我们都知道+load方法.
你是不是以为+load方法的底层也是消息发送机制，通过objc_msgSend来调用？
你有没有搞清楚，为什么同样的+load方法，为什么先执行类的+load方法在执行分类的方法？
你有没有搞清楚，为什么同样的+load方法，为什么先执行父类的+load方法在执行子类的方法？
你有没搞清楚，+load方法的整个调用流程？

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下面跟着思路一步一步来吧

** 我们直接从底层代码看起**

_objc_init 初始化函数

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

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();
    static_init();
    lock_init();
    exception_init();
    _dyld_objc_notify_register(&map_images, load_images, unmap_image);
}

load_images函数

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
extern bool hasLoadMethods(const headerType *mhdr);
extern void prepare_load_methods(const headerType *mhdr);

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    // 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
    {
        rwlock_writer_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);  //从这里我们可以看出来是准备load方法
    }

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

prepare_load_methods 函数

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

    runtimeLock.assertWriting();

    classref_t *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);//拿取所有类的列表
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i])); //从这里可以看到规划类的load方法
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);//拿取所有分类的列表
    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
        realizeClass(cls);
        assert(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}

schedule_class_load 这里就解释了为什么先调用父类的方法，然后才调用子类，请仔细看

/***********************************************************************
* prepare_load_methods
* Schedule +load for classes in this image, any un-+load-ed 
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
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->superclass); //通过递归，先把类的父类的方法添加进去

    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED); 
}
/***********************************************************************
* add_class_to_loadable_list
* Class cls has just become connected. Schedule it for +load if
* it implements a +load method.
**********************************************************************/
void add_class_to_loadable_list(Class cls)
{
    IMP method;

    loadMethodLock.assertLocked();

    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) { //开辟空间
        loadable_classes_allocated = loadable_classes_allocated*2 + 16;
        loadable_classes = (struct loadable_class *)
            realloc(loadable_classes,
                              loadable_classes_allocated *
                              sizeof(struct loadable_class));
    }
    
    loadable_classes[loadable_classes_used].cls = cls;
    loadable_classes[loadable_classes_used].method = method;
    loadable_classes_used++;
}

方法刚被调用时：
会从 class 中获取 load 方法： method = cls->getLoadMethod();
判断当前 loadable_classes 这个数组是否已经被全部占用了：loadable_classes_used == loadable_classes_allocated
在当前数组的基础上扩大数组的大小：realloc
把传入的 class 以及对应的方法的实现加到列表中

call_load_methods 调用load方法 这里解释了为什么先调用类的+load方法后调用分类的+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;
}

call_class_loads函数 这个地方可以看到为什么+load方法不是通过objc_msgSend调用的

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;//直接拿到+load方法的指针进行调用
        if (!cls) continue; 

        if (PrintLoading) {
            _objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
        }
        (*load_method)(cls, SEL_load);//进行调用
    }
    
    // Destroy the detached list.
    if (classes) free(classes);
}

call_category_loads分类的方法如何调用

static bool call_category_loads(void)
{
    int i, shift;
    bool new_categories_added = NO;
    
    // Detach current loadable list.
 // 1. 获取当前可以加载的分类列表
    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;
 // 2. 如果当前类是可加载的 `cls  &&  cls->isLoadable()` 就会调用分类的 load 方法
        cls = _category_getClass(cat);
        if (cls  &&  cls->isLoadable()) {
            if (PrintLoading) {
                _objc_inform("LOAD: +[%s(%s) load]\n", 
                             cls->nameForLogging(), 
                             _category_getName(cat));
            }
            (*load_method)(cls, SEL_load);
            cats[i].cat = nil;
        }
    }
// 3. 将所有加载过的分类移除 `loadable_categories` 列表
    // Compact detached list (order-preserving)
    shift = 0;
    for (i = 0; i < used; i++) {
        if (cats[i].cat) {
            cats[i-shift] = cats[i];
        } else {
            shift++;
        }
    }
    used -= shift;
 // 4. 为 `loadable_categories` 重新分配内存，并重新设置它的值
    // Copy any new +load candidates from the new list to the detached list.
    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];
    }

    // Destroy the new list.
    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) {
        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;
    }

    if (PrintLoading) {
        if (loadable_categories_used != 0) {
            _objc_inform("LOAD: %d categories still waiting for +load\n",
                         loadable_categories_used);
        }
    }

    return new_categories_added;
}

获取当前可以加载的分类列表
如果当前类是可加载的 cls && cls->isLoadable() 就会调用分类的 load 方法
将所有加载过的分类移除 loadable_categories 列表
为 loadable_categories 重新分配内存，并重新设置它的值

总结一下:
+load方法会在runtime加载类、分类时调用
每个类、分类的+load，在程序运行过程中只调用一次
调用顺序
先调用类的+load
按照编译先后顺序调用（先编译，先调用）
调用子类的+load之前会先调用父类的+load
再调用分类的+load
按照编译先后顺序调用（先编译，先调用）