- +load方法会在程序初始化时候调用
- 每个类和分类的+load方法都会调用，并且只调用一次
-
调用顺序
1. 先调用类中的+load
按照编译顺序调用，先编译的先调用
父类优先调用。调用子类的+load方法之前会先调用父类的+load
2. 后调用category中的+load
按照编译顺序调用，先编译的先调用
源码分析
源码版本objc4-781.tar.gz
入口：objc-os.mm
void _objc_init(void)
void load_images(const char *path __unused, const struct mach_header *mh)
void prepare_load_methods(const headerType *mhdr)
void call_load_methods(void)
static void call_class_loads(void)
static bool call_category_loads(void)
先看调用情况，查看void call_load_methods(void)方法可知，先调用了类的+load方法，再调用category中的+load方法。
/***********************************************************************
* call_load_methods
* Call all pending class and category +load methods.
* Class +load methods are called superclass-first.
* Category +load methods are not called until after the parent class's +load.
*
* This method must be RE-ENTRANT, because a +load could trigger
* more image mapping. In addition, the superclass-first ordering
* must be preserved in the face of re-entrant calls. Therefore,
* only the OUTERMOST call of this function will do anything, and
* that call will handle all loadable classes, even those generated
* while it was running.
*
* The sequence below preserves +load ordering in the face of
* image loading during a +load, and make sure that no
* +load method is forgotten because it was added during
* a +load call.
* Sequence:
* 1. Repeatedly call class +loads until there aren't any more
* 2. Call category +loads ONCE.
* 3. Run more +loads if:
* (a) there are more classes to load, OR
* (b) there are some potential category +loads that have
* still never been attempted.
* Category +loads are only run once to ensure "parent class first"
* ordering, even if a category +load triggers a new loadable class
* and a new loadable category attached to that class.
*
* Locking: loadMethodLock must be held by the caller
* All other locks must not be held.
**********************************************************************/
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;
}
static void call_class_loads(void) 调用类的+load。源码中比较关键点是loadable_classes这个列表，其中保存了类的+load方法，循环获取到+load方法后直接调用。所以了解了loadable_classes的初始化，就知道了类中+load方法调用的顺序。
/***********************************************************************
* call_class_loads
* Call all pending class +load methods.
* If new classes become loadable, +load is NOT called for them.
*
* Called only by call_load_methods().
**********************************************************************/
static void call_class_loads(void)
{
int i;
// Detach current loadable list.
struct loadable_class *classes = loadable_classes; // load方法列表
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;
// 得到+load方法
load_method_t load_method = (load_method_t)classes[i].method;
if (!cls) continue;
if (PrintLoading) {
_objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
}
// 直接调用+load方法
(*load_method)(cls, @selector(load));
}
// Destroy the detached list.
if (classes) free(classes);
}
在 static bool call_category_loads(void)方法中的调用情况类似，其中有个名为loadable_categories的列表，循环获取到category中的+load方法后直接调用。所以了解了loadable_categories的初始化，就知道了category中+load方法调用的顺序。
/***********************************************************************
* call_category_loads
* Call some pending category +load methods.
* The parent class of the +load-implementing categories has all of
* its categories attached, in case some are lazily waiting for +initalize.
* Don't call +load unless the parent class is connected.
* If new categories become loadable, +load is NOT called, and they
* are added to the end of the loadable list, and we return TRUE.
* Return FALSE if no new categories became loadable.
*
* Called only by call_load_methods().
**********************************************************************/
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));
}
(*load_method)(cls, @selector(load));
cats[i].cat = nil;
}
}
... 以下省略 ...
... ...
... ...
}
void prepare_load_methods(const headerType *mhdr)该方法就是用来初始化loadable_classes和loadable_categories的。
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");
}
realizeClassWithoutSwift(cls, nil);
ASSERT(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
其中首先调用了static void schedule_class_load(Class cls)方法，加载类中的+load。并且该方法中存在递归调用，在当前的类的+load方法添加到数组之前，会先将父类作为参数递归调用该方法，如果父类没有加载过，就先加载父类。确保父类比子类先加载
void add_class_to_loadable_list(Class cls)中直接将类赋值给类数组，所以先加载的在数组的前面，这样就可以确保父类的+load方法在子类之前。保证了后面的调用顺序
/***********************************************************************
* 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);
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) {
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++;
}
加载category中的+load方法则比较简单,调用void add_category_to_loadable_list(Category cat)，直接按照编译的顺序，先编译的在前面
/***********************************************************************
* add_category_to_loadable_list
* Category cat's parent class exists and the category has been attached
* to its class. Schedule this category for +load after its parent class
* becomes connected and has its own +load method called.
**********************************************************************/
void add_category_to_loadable_list(Category cat)
{
IMP method;
loadMethodLock.assertLocked();
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));
}
loadable_categories[loadable_categories_used].cat = cat;
loadable_categories[loadable_categories_used].method = method;
loadable_categories_used++;
}
