本文擴(kuò)展自：http://blog.sunnyxx.com/2014/08/30/objc-PRe-main/ 這篇文章主要描述了iOS平臺上main函數(shù)調(diào)用之前所發(fā)生的事。我們從這里開始講述category是如何加載的。
main函數(shù)開始之前，在一些準(zhǔn)備工作之后，libSystem會(huì)調(diào)用
void _objc_init(void)函數(shù)，這里便是runtime的入口，也就是這時(shí)候啟動(dòng)了runtime。蘋果自己的注釋也描述的很清楚
/************************************************************* * _objc_init * Bootstrap initialization. Registers our image notifier with dyld. * Called by libSystem BEFORE library initialization time
蘋果在這個(gè)函數(shù)里面做了一些初始化的工作，包括每個(gè)類+load方法的調(diào)用，關(guān)于load方法的調(diào)用順序如下：
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);接下來蘋果調(diào)用了map_2_images函數(shù)，函數(shù)里面在最后又調(diào)用了_read_images函數(shù),該函數(shù)通過_objc_read_categories_from_image實(shí)現(xiàn)category的加載。該函數(shù)的實(shí)現(xiàn)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;}首先判斷該類是否忽略加載category，如果忽略就直接返回；然后從傳入的hi（其中存儲的是各個(gè)加載的模塊的信息）參數(shù)中遍歷要加載的category的所有模塊；首先判斷模塊的符號表是否為空
// Nothing to do for a module without a symbol table if (mods[midx].symtab == nil) continue;如果為空就不需要加載了；接下來從該模塊的符號表中取出category的數(shù)量和類的數(shù)量，然后遍歷該模塊所有的category，調(diào)用_objc_register_category函數(shù)進(jìn)行加載。該函數(shù)的實(shí)現(xiàn)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;}蘋果對該函數(shù)也進(jìn)行了說明：
/************************************************************************ _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對應(yīng)的類存在，那么就立即加載category；如果對應(yīng)的類不存在，那么將category掛起等待后續(xù)加載；如果類的一些方法緩存需要刷新，但是刷新行為不在該函數(shù)中完成，該函數(shù)返回YES。 _objc_register_category函數(shù)首先去尋找category對應(yīng)的類
// 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，就加載category到對應(yīng)的類，如果沒找到，就建立一個(gè)hash表，將這個(gè)category存起來，待之后系統(tǒng)去加載。 category的加載通過_objc_add_category_flush_caches函數(shù)實(shí)現(xiàn)：
/************************************************************************ _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;}這里面主要就是調(diào)用_objc_add_category函數(shù)，該函數(shù)的實(shí)現(xiàn)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); } }}這里主要是做了三件事： 1、將category的實(shí)例方法和類方法添加到類中 2、將category的protocol添加到類中 3、將category的實(shí)例對象和類對象添加到類中
至此，runtime幫助我們完成了所有的category的加載。
最后附上category的結(jié)構(gòu)，其實(shí)也十分簡單：
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); }};新聞熱點(diǎn)
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