erase(i);
}
}
// the calls to releaseValue() happen outside of the lock.
for_each(elements.begin(), elements.end(), ReleaseValue());
}
关联对象如何实现weak存储
runtime暴露的添加关联对象接口中不能指定weak存储类型
typedef OBJC_ENUM(uintptr_t, objc_AssociationPolicy) {
OBJC_ASSOCIATION_ASSIGN = 0,
OBJC_ASSOCIATION_RETAIN_NONATOMIC = 1,
OBJC_ASSOCIATION_COPY_NONATOMIC = 3,
OBJC_ASSOCIATION_RETAIN = 01401,
OBJC_ASSOCIATION_COPY = 01403
};
那我们如何自己实现weak效果呢？
思路是这样的，虽然runtime没有开放weak解决方案，但OBJC对象是可以实现weak的，所以让需要被修饰的对象去持有一个strong的对象，然后当被修饰的对象被释放的时候，持有的对象也会被释放，那么我们就可以捕捉到释放的事件，进而使用OBJC_ASSOCIATION_ASSIGN 来实现弱引用，在释放事件里面再将其释放掉，进而实现weak功能。
// 定义一个对象，使用block来回调析构函数。
typedef void (^DeallocBlock)();
@interface OriginalObject : NSObject
@property (nonatomic, copy) DeallocBlock block;
- (instancetype)initWithBlock:(DeallocBlock)block;
@end
@implementation OriginalObject
- (instancetype)initWithBlock:(DeallocBlock)block{
self = [super init];
if (self) {
self.block = block;
}
return self;
}
- (void)dealloc {
self.block ? self.block() : nil;
}
@end
@interface NSObject (property)
@property (nonatomic, weak) id objc_weak_id;
@end
@implementation NSObject (property)
- (id)objc_weak_id {
return objc_getAssociatedObject(self, _cmd);
}
- (void)setObjc_weak_id:(id)objc_weak_id {
OriginalObject *ob = [[OriginalObject alloc] initWithBlock:^{
objc_setAssociatedObject(self, @selector(objc_weak_id), nil, OBJC_ASSOCIATION_ASSIGN);
}];
// 这里关联的key必须唯一，如果使用_cmd，对一个对象多次关联的时候，前面的对象关联会失效。
objc_setAssociatedObject(objc_weak_id, (__bridge const void *)(ob.block), ob, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
objc_setAssociatedObject(self, @selector(objc_weak_id), objc_weak_id, OBJC_ASSOCIATION_ASSIGN);
}
@end
关联对象需要手动清理吗？
不需要手动清理，runtime的销毁对象函数objc_destructInstance里面会判断这个对象有没有关联对象，如果有，会调用_object_remove_assocations做关联对象的清理工作。
- (void)dealloc {
_objc_rootDealloc(self);
}
void _objc_rootDealloc(id obj)
{
assert(obj);
obj->rootDealloc();
}
inline void objc_object::rootDealloc()
{
if (isTaggedPointer()) return;
object_dispose((id)this);
}
id object_dispose(id obj)
{
if (!obj) return nil;
objc_destructInstance(obj);
free(obj);
return nil;
}
void *objc_destructInstance(id obj)
{
if (obj) {
Class isa = obj->getIsa();
if (isa->hasCxxDtor()) {
object_cxxDestruct(obj);
}
if (isa->instancesHaveAssociatedObjects()) {
_object_remove_assocations(obj);
}
objc_clear_deallocating(obj);
}
return obj;
}
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