流程分析

收到回調(diào)

    _dyld_objc_notify_register(&map_images, load_images, unmap_image);

用到的數(shù)據(jù)結構：

struct loadable_class {
    Class cls;  // may be nil
    IMP method;
};


// List of classes that need +load called (pending superclass +load)
// This list always has superclasses first because of the way it is constructed
static struct loadable_class *loadable_classes = nil;
static int loadable_classes_used = 0;
static int loadable_classes_allocated = 0;

• 當自定義類，沒有實現(xiàn)load方法，而只有它的分類實現(xiàn)，那么不會加入到_objc_catlist數(shù)據(jù)段中
• 當自定義類和分類都實現(xiàn)了load方法，分類會加入到_objc_catlist數(shù)據(jù)段中

read_image會將nlclslist里的類進行realizeClassWithoutSwift。這樣如果一個自定義類，實現(xiàn)了load方法，在鏡像映射完成的時候，也就被填充了內(nèi)部信息。（只是類，不是分類這一步）
load_image會首先加載共享緩存中的分類
然后，準備那些實現(xiàn)了load方法的類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++) {
        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");
        }
        printf("------------%s\n", cat->name);
        realizeClassWithoutSwift(cls, nil);
        ASSERT(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}
    

這里有一個關鍵的方法是schedule_class_load,它會先將父類的class的load方法加入loadable_classes

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

    if (cls->data()->flags & RW_LOADED) return;

    // Ensure superclass-first ordering
    schedule_class_load(cls->getSuperclass());

    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED); 
}

schedule_class_load方法將有load方法的類加入loadable_classes

 category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);

這個方法獲取__objc_catlist2數(shù)據(jù)段下的分類，通過add_category_to_loadable_list(cat);方法加入loadable_categories中

然后接下來通過call_load_methods()方法調(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) {
            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);

    objc_autoreleasePoolPop(pool);

    loading = NO;
}

循環(huán)調(diào)用load方法，并調(diào)用分類的load方法

realizeClassWithoutSwift學習

realizeClassWithoutSwift這方法很重要，有空看

/***********************************************************************
* realizeClassWithoutSwift
* Performs first-time initialization on class cls, 
* including allocating its read-write data.
* Does not perform any Swift-side initialization.
* Returns the real class structure for the class. 
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/