iOS中的Category详解

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

  1. 将category的实例方法、属性添加到主类中
  2. 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

这里可以看到源码

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