简介
当我们功能越来越多时，单一文件的体积就会变大，甚至臃肿。而多人配合开发一个文件时，还会出现不少冲突，这时就要用相应的解决方法，就是category。
这篇文章将会从几个方向出发，详细的了解category的实现机制。
1. category简介。
2. category与extension的区别
3. category如何加载并且添加到类
4. category对象关联
category简介
category是在2.0之后添加的特性，作用是可以为存在的文件添加方法，我们列举几个常用的场景：
1. 模拟多继承
2. 将私有API公开
3. 将一个类中的代码分散出来管理
4. 私有方法
5. ......
category有很多可以挖掘的地方，下面我们就来一步一步揭开它的面纱
category与extension的区别
extension是在编译期决定，category由运行期决定，这就是他们不同的根本之处。它决定了他们之间的分工与区别。
extension的生命周期跟随主类，用于隐藏私有信息，你必须拥有这个类的实现/源码，你才可以为它添加extension。见extension
category无法添加实例变量，在运行期间，对象内存布局已经确认，这时你无法破坏已经存在的内存空间，所以无法进行实例变量的添加。
category如何加载并且添加到类
在OC的runtime层中，都是用struct来表示，category由结构体category_t表示，(objc-runtime-new.h中可以找到)，
typedef struct category_t {
const char *name; //类名
classref_t cls; //类
struct method_list_t *instanceMethods; //添加的实例方法列表
struct method_list_t *classMethods; //添加的类方法列表
struct protocol_list_t *protocols; //添加的协议列表
struct property_list_t *instanceProperties; //添加的所有属性
} category_t;
那么category如何加载？
在OC运行时中，入口方法如下(在objc-os.mm)
void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
lock_init();
exception_init();
// Register for unmap first, in case some +load unmaps something
_dyld_register_func_for_remove_image(&unmap_image);
dyld_register_image_state_change_handler(dyld_image_state_bound,
1/*batch*/, &map_images);
dyld_register_image_state_change_handler(dyld_image_state_dependents_initialized, 0/*not batch*/, &load_images);
}
category被附加到类上面是在map_images时发送，在new-ABI的标准下，map_images最终会调用objc-runtime-new.mm文件中_read_images方法，我们来看一下：
void _read_images(header_info **hList, uint32_t hCount)
{
...
_free_internal(resolvedFutureClasses);
}
// Discover categories.
for (EACH_HEADER) {
category_t **catlist =
_getObjc2CategoryList(hi, &count);
for (i = 0; i < count; i++) {
category_t *cat = catlist[i];
Class cls = remapClass(cat->cls);
if (!cls) {
// Category's target class is missing (probably weak-linked).
// Disavow any knowledge of this category.
catlist[i] = nil;
if (PrintConnecting) {
_objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "
"missing weak-linked target class",
cat->name, cat);
}
continue;
}
// Process this category.
// First, register the category with its target class.
// Then, rebuild the class's method lists (etc) if
// the class is realized.
BOOL classExists = NO;
if (cat->instanceMethods || cat->protocols
|| cat->instanceProperties)
{
addUnattachedCategoryForClass(cat, cls, hi);
if (cls->isRealized()) {
remethodizeClass(cls);
classExists = YES;
}
if (PrintConnecting) {
_objc_inform("CLASS: found category -%s(%s) %s",
cls->nameForLogging(), cat->name,
classExists ? "on existing class" : "");
}
}
if (cat->classMethods || cat->protocols
/* || cat->classProperties */)
{
addUnattachedCategoryForClass(cat, cls->ISA(), hi);
if (cls->ISA()->isRealized()) {
remethodizeClass(cls->ISA());
}
if (PrintConnecting) {
_objc_inform("CLASS: found category +%s(%s)",
cls->nameForLogging(), cat->name);
}
}
}
}
// Category discovery MUST BE LAST to avoid potential races
// when other threads call the new category code before
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
...
