思考：如何实现给分类“添加成员变量”？
默认情况下，因为分类底层结构的限制，不能添加成员变量到分类中。但可以通过关联对象来间接实现
那如何为分类添加属性呢？
为 NSObject 添加name属性，创建NSObject的分类，使用@property给分类添加属性
@property(nonatomic,strong)NSString *name;
通过探寻Category本质了解到分类的底层结构中不包含成员变量，虽然在分类中可以写@property
添加属性，但是不会自动生成私有成员变量，也不会生成set,get方法的实现，只会生成set,get的声明，需要我们自己去实现。
方法一：我们可以通过使用静态全局变量给分类添加成员变量
static NSString *_name;
-(void)setName:(NSString *)name
{
_name = name;
}
-(NSString *)name
{
return _name;
}
但是这样_name静态全局变量与类并没有关联，无论对象创建与销毁，只要程序在运行_name变量就存在，并不是真正意义上的成员变量。
方法二：通过关联对象简接添加成员变量
-(void)setName:(NSString *)name
{
objc_setAssociatedObject(self, @"name",name, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
-(NSString *)name
{
return objc_getAssociatedObject(self, @"name");
}
关联对象提供了以下API
/*添加关联对象
* id object : 给哪个对象添加属性，这里要给自己添加属性，用self。
* const void * id key : 根据key获取关联对象的属性的值，在objc_getAssociatedObject中通过次key获得属性的值并返回。
* id value : 关联的值，也就是set方法传入的值给属性去保存。
* objc_AssociationPolicy policy : 策略，属性以什么形式保存。
*/
void objc_setAssociatedObject(id object,
const void * key,
id value,
objc_AssociationPolicy policy)
/*获得关联对象
* id object : 取哪个对象的关联属性
* const void * key：与objc_setAssociatedObject中的key相对应，即通过key值取出value
*/
id objc_getAssociatedObject(id object, const void * key)
/* 移除所有的关联对象*/
void objc_removeAssociatedObjects(id object)
key的常见用法
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))
一般都是都是使用属性名作为 key
objc_AssociationPolicy 有以下几种
objc_AssociationPolicy 策略
关联对象的原理
实现关联对象技术的核心对象有
AssociationsManager
AssociationsHashMap //Map 的结构跟字典类似
ObjectAssociationMap
ObjcAssociation
objc_setAssociatedObject函数
在 objc-runtime.mm 中找到 objc_setAssociatedObject 函数
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
可以看到其内部调用的是 _object_set_associative_reference 函数，我们来到 _object_set_associative_reference 函数中
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);
}
_object_set_associative_reference函数内部可以全部找到我们上面说过的实现关联对象技术的核心对象。接下来一个一个看其内部实现原理探寻他们之间的关系。
AssociationsManager
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;
}
};
可以看到 AssociationsManager 内部有一个 AssociationsHashMap 对象。
AssociationsHashMap
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); }
};
可以发现 AssociationsHashMap 继承于 unordered_map , 进入 unordered_map 中发现其中_Key和_Tp 这两个参数对应着map中的Key和Value，对照AssociationsHashMap内源码发现_Key中传入的是disguised_ptr_t，_Tp中传入的值则为ObjectAssociationMap*。
** ObjectAssociationMap**
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); }
};
发现ObjectAssociationMap 继承于 std:map ，点击进入map 可以发现中同样以key、Value的方式存储着ObjcAssociation，对照ObjectAssociationMap内源码可知 key 为 void* ，value 是 ObjcAssociation
ObjcAssociation
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; }
};
发现ObjcAssociation存储着_policy、_value，而这两个值我们可以发现正是我们调用objc_setAssociatedObject函数传入的值，也就是说我们在调用objc_setAssociatedObject函数中传入的value和policy这两个值最终是存储在ObjcAssociation中的。
下面我们看一下objc_setAssociatedObject函数中传入的四个参数分别放在哪个对象中充当什么作用
重新回到_object_set_associative_reference函数实现中
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);
}
细读上述源码可以发现，首先根据我们传入的value经过acquireValue函数处理获取new_value。acquireValue函数内部其实是通过对策略的判断返回不同的值
static id acquireValue(id value, uintptr_t policy) {
switch (policy & 0xFF) {
case OBJC_ASSOCIATION_SETTER_RETAIN:
return objc_retain(value);
case OBJC_ASSOCIATION_SETTER_COPY:
return ((id(*)(id, SEL))objc_msgSend)(value, SEL_copy);
}
return value;
}
之后创建AssociationsManager manager;以及拿到manager内部的AssociationsHashMap即associations。
之后我们看到了我们传入的第一个参数object
object经过DISGUISE函数被转化为了disguised_ptr_t类型的disguised_object。
typedef uintptr_t disguised_ptr_t;
inline disguised_ptr_t DISGUISE(id value) { return ~uintptr_t(value); }
inline id UNDISGUISE(disguised_ptr_t dptr) { return id(~dptr); }
DISGUISE函数其实仅仅对object做了位运算
之后我们看到被处理成new_value的value，同policy被存入了ObjcAssociation中。
而ObjcAssociation对应我们传入的key被存入了ObjectAssociationMap中。
disguised_object和ObjectAssociationMap则以key-value的形式对应存储在associations中也就是AssociationsHashMap中。
// create the new association (first time).
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
如果我们value设置为nil的话那么会执行下面的代码
// 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);
}
}
从上述代码中可以看出，如果我们设置value为nil时，就会将关联对象从ObjectAssociationMap中移除。
现在通过一张图可以很清晰的理清楚其中的关系
关联对象技术的核心对象关系
通过上图可得一个实例对象就对应一个ObjectAssociationMap，而ObjectAssociationMap中存储着多个此实例对象的关联对象的key以及ObjcAssociation，为ObjcAssociation中存储着关联对象的value和policy策略。
由此我们可以知道关联对象并不是放在了原来的对象里面，而是自己维护了一个全局的map用来存放每一个对象及其对应关联属性表格。
objc_getAssociatedObject函数
id objc_getAssociatedObject(id object, const void *key) {
return _object_get_associative_reference(object, (void *)key);
}
_object_get_associative_reference函数
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;
}
从_object_get_associative_reference函数内部可以看出，像set方法中那样，反向将value一层一层取出最后return出去。
objc_getAssociatedObject函数
objc_removeAssociatedObjects用来删除所有的关联对象
void objc_removeAssociatedObjects(id object)
{
if (object && object->hasAssociatedObjects()) {
_object_remove_assocations(object);
}
}
_object_remove_assocations
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());
}
上述源码可以看出_object_remove_assocations函数将object对象向对应的所有关联对象全部删除。
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