实现过程
- 在类中声明属性，会生成setter、getter、成员变量
- 在分类中声明属性，只会生成setter个getter，不会生成成员变量
- 从分类的底层结构也可以看出，没有成员变量的属性
struct category_t {
const char *name;
classref_t cls;
struct method_list_t *instanceMethods; /// 实例方法列表
struct method_list_t *classMethods; /// 类方法列表
struct protocol_list_t *protocols; // 协议列表
struct property_list_t *instanceProperties; // 实例属性列表
// Fields below this point are not always present on disk.
struct property_list_t *_classProperties;
method_list_t *methodsForMeta(bool isMeta) {
if (isMeta) return classMethods;
else return instanceMethods;
}
property_list_t *propertiesForMeta(bool isMeta, struct header_info *hi);
};
- 但可以通过关联对象来简介实现成员变量
// 添加关联对象
void objc_setAssociatedObject(id _Nonnull object, const void * _Nonnull key,
id _Nullable value, objc_AssociationPolicy policy)
// 获取关联对象
id objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key)
// 移除所有的关联对象
void objc_removeAssociatedObjects(id _Nonnull object)
- 完整代码如下
@interface MJPerson (Associate)
@property (nonatomic, copy) NSString *nickiname;
@end
@implementation MJPerson (Associate)
- (void)setNickiname: (NSString*)nickiname {
objc_setAssociatedObject(self, @selector(nickiname), nickiname, OBJC_ASSOCIATION_COPY);
}
- (NSString*)nickiname {
return objc_getAssociatedObject(self, @selector(nickiname));
}
@end
- 关联对象所需要的key常见写法如下：
- 使用地址值作为Key
static void *MyKey = &MyKey; objc_setAssociatedObject(obj, MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, MyKey)static char MyKey; objc_setAssociatedObject(obj, &MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, &MyKey)- 使用属性名作为key
objc_setAssociatedObject(obj, @"property", value, OBJC_ASSOCIATION_RETAIN_NONATOMIC); objc_getAssociatedObject(obj, @"property");- 使用get方法的@selecor作为key
objc_setAssociatedObject(obj, @selector(getter), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, @selector(getter)) -
objc_AssociationPolicy对应的修饰符
image.png
关联对象实现属性的原理，源码解读
- 实现关联对象技术的核心对象有:
-
AssociationsManager负责处理AssociationsHashMap的存储逻辑 -
AssociationsHashMap保存程序中所有的被关联对象（键值对的形式存储） -
ObjectAssociationMap保存被关联对象的所有属性（键值对的形式存储） -
ObjcAssociation保存被关联对象的某一个属性值
-
- objc4源码解读，objc-reference.mm
class AssociationsManager { // associative references: object pointer -> PtrPtrHashMap. static AssociationsHashMap *_map; class AssociationsManager { // associative references: object pointer -> PtrPtrHashMap. static AssociationsHashMap *_map; public: AssociationsManager() { AssociationsManagerLock.lock(); } ~AssociationsManager() { AssociationsManagerLock.unlock(); } AssociationsHashMap &associations() { if (_map == NULL) _map = new AssociationsHashMap(); return *_map; } }; class AssociationsHashMap : public unordered_map<disguised_ptr_t, ObjectAssociationMap *, DisguisedPointerHash, DisguisedPointerEqual, AssociationsHashMapAllocator> { public: void *operator new(size_t n) { return ::malloc(n); } void operator delete(void *ptr) { ::free(ptr); } }; class ObjectAssociationMap : public std::map<void *, ObjcAssociation, ObjectPointerLess, ObjectAssociationMapAllocator> { public: void *operator new(size_t n) { return ::malloc(n); } void operator delete(void *ptr) { ::free(ptr); } }; class ObjcAssociation { uintptr_t _policy; id _value; public: ObjcAssociation(uintptr_t policy, id value) : _policy(policy), _value(value) {} ObjcAssociation() : _policy(0), _value(nil) {} uintptr_t policy() const { return _policy; } id value() const { return _value; } bool hasValue() { return _value != nil; } };- 关联对象并不是存储在被关联对象本身内存中
- 关联对象存储在全局的统一的一个AssociationsManager中
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
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;
// 所有被关联的对象的哈希表
AssociationsHashMap &associations(manager.associations());
// 用被关联对象作为Key
disguised_ptr_t disguised_object = DISGUISE(object);
if (new_value) {
// break any existing association.
// 找到当前被关联的对象
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) { // 如果mannager中没有设置被关联对象，则创建一个ObjectAssociationMap（哈希表），用于存储当前关联对象中所有的关联属性
// secondary table exists
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
// 如果不存在当前Key，则添加
old_association = j->second;
j->second = ObjcAssociation(policy, new_value);
} else {
// 如果存在当前key，则直接替换值
(*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);
}
关联对象的实现逻辑
