在objc_init方法中設(shè)置了load_images回調(diào)

void _objc_init(void)
{
···
    _dyld_objc_notify_register(&map_images, load_images, unmap_image);
···
}

load_images

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    if (!didInitialAttachCategories && didCallDyldNotifyRegister) {
        didInitialAttachCategories = true;
// 加載分類
        loadAllCategories();
    }

    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        mutex_locker_t lock2(runtimeLock);
// 準(zhǔn)備調(diào)用load方法,會(huì)把類的load方法和分類的load方法存儲(chǔ)起來(lái)
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
// 調(diào)用load方法
    call_load_methods();
}

添加load方法到集合

prepare_load_methods

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, i;

    runtimeLock.assertLocked();

    classref_t const *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i < count; i++) {
// 增加類的load方法
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        if (cls->isSwiftStable()) {
            _objc_fatal("Swift class extensions and categories on Swift "
                        "classes are not allowed to have +load methods");
        }
        realizeClassWithoutSwift(cls, nil);
        ASSERT(cls->ISA()->isRealized());
// 增加分類的load方法
        add_category_to_loadable_list(cat);
    }
}

schedule_class_load

static void schedule_class_load(Class cls)
{
    if (!cls) return;
    ASSERT(cls->isRealized());  // _read_images should realize

// 判斷是否需要添加
    if (cls->data()->flags & RW_LOADED) return;

    // 遞歸添加父類
    schedule_class_load(cls->getSuperclass());
    // 添加類的load方法
    add_class_to_loadable_list(cls);
// 設(shè)置標(biāo)識(shí),防止重復(fù)添加
    cls->setInfo(RW_LOADED); 
}

add_class_to_loadable_list

void add_class_to_loadable_list(Class cls)
{
    IMP method;

    loadMethodLock.assertLocked();
// 通過(guò)ro列表獲取類的load方法
    method = cls->getLoadMethod();
    if (!method) return;  // Don't bother if cls has no +load method
    
    if (PrintLoading) {
        _objc_inform("LOAD: class '%s' scheduled for +load", 
                     cls->nameForLogging());
    }
    
    if (loadable_classes_used == loadable_classes_allocated) {
        loadable_classes_allocated = loadable_classes_allocated*2 + 16;
        loadable_classes = (struct loadable_class *)
            realloc(loadable_classes,
                              loadable_classes_allocated *
                              sizeof(struct loadable_class));
    }
    // 添加load方法
    loadable_classes[loadable_classes_used].cls = cls;
    loadable_classes[loadable_classes_used].method = method;
    loadable_classes_used++;
}

add_category_to_loadable_list 添加分類的load方法

void add_category_to_loadable_list(Category cat)
{
    IMP method;

    loadMethodLock.assertLocked();
// 獲取分類的load方法
    method = _category_getLoadMethod(cat);

    // Don't bother if cat has no +load method
    if (!method) return;

    if (PrintLoading) {
        _objc_inform("LOAD: category '%s(%s)' scheduled for +load", 
                     _category_getClassName(cat), _category_getName(cat));
    }
    
    if (loadable_categories_used == loadable_categories_allocated) {
        loadable_categories_allocated = loadable_categories_allocated*2 + 16;
        loadable_categories = (struct loadable_category *)
            realloc(loadable_categories,
                              loadable_categories_allocated *
                              sizeof(struct loadable_category));
    }
// 增加分類的load方法
    loadable_categories[loadable_categories_used].cat = cat;
    loadable_categories[loadable_categories_used].method = method;
    loadable_categories_used++;
}

調(diào)用load方法

void call_load_methods(void)
{
    static bool loading = NO;
    bool more_categories;

    loadMethodLock.assertLocked();

    // Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

    void *pool = objc_autoreleasePoolPush();

    do {
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
    //調(diào)用類的load方法
            call_class_loads();
        }

        // 2. Call category +loads ONCE
// 調(diào)用分類的load方法
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    objc_autoreleasePoolPop(pool);

    loading = NO;
}

call_class_loads 調(diào)用類的load方法

static void call_class_loads(void)
{
    int i;
    
    // Detach current loadable list.
    struct loadable_class *classes = loadable_classes;
    int used = loadable_classes_used;
    loadable_classes = nil;
    loadable_classes_allocated = 0;
    loadable_classes_used = 0;
    
    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        load_method_t load_method = (load_method_t)classes[i].method;
        if (!cls) continue; 

        if (PrintLoading) {
            _objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
        }
// 調(diào)用方法
        (*load_method)(cls, @selector(load));
    }
    
    // Destroy the detached list.
    if (classes) free(classes);
}

call_category_loads 調(diào)用分類的load方法

static bool call_category_loads(void)
{
    int i, shift;
    bool new_categories_added = NO;
    
    // Detach current loadable list.
    struct loadable_category *cats = loadable_categories;
    int used = loadable_categories_used;
    int allocated = loadable_categories_allocated;
    loadable_categories = nil;
    loadable_categories_allocated = 0;
    loadable_categories_used = 0;

    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Category cat = cats[i].cat;
        load_method_t load_method = (load_method_t)cats[i].method;
        Class cls;
        if (!cat) continue;

        cls = _category_getClass(cat);
        if (cls  &&  cls->isLoadable()) {
            if (PrintLoading) {
                _objc_inform("LOAD: +[%s(%s) load]\n", 
                             cls->nameForLogging(), 
                             _category_getName(cat));
            }
// 調(diào)用方法
            (*load_method)(cls, @selector(load));
            cats[i].cat = nil;
        }
    }

    // Compact detached list (order-preserving)
// 刪除空的分類
    shift = 0;
    for (i = 0; i < used; i++) {
        if (cats[i].cat) {
            cats[i-shift] = cats[i];
        } else {
            shift++;
        }
    }
    used -= shift;

    // Copy any new +load candidates from the new list to the detached list.
// 新增加的  分類列表
    new_categories_added = (loadable_categories_used > 0);
    for (i = 0; i < loadable_categories_used; i++) {
        if (used == allocated) {
            allocated = allocated*2 + 16;
            cats = (struct loadable_category *)
                realloc(cats, allocated *
                                  sizeof(struct loadable_category));
        }
        cats[used++] = loadable_categories[i];
    }

    // 銷毀新增加的分類列表
    if (loadable_categories) free(loadable_categories);

    // Reattach the (now augmented) detached list. 
    // But if there's nothing left to load, destroy the list.
    if (used) { // 有使用的分類
        loadable_categories = cats;
        loadable_categories_used = used;
        loadable_categories_allocated = allocated;
    } else {
        if (cats) free(cats);
        loadable_categories = nil;
        loadable_categories_used = 0;
        loadable_categories_allocated = 0;
    }
···
// 返回在調(diào)用過(guò)程中是否有新增加的分類
    return new_categories_added;
}