Category的底层结构
定义在objc-runtime-new.h中
struct category_t {
const char *name;
classref_t cls;
struct method_list_t *instanceMethods;
struct method_list_t *classMethods;
struct protocol_list_t *protocols;
struct property_list_t *instanceProperties;
// 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);
};
Category的加载处理过程
1.通过Runtime加载某个类的所有Category数据
2.把所有Category的方法、属性、协议数据，合并到一个大数组中
后面参与编译的Category数据，会在数组的前面
3.将合并后的分类数据（方法、属性、协议），插入到类原来数据的前面
// Attach method lists and properties and protocols from categories to a class.
// Assumes the categories in cats are all loaded and sorted by load order,
// oldest categories first.
static void
attachCategories(Class cls, category_list *cats, bool flush_caches)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
bool isMeta = cls->isMetaClass();
// fixme rearrange to remove these intermediate allocations
method_list_t **mlists = (method_list_t **)
malloc(cats->count * sizeof(*mlists));
property_list_t **proplists = (property_list_t **)
malloc(cats->count * sizeof(*proplists));
protocol_list_t **protolists = (protocol_list_t **)
malloc(cats->count * sizeof(*protolists));
// Count backwards through cats to get newest categories first
int mcount = 0;
int propcount = 0;
int protocount = 0;
int i = cats->count;
bool fromBundle = NO;
while (i--) {
auto& entry = cats->list[I];
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
mlists[mcount++] = mlist;
fromBundle |= entry.hi->isBundle();
}
property_list_t *proplist =
entry.cat->propertiesForMeta(isMeta, entry.hi);
if (proplist) {
proplists[propcount++] = proplist;
}
protocol_list_t *protolist = entry.cat->protocols;
if (protolist) {
protolists[protocount++] = protolist;
}
}
//得到类对象里面的数据
auto rw = cls->data();
//将所有分类的对象方法，附加到类对象的方法列表中
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
rw->methods.attachLists(mlists, mcount);
free(mlists);
if (flush_caches && mcount > 0) flushCaches(cls);
//将所有分类的属性，附加到类对象的属性列表中
rw->properties.attachLists(proplists, propcount);
free(proplists);
//将所有分类的协议，附加到类对象的协议列表中
rw->protocols.attachLists(protolists, protocount);
free(protolists);
}
void attachLists(List* const * addedLists, uint32_t addedCount) {
if (addedCount == 0) return;
if (hasArray()) {
// many lists -> many lists
uint32_t oldCount = array()->count;
uint32_t newCount = oldCount + addedCount;
setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
array()->count = newCount;
//array()->lists 原来的方法列表
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
else if (!list && addedCount == 1) {
// 0 lists -> 1 list
list = addedLists[0];
}
else {
// 1 list -> many lists
List* oldList = list;
uint32_t oldCount = oldList ? 1 : 0;
uint32_t newCount = oldCount + addedCount;
setArray((array_t *)malloc(array_t::byteSize(newCount)));
array()->count = newCount;
if (oldList) array()->lists[addedCount] = oldList;
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
}
+load方法
+load方法会在runtime加载类、分类时调用
每个类、分类的+load，在程序运行过程中只调用一次
调用顺序
先调用类的+load
按照编译先后顺序调用（先编译，先调用）
调用子类的+load之前会先调用父类的+load
再调用分类的+load
按照编译先后顺序调用（先编译，先调用）
objc4源码解读过程：objc-os.mm
_objc_init
load_images
prepare_load_methods
schedule_class_load
add_class_to_loadable_list
add_category_to_loadable_list
call_load_methods
call_class_loads
call_category_loads
(*load_method)(cls, SEL_load)
+load方法是根据方法地址直接调用，并不是经过objc_msgSend函数调用
+initialize方法
+initialize方法会在类第一次接收到消息时调用
调用顺序
先调用父类的+initialize，再调用子类的+initialize
(先初始化父类，再初始化子类，每个类只会初始化1次)
objc4源码解读过程
objc-msg-arm64.s
objc_msgSend
objc-runtime-new.mm
class_getInstanceMethod
lookUpImpOrNil
lookUpImpOrForward
_class_initialize
callInitialize
objc_msgSend(cls, SEL_initialize)
+initialize和+load的很大区别是，+initialize是通过objc_msgSend进行调用的，所以有以下特点
如果子类没有实现+initialize，会调用父类的+initialize（所以父类的+initialize可能会被调用多次）
如果分类实现了+initialize，就覆盖类本身的+initialize调用
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
关联对象提供了以下API
添加关联对象
void objc_setAssociatedObject(id object, const void * key,
id value, objc_AssociationPolicy policy)
获得关联对象
id objc_getAssociatedObject(id object, const void * key)
移除所有的关联对象
void objc_removeAssociatedObjects(id object)
void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
// This code used to work when nil was passed for object and key. Some code
// probably relies on that to not crash. Check and handle it explicitly.
// rdar://problem/44094390
if (!object && !value) return;
assert(object);
if (object->getIsa()->forbidsAssociatedObjects())
_objc_fatal("objc_setAssociatedObject called on instance (%p) of class %s which does not allow associated objects", object, object_getClassName(object));
// 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);
}
通过上面的源码我们可以总结一下：
实现关联对象技术的核心对象有
AssociationsManager
AssociationsHashMap
ObjectAssociationMap
ObjcAssociation
objc4源码解读：objc-references.mm
image.png
关联对象的原理
image.png
void objc_setAssociatedObject(id object, const void * key,
id value,
objc_AssociationPolicy policy)
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