}
这里我们可以看出
- 将category的实例方法、属性添加到主类中
- 将category的类方法添加到元类(metaclass)中
无论哪种情况，最后都是通过调用static void remethodizeClass(Class cls)函数来重新整理类数据。
static void remethodizeClass(class_t *cls)
{
category_list *cats;
BOOL isMeta;
rwlock_assert_writing(&runtimeLock);
isMeta = isMetaClass(cls);
// Re-methodizing: check for more categories
if ((cats = unattachedCategoriesForClass(cls))) {
chained_property_list *newproperties;
const protocol_list_t **newprotos;
if (PrintConnecting) {
_objc_inform("CLASS: attaching categories to class '%s' %s",
getName(cls), isMeta ? "(meta)" : "");
}
// Update methods, properties, protocols
BOOL vtableAffected = NO;
attachCategoryMethods(cls, cats, &vtableAffected);
newproperties = buildPropertyList(NULL, cats, isMeta);
if (newproperties) {
newproperties->next = cls->data()->properties;
cls->data()->properties = newproperties;
}
newprotos = buildProtocolList(cats, NULL, cls->data()->protocols);
if (cls->data()->protocols && cls->data()->protocols != newprotos) {
_free_internal(cls->data()->protocols);
}
cls->data()->protocols = newprotos;
_free_internal(cats);
// Update method caches and vtables
flushCaches(cls);
if (vtableAffected) flushVtables(cls);
}
}
此函数的作用是将category中的方法、属性、协议整合到主类/元类中，更新数据字段 data()中的 method_lists、properties、protocols， 对于添加实例方法，则会调用 attachCategoryMethods，
static void
attachCategoryMethods(class_t *cls, category_list *cats,
BOOL *inoutVtablesAffected)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
BOOL isMeta = isMetaClass(cls);
method_list_t **mlists = (method_list_t **)
_malloc_internal(cats->count * sizeof(*mlists));
// Count backwards through cats to get newest categories first
int mcount = 0;
int i = cats->count;
BOOL fromBundle = NO;
while (i--) {
method_list_t *mlist = cat_method_list(cats->list[i].cat, isMeta);
if (mlist) {
mlists[mcount++] = mlist;
fromBundle |= cats->list[i].fromBundle;
}
}
attachMethodLists(cls, mlists, mcount, NO, fromBundle, inoutVtablesAffected);
_free_internal(mlists);
}
它的工作可以看成将category的实例方法列表拼成一个实例方法列表，然后调用attachMethodLists进行处理。
for (uint32_t m = 0;
(scanForCustomRR || scanForCustomAWZ) && m < mlist->count;
m++)
{
SEL sel = method_list_nth(mlist, m)->name;
if (scanForCustomRR && isRRSelector(sel)) {
cls->setHasCustomRR();
scanForCustomRR = false;
} else if (scanForCustomAWZ && isAWZSelector(sel)) {
cls->setHasCustomAWZ();
scanForCustomAWZ = false;
}
}
// Fill method list array
newLists[newCount++] = mlist;
.
.
.
// Copy old methods to the method list array
for (i = 0; i < oldCount; i++) {
newLists[newCount++] = oldLists[i];
}
这里我们需要注意，category并没有完全的替换掉原有类的同名方法，category的方法被放置在新方法列表的前面，而原来类的方法被放到后面，在runtime中，遍历方法列表查找时，找到了category的方法后，就会停止遍历，这就是我们平时所说的“覆盖”方法。
找到源方法很简单，只需要遍历方法列表，找到最后的一个对应名字方法即可，(摘自：美团技术博客)
Class currentClass = [MyClass class];
MyClass *my = [[MyClass alloc] init];
if (currentClass) {
unsigned int methodCount;
Method *methodList = class_copyMethodList(currentClass, &methodCount);
IMP lastImp = NULL;
SEL lastSel = NULL;
for (NSInteger i = 0; i < methodCount; i++) {
Method method = methodList[i];
NSString *methodName = [NSString stringWithCString:sel_getName(method_getName(method))
encoding:NSUTF8StringEncoding];
if ([@"printName" isEqualToString:methodName]) {
lastImp = method_getImplementation(method);
lastSel = method_getName(method);
}
}
typedef void (*fn)(id,SEL);
if (lastImp != NULL) {
fn f = (fn)lastImp;
f(my,lastSel);
}
free(methodList);
}
category对象关联
那么当在 category 中使用对象关联，那么相应的存储位置，生命周期是怎么样的？
去探索一下源码，在objc-references.mm文件中void _object_set_associative_reference方法
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);
}
我们可以看出，关联对象是由AssociationsManager管理，那么它是什么呢？
/ class AssociationsManager manages a lock / hash table singleton pair.
// Allocating an instance acquires the lock, and calling its assocations()
// method lazily allocates the hash table.
spinlock_t AssociationsManagerLock;
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 *AssociationsManager::_map = NULL;
AssociationsManager 是一个静态的全局 AssociationsHashMap，用来存储所有的关联对象，key是对象的内存地址，value则是另一个 AssociationsHashMap，其中存储了关联对象的kv，对象销毁的工作则交给 objc_destructInstance
void *objc_destructInstance(id obj)
{
if (obj) {
Class isa_gen = _object_getClass(obj);
class_t *isa = newcls(isa_gen);
// Read all of the flags at once for performance.
bool cxx = hasCxxStructors(isa);
bool assoc = !UseGC && _class_instancesHaveAssociatedObjects(isa_gen);
// This order is important.
if (cxx) object_cxxDestruct(obj);
if (assoc) _object_remove_assocations(obj);
if (!UseGC) objc_clear_deallocating(obj);
}
return obj;
}
如有错误请指正~
参考链接：
https://tech.meituan.com/DiveIntoCategory.html
http://blog.leichunfeng.com/blog/2015/05/18/objective-c-category-implementation-principle/
https://developer.apple.com/library/ios/documentation/General/Conceptual/DevPedia-CocoaCore/Category.html#//apple_ref/doc/uid/TP40008195-CH5-SW1 http://stackoverflow.com/questions/5272451/overriding-methods-using-categories-in-objective-c
