当一个东西你只会用而不清除底层的怎么实现的时候,往往在别人眼里,会很菜吧。
objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy)
id objc_getAssociatedObject(id object, const void *key)
我们知道category分类不能添加属性,所以如果要为分类添加属性,那么就要采取这种关联对象的方式。
但是曾经有人问过我这么一个问题,还有没有别的方法可以实现同样的功能。
开胃菜
利用NSMutableDictionary来为分类添加属性。
基本原理:在分类中针对每一个属性写一个全局的NSMutableDictionary,让它一直存在而不会被释放掉,然后以当前类的self地址作为k,分别存储这些对象。
#import "XHPerson.h"
@interface XHPerson (Test)
@property (assign, nonatomic) int weight;
@property (copy, nonatomic) NSString* name;
@end
#import "XHPerson+Test.h"
#define XHKey [NSString stringWithFormat:@"%p", self]
@implementation XHPerson (Test)
NSMutableDictionary *names_;
NSMutableDictionary *weights_;
+ (void)load
{
weights_ = [NSMutableDictionary dictionary];
names_ = [NSMutableDictionary dictionary];
}
- (void)setName:(NSString *)name
{
// NSString *key = [NSString stringWithFormat:@"%p", self];
names_[XHKey] = name;
}
- (NSString *)name
{
// NSString *key = [NSString stringWithFormat:@"%p", self];
return names_[XHKey];
}
- (void)setWeight:(int)weight
{
// NSString *key = [NSString stringWithFormat:@"%p", self];
weights_[XHKey] = @(weight);
}
- (int)weight
{
// NSString *key = [NSString stringWithFormat:@"%p", self];
return [weights_[XHKey] intValue];
}
@end
上面的方法就是具体的实现,当然这只是一个简化版,还存在一些问题,比如说线程安全方面的问题。
objc_setAssociatedObject的底层
我们直奔源码
objc_setAssociatedObject函数
四个参数分别是:需要关联的对象,指针,关联的属性,关联协议
void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
_object_set_associative_reference(object, (void *)key, value, policy);
}
_object_set_associative_reference函数
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);
}
通过上面的函数,我们可以看到这么几个东西 AssociationsManager AssociationsHashMap ObjectAssociationMap ObjcAssociation 先来看下分别是什么。
AssociationsManager 可以看到里面包含了一个AssociationsHashMap
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;
}
};
AssociationsHashMap 可以看到 key是disguised_ptr_t value 是ObjectAssociationMap
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); }
};
*ObjectAssociationMap 可以看到key是 void 的指针,value是ObjcAssociation
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); }
};
** ObjcAssociation _policy关联的协议 和 value关联的对象属性**
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; }
};
他们的关系可以用下面这个图来表示

上面看完objc_setAssociatedObject的源码,那么下面再来看objc_getAssociatedObject,objc_removeAssociatedObjects的源码
objc_getAssociatedObject
id objc_getAssociatedObject(id object, const void *key) {
return _object_get_associative_reference(object, (void *)key);
}
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;
}
objc_removeAssociatedObjects源码
void objc_removeAssociatedObjects(id object)
{
if (object && object->hasAssociatedObjects()) {
_object_remove_assocations(object);
}
}
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());
}
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