runtime源码探究（五）category的加载

本文扩展自：http://blog.sunnyxx.com/2014/08/30/objc-pre-main/ 这篇文章主要描述了iOS平台上main函数调用之前所发生的事。我们从这里开始讲述category是如何加载的。

main函数开始之前，在一些准备工作之后，libSystem会调用

void _objc_init(void)

函数，这里便是runtime的入口，也就是这时候启动了runtime。苹果自己的注释也描述的很清楚

/*************************************************************
* _objc_init
* Bootstrap initialization. Registers our image notifier with dyld.
* Called by libSystem BEFORE library initialization time

---

苹果在这个函数里面做了一些初始化的工作，包括每个类+load方法的调用，关于load方法的调用顺序如下：

do {        // 1. Repeatedly call class +loads until there         aren't any more        while (loadable_classes_used > 0) {            call_class_loads();        }        // 2. Call category +loads ONCE        more_categories = call_category_loads();        // 3. Run more +loads if there are classes OR more untried categories    } while (loadable_classes_used > 0  ||  more_categories);

接下来苹果调用了map_2_images函数，函数里面在最后又调用了_read_images函数,该函数通过_objc_read_categories_from_image实现category的加载。该函数的实现如下：

static bool _objc_read_categories_from_image (header_info *  hi){    Module      mods;    size_t  midx;    bool needFlush = NO;    if (hi->info()->isReplacement()) {        // Ignore any categories in this image        return NO;    }    // Major loop - process all modules in the header    mods = hi->mod_ptr;    // NOTE: The module and category lists are traversed backwards     // to preserve the pre-10.4 processing order. Changing the order     // would have a small chance of introducing binary compatibility bugs.    midx = hi->mod_count;    while (midx-- > 0) {        unsigned int    index;        unsigned int    total;        // Nothing to do for a module without a symbol table        if (mods[midx].symtab == nil)            continue;        // Total entries in symbol table (class entries followed        // by category entries)        total = mods[midx].symtab->cls_def_cnt +            mods[midx].symtab->cat_def_cnt;        // Minor loop - register all categories from given module        index = total;        while (index-- > mods[midx].symtab->cls_def_cnt) {            old_category *cat = (old_category *)mods[midx].symtab->defs[index];            needFlush |= _objc_register_category(cat, (int)mods[midx].version);        }    }    return needFlush;}

首先判断该类是否忽略加载category，如果忽略就直接返回；然后从传入的hi（其中存储的是各个加载的模块的信息）参数中遍历要加载的category的所有模块；首先判断模块的符号表是否为空

// Nothing to do for a module without a symbol table        if (mods[midx].symtab == nil)            continue;

如果为空就不需要加载了；接下来从该模块的符号表中取出category的数量和类的数量，然后遍历该模块所有的category，调用_objc_register_category函数进行加载。该函数的实现如下：

static bool _objc_register_category(old_category *cat, int version){    _objc_unresolved_category * new_cat;    _objc_unresolved_category * old;    Class theClass;    // If the category's class exists, attach the category.    if ((theClass = objc_lookUpClass(cat->class_name))) {        return _objc_add_category_flush_caches(theClass, cat, version);    }    // If the category's class exists but is unconnected,     // then attach the category to the class but don't bother     // flushing any method caches (because they must be empty).    // YES unconnected, NO class_handler    if ((theClass = look_up_class(cat->class_name, YES, NO))) {        _objc_add_category(theClass, cat, version);        return NO;    }    // Category's class does not exist yet.     // Save the category for later attachment.    if (PrintConnecting) {        _objc_inform("CONNECT: pending category '%s (%s)'", cat->class_name, cat->category_name);    }    // Create category lookup table if needed    if (!category_hash)        category_hash = NXCreateMapTable(NXStrValueMapPrototype, 128);    // Locate an existing list of categories, if any, for the class.    old = (_objc_unresolved_category *)        NXMapGet (category_hash, cat->class_name);    // Register the category to be fixed up later.    // The category list is built backwards, and is reversed again     // by resolve_categories_for_class().    new_cat = (_objc_unresolved_category *)        malloc(sizeof(_objc_unresolved_category));    new_cat->next    = old;    new_cat->cat     = cat;    new_cat->version = version;    (void) NXMapKeyCopyingInsert (category_hash, cat->class_name, new_cat);    return NO;}

