分类中不能添加成员变量
默认情况下，因为分类底层结构的限制，分类中不能添加成员变量。如果强行添加会报错“Instance variables may not be placed in categories实例变量不能放在类别中实例变量不能放在类别中”如下图
image.png
如果通过@property (assign, nonatomic) int a;不会添加成员变量，只会生成(没有实现设值和取值)的setter和getter方法.
通过关联对象变相的给分类“添加成员变量”
设置关联对象
objc_setAssociatedObject(id object, const void * key, id value, objc_AssociationPolicy policy)
获取关联对象
objc_getAssociatedObject(id object, const void * key)
移除类所有的关联对象
objc_removeAssociatedObjects(id object)
key的常见用法,保证key的唯一性
取唯一的指针地址值 static void *MyKey = &MyKey; objc_setAssociatedObject(obj, MyKey, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, MyKey) static NSString *MyKey1; objc_setAssociatedObject(obj, &MyKey1, value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, &MyKey1) 使用属性名作为key objc_setAssociatedObject(obj, @"property", value, OBJC_ASSOCIATION_RETAIN_NONATOMIC); objc_getAssociatedObject(obj, @"property"); 使用get方法的方法编号@selecor作为key objc_setAssociatedObject(obj, @selector(getter), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC) objc_getAssociatedObject(obj, @selector(getter))
objc_AssociationPolicy
OBJC_ASSOCIATION_ASSIGN//assign OBJC_ASSOCIATION_RETAIN_NONATOMIC//strong,nonatomic OBJC_ASSOCIATION_COPY_NONATOMIC//copy, nonatomic OBJC_ASSOCIATION_RETAIN//strong,atomic OBJC_ASSOCIATION_COPY//copy,atomic
关联对象的原理
objc4源码解读`objc_setAssociatedObject(id object, const void * key, id value, objc_AssociationPolicy policy)
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) { // 初始化第三个表。 ObjcAssociation old_association(0, nil); id new_value = value ? acquireValue(value, policy) : nil; { AssociationsManager manager; 第一个表AssociationsHashMap AssociationsHashMap &associations(manager.associations()); disguised_ptr_t disguised_object = DISGUISE(object);//object作为key if (new_value) { //通过disguised_object取value值ObjectAssociationMap AssociationsHashMap::iterator i = associations.find(disguised_object); if (i != associations.end()) {//i不是最后一个 // 第二个表ObjectAssociationMap类表 ObjectAssociationMap *refs = i->second; //ObjectAssociationMap对象表中通过key取出value值ObjcAssociation ObjectAssociationMap::iterator j = refs->find(key); if (j != refs->end()) {//j不是最后一个 //第三个表，ObjcAssociation类表 old_association = j->second; //第三个表设置新值 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 { // 设置关联为nil会移除关联值。 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);//擦除 } } } } // release the old value (outside of the lock). if (old_association.hasValue()) ReleaseValue()(old_association); }
实现关联对象技术的核心对象有
AssociationsManager
AssociationsHashMap//类表1
ObjectAssociationMap//类表2
ObjcAssociatio//类表3
class AssociationsManager { static AssociationsHashMap *_map }; 类表1 class AssociationsHashMap : public unordered_map<disguised_ptr_t(key), ObjectAssociationMap *(value)> 类表2 class ObjectAssociationMap : public std::map<void *(key), ObjcAssociation(value)> 类表3 class ObjcAssociation { uintptr_t _policy;//key id _value;//value };
objc4源码解读：objc-references.mm
原理图解：
image.png
