重新认识 load 方法

https://developer.apple.com/documentation/objectivec/nsobject/1418815-load?language=objc

The load message is sent to classes and categories that are both dynamically loaded and statically linked, but only if the newly loaded class or category implements a method that can respond.

The order of initialization is as follows:

All initializers in any framework you link to.

All +load methods in your image.

All C++ static initializers and C/C++ __attribute__(constructor) functions in your image.

All initializers in frameworks that link to you.

opensource.apple.com

https://opensource.apple.com/tarballs/objc4/objc4-818.2.tar.gz

直接通过函数地址调用，而不是objc_msgSend，所以分类和主类的load、子类和父类的load互不影响。



load_method_t load_method = (load_method_t)classes[i].method;
(*load_method)(cls, @selector(load));


load_method_t load_method = (load_method_t)cats[i].method;
cls = _category_getClass(cat);
(*load_method)(cls, @selector(load));

先执行主类的load，再执行分类的load


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);

先执行父类的load，再执行子类的load


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

按照编译顺序，先编译先执行（父类的分类的加载与子类的分类的加载的先后也取决于编译顺序）


classref_t const *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);

load的执行时机。动态库或可执行文件加载时执行。


/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
**********************************************************************/
void
load_images(const char *path __unused, const struct mach_header *mh)

通过类结构里的标志位，保证每个类的load只加入列表一次。如果没有这个标志位判断，那么沿着superclass调用时应该会导致重复加入列表。分类没有这个标志位，是因为分类不需要superclass-first。


// class +load has been called
#define RW_LOADED             (1<<23)

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); 
}

类和分类的load方法的从准备到执行，而且只执行一次，是通过动态数组来维护的。


// 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;

// List of categories that need +load called (pending parent class +load)
static struct loadable_category *loadable_categories = nil;
static int loadable_categories_used = 0;
static int loadable_categories_allocated = 0;