苹果对该函数也进行了说明：

/************************************************************************ _objc_register_category.* Process a category read from an image. * If the category's class exists, attach the category immediately. *   Classes that need cache flushing are marked but not flushed.* If the category's class does not exist yet, pend the category for *   later attachment. Pending categories are attached in the order *   they were discovered.* Returns YES if some method caches now need to be flushed.**********************************************************************/

如果category对应的类存在，那么就立即加载category；如果对应的类不存在，那么将category挂起等待后续加载；如果类的一些方法缓存需要刷新，但是刷新行为不在该函数中完成，该函数返回YES。
_objc_register_category函数首先去寻找category对应的类

// If the category's class exists, attach the category.    if ((theClass = objc_lookUpClass(cat->class_name))) {        return _objc_add_category_flush_caches(theClass, cat, version);    }    // If the category's class exists but is unconnected,     // then attach the category to the class but don't bother     // flushing any method caches (because they must be empty).    // YES unconnected, NO class_handler    if ((theClass = look_up_class(cat->class_name, YES, NO))) {        _objc_add_category(theClass, cat, version);        return NO;    }

如果找到了，就加载category到对应的类，如果没找到，就建立一个hash表，将这个category存起来，待之后系统去加载。
category的加载通过_objc_add_category_flush_caches函数实现：

/************************************************************************ _objc_add_category_flush_caches.  Install the specified category's * methods into the class it augments, and flush the class' method cache.* Return YES if some method caches now need to be flushed.**********************************************************************/static bool _objc_add_category_flush_caches(Class cls, old_category *category, int version){    bool needFlush = NO;    // Install the category's methods into its intended class    {        mutex_locker_t lock(methodListLock);        _objc_add_category (cls, category, version);    }    // Queue for cache flushing so category's methods can get called    if (category->instance_methods) {        cls->setInfo(CLS_FLUSH_CACHE);        needFlush = YES;    }    if (category->class_methods) {        cls->ISA()->setInfo(CLS_FLUSH_CACHE);        needFlush = YES;    }    return needFlush;}

这里面主要就是调用_objc_add_category函数，该函数的实现如下：

/************************************************************************ _objc_add_category.  Install the specified category's methods and* protocols into the class it augments.* The class is assumed not to be in use yet: no locks are taken and * no method caches are flushed.**********************************************************************/static inline void _objc_add_category(Class cls, old_category *category, int version){    if (PrintConnecting) {        _objc_inform("CONNECT: attaching category '%s (%s)'", cls->name, category->category_name);    }    // Augment instance methods    if (category->instance_methods)        _objc_insertMethods (cls, category->instance_methods, category);    // Augment class methods    if (category->class_methods)        _objc_insertMethods (cls->ISA(), category->class_methods, category);    // Augment protocols    if ((version >= 5) && category->protocols)    {        if (cls->ISA()->version >= 5)        {            category->protocols->next = cls->protocols;            cls->protocols            = category->protocols;            cls->ISA()->protocols       = category->protocols;        }        else        {            _objc_inform ("unable to add protocols from category %s...\n", category->category_name);            _objc_inform ("class `%s' must be recompiled\n", category->class_name);        }    }    // Augment instance properties    if (version >= 7  &&  category->instance_properties) {        if (cls->ISA()->version >= 6) {            _class_addProperties(cls, category->instance_properties);        } else {            _objc_inform ("unable to add instance properties from category %s...\n", category->category_name);            _objc_inform ("class `%s' must be recompiled\n", category->class_name);        }    }    // Augment class properties    if (version >= 7  &&  category->hasClassPropertiesField()  &&          category->class_properties)     {        if (cls->ISA()->version >= 6) {            _class_addProperties(cls->ISA(), category->class_properties);        } else {            _objc_inform ("unable to add class properties from category %s...\n", category->category_name);            _objc_inform ("class `%s' must be recompiled\n", category->class_name);        }    }}

这里主要是做了三件事：
1、将category的实例方法和类方法添加到类中
2、将category的protocol添加到类中
3、将category的实例对象和类对象添加到类中

至此，runtime帮助我们完成了所有的category的加载。

最后附上category的结构，其实也十分简单：

struct old_category {    char *category_name;    char *class_name;    struct old_method_list *instance_methods;    struct old_method_list *class_methods;    struct old_protocol_list *protocols;    // Fields below this point are in version 7 or later only.    uint32_t size;    struct old_property_list *instance_properties;    // Check size for fields below this point.    struct old_property_list *class_properties;    bool hasClassPropertiesField() const {         return size >= offsetof(old_category, class_properties) + sizeof(class_properties);    }};