Category使用关联对象生成属性的原理
Category关联对象：
main.m文件：
#import
#import "NSString+Category.h"int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
NSLog(@"Hello, World!");
NSString *string = [NSString new];
string.newString = @"newString";
NSLog(@"%@", string.newString);
}
return 0;
}
NSString+Category.h文件：
#import NS_ASSUME_NONNULL_BEGIN@interface NSString (Category)
@property (nonatomic, copy) NSString *newString;
@endNS_ASSUME_NONNULL_END
NSString+Category.m文件:
#import "NSString+Category.h"
#import @implementation NSString (Category)
- (NSString *)newString {
return objc_getAssociatedObject(self, @"newString");
}
- (void)setNewString:(NSString *)newString {
objc_setAssociatedObject(self, @"newString", newString, OBJC_ASSOCIATION_COPY);
}
@end
通过关联对象的方式在运行添加处为类别来添加实例对象、getter和setter方法。
接下来从底层原理看一下，它是如何做到这一点的。
在runtime.h文件中第1658行
/**
* Sets an associated value for a given object using a given key and association policy.
*
* @param object The source object for the association.
* @param key The key for the association.
* @param value The value to associate with the key key for object. Pass nil to clear an existing association.
* @param policy The policy for the association. For possible values, see “Associative Object Behaviors.”
*
* @see objc_setAssociatedObject
* @see objc_removeAssociatedObjects
*/
OBJC_EXPORT void
objc_setAssociatedObject(id _Nonnull object, const void * _Nonnull key,
id _Nullable value, objc_AssociationPolicy policy)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);
/**
* Returns the value associated with a given object for a given key.
*
* @param object The source object for the association.
* @param key The key for the association.
*
* @return The value associated with the key \e key for \e object.
*
* @see objc_setAssociatedObject
*/
OBJC_EXPORT id _Nullable
objc_getAssociatedObject(id _Nonnull object, const void * _Nonnull key)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);
/**
* Removes all associations for a given object.
*
* @param object An object that maintains associated objects.
*
* @note The main purpose of this function is to make it easy to return an object
*to a "pristine state”. You should not use this function for general removal of
*associations from objects, since it also removes associations that other clients
*may have added to the object. Typically you should use \c objc_setAssociatedObject
*with a nil value to clear an association.
*
* @see objc_setAssociatedObject
* @see objc_getAssociatedObject
*/
OBJC_EXPORT void
objc_removeAssociatedObjects(id _Nonnull object)
OBJC_AVAILABLE(10.6, 3.1, 9.0, 1.0, 2.0);
从这里能看到函数的定义和函数注释，接下来我们看看函数的实现：
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);
}
}
我们先来看一下set方法，objc_setAssociatedObject这个方法调用的是_object_set_associative_reference，以下是_object_set_associative_reference的源代码实现:
_object_set_associative_reference
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 = https://www.it610.com/article/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);
}
【Category使用关联对象生成属性的原理】我们来看个关键的地方：
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);
}
}
}
从源代码中看，有个地方不太好理解：
AssociationsManager manager;
AssociationsHashMap &associations(manager.associations());
每次函数判断完是否有新值之后，都会实例化一个manager，那么是怎么管理设置进入的属性呢，因为AssociationsHashMap的获取是一个单例对象，这样就可以管理以往加入的属性。
接下来看一下源码中，关联关系的几个类：
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;
}
};
在associations函数中可以看到那个单例HashMap, 还有AssociationsManagerLock这个自旋锁，用来保证线程安全。
AssociationsHashMap
class AssociationsHashMap : public unordered_map {
public:
void *operator new(size_t n) { return ::malloc(n);
}
void operator delete(void *ptr) { ::free(ptr);
}
};
用来存储disguised_ptr_t和ObjectAssociationMap之间的映射。
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;
}
};
这里有两个非常重要的变量：_policy和_value
_policy:用来存储关联的策略。例如OBJC_ASSOCIATION_COPY
_value:用来存储关联的对象。
根据以上几个类，我们就可以大概分析出_object_set_associative_reference的源代码流程,下图借鉴与此：http://blog.leichunfeng.com/blog/2015/06/26/objective-c-associated-objects-implementation-principle/
文章图片
image.png
接下来，继续看_object_set_associative_reference源代码：
_object_get_associative_reference
id _object_get_associative_reference(id object, void *key) {
id value = https://www.it610.com/article/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 = https://www.it610.com/article/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;
}
通过分析过set函数之后，get函数逻辑就很简单了，首先实例一个AssociationsManager对象，然后去获取AssociationsHashMap对象，在通过AssociationsHashMap找到对应的ObjecAssociation,最后再从ObjcAssociation中取出对应的值，如果没有对应的关联的对象，那么返回nil。
还有一个函数，但是这个函数一般情况下，我们用不到，即使用不到，理解其源码，也能帮我们更好的理解其他相关联的源代码。
objc_removeAssociatedObjects
void objc_removeAssociatedObjects(id object)
{
if (object && object->hasAssociatedObjects()) {
_object_remove_assocations(object);
}
}
首先判断对象是否存在并且判断对象是否有没有associatedObject,具体hasAssociatedObjects实现原理，可以查看这个文章：https://juejin.im/post/5cf4875651882544171c60d9
如果通过if判断之后，就进入了_object_remove_assocations函数：
void _object_remove_assocations(id object) {
vector< ObjcAssociation,ObjcAllocator > 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());
}
跟以上两函数一样，但是一开始要创建一个vector，这个在后面游泳，接下来，实例一个AssociationsManager对象，通过AssociationsManager对象获取到AssociationsHashMap单例对象，通过对象获取disguised_ptr_t的值，然后在AssociationsHashMap中通过disguised_ptr_t对象找到ObjectAssociationMap，然后遍历ObjectAssociationMap，通过每个元素，找到最终的对象值。然后放到刚开始创建的vector里面，然后释放ObjectAssociationMap对象。释放完ObjectAssociationMap对象之后，将关联关系从associations中解除。最后把vector里面的对象都释放掉。
到此为止，Category关联对象生成属性的原理就基本分析完了，下面提供一个关联策略表格：
| 关联策略 | 等价属性 | 说明 |
|---|---|---|
| OBJC_ASSOCIATION_ASSIGN | @property(assign) | 弱引用关联对象 |
| OBJC_ASSOCIATION_RETAIN_NONATOMIC | @property(strong, nonatomic) | 非原子操作的强引用关联对象 |
| OBJC_ASSOCIATION_RETAIN | @property(strong, atomic) | 原子操作的强引用关联对象 |
| OBJC_ASSOCIATION_COPY | @property(copy, atomic) | 原子操作的复制关联对象 |
| OBJC_ASSOCIATION_COPY_NONATOMIC | @property(copy, nonatomic) | 非原子操作的复制关联对象 |
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