+load方法与+initialize方法的区别
2018-11-20 本文已影响0人
CoderLS
两个方法的区别
1.两个方法的调用方式
load是拿到函数地址直接进行调用
initialize是通过objc_msgSend()进行调用的
2.两个方法的调用时机
load是runtime加载类，分类的时候调用的(只调用一次)
initialize是类第一次接收到消息时调用的，而且每个类只能被初始化一次(父类initialize方法可能被调用多次)
3.两个方法的调用顺序
-
load
- 先调用类的load方法
先编译的类优先调用
调用子类的load的之前，会先调用父类的load方法 - 再调用分类的load方法
先编译的分类优先先调用(只看编译顺序，不区分是父类的分类还是子类的分类)
- 先调用类的load方法
-
initialize
先初始化父类
再初始化子类(可能最终调用的是父类的initialize方法)
load源码分析步骤:
objc4源码解读过程：objc-os.mm
_objc_init
load_images
prepare_load_methods
schedule_class_load
add_class_to_loadable_list
add_category_to_loadable_list
call_load_methods
call_class_loads
call_category_loads
(*load_method)(cls, SEL_load)
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());
}
(*load_method)(cls, SEL_load);
}
// Destroy the detached list.
if (classes) free(classes);
}
从上面看到+load方法是根据方法地址直接调用，并不是经过objc_msgSend函数调用，loadable_classes里存放着很多下面这种结构体：
cls也就是哪个类，method就是load方法
struct loadable_class {
Class cls;
IMP method;
};
遍历loadable_classes数组，拿到load方法地址直接调用,那么也就是数组的顺序就是调用load方法的顺序，那就接着看看这个数组是怎么添加的，看
prepare_load_methods这个函数
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertWriting();
classref_t *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
for (i = 0; i < count; i++) {
schedule_class_load(remapClass(classlist[i]));
}
category_t **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
realizeClass(cls);
assert(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
看到这个函数schedule_class_load,再接着深入看
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);
}
可以看到schedule_class_load(cls->superclass);这行代码会递归将父类填到到这个列表，然后再添加当前类，
if (cls->data()->flags & RW_LOADED) return;
这行代码是判断是否添加过，如果添加过直接return，保证每个类只调用一次load方法
cls->setInfo(RW_LOADED); 这行代码是将类添加到数组里时设置的标识位用来判断是否添加过
而分类就是正常遍历直接添加，所以就是按照编译顺序调用的，先编译先调用
综上大家应该了解了+load方法调用的底层结构了，如果手动调用[self load]方法，调用的还是objc_msgSend(),则按照正常方法调用步骤，找isa，superclass，一步步寻找方法进行调用，load方法可能会被分类覆盖，但是一般不需要手动调用，
+initialize源码分析步骤：
objc4源码解读过程
objc-msg-arm64.s
objc_msgSend
objc-runtime-new.mm
class_getInstanceMethod
lookUpImpOrNil
lookUpImpOrForward
_class_initialize
callInitialize
objc_msgSend(cls, SEL_initialize)
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil;
bool triedResolver = NO;
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (cache) {
imp = cache_getImp(cls, sel);
if (imp) return imp;
}
// runtimeLock is held during isRealized and isInitialized checking
// to prevent races against concurrent realization.
// runtimeLock is held during method search to make
// method-lookup + cache-fill atomic with respect to method addition.
// Otherwise, a category could be added but ignored indefinitely because
// the cache was re-filled with the old value after the cache flush on
// behalf of the category.
runtimeLock.read();
if (!cls->isRealized()) {
// Drop the read-lock and acquire the write-lock.
// realizeClass() checks isRealized() again to prevent
// a race while the lock is down.
runtimeLock.unlockRead();
runtimeLock.write();
realizeClass(cls);
runtimeLock.unlockWrite();
runtimeLock.read();
}
if (initialize && !cls->isInitialized()) {
runtimeLock.unlockRead();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.read();
// If sel == initialize, _class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
retry:
runtimeLock.assertReading();
// Try this class's cache.
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.
{
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel);
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
// Superclass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
// No implementation found. Try method resolver once.
if (resolver && !triedResolver) {
runtimeLock.unlockRead();
_class_resolveMethod(cls, sel, inst);
runtimeLock.read();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlockRead();
return imp;
}
- 看上面的源码可以看到下面片段代码，会先判断当前类是否初始化过，initialize参数系统传的是YES,所以当前类没初始化过就调用
_class_initialize()
if (initialize && !cls->isInitialized()) {
runtimeLock.unlockRead();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.read();
// If sel == initialize, _class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
_class_initialize源码如下
void _class_initialize(Class cls)
{
assert(!cls->isMetaClass());
Class supercls;
bool reallyInitialize = NO;
// Make sure super is done initializing BEFORE beginning to initialize cls.
// See note about deadlock above.
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
// Try to atomically set CLS_INITIALIZING.
{
monitor_locker_t lock(classInitLock);
if (!cls->isInitialized() && !cls->isInitializing()) {
cls->setInitializing();
reallyInitialize = YES;
}
}
if (reallyInitialize) {
// We successfully set the CLS_INITIALIZING bit. Initialize the class.
// Record that we're initializing this class so we can message it.
_setThisThreadIsInitializingClass(cls);
if (MultithreadedForkChild) {
// LOL JK we don't really call +initialize methods after fork().
performForkChildInitialize(cls, supercls);
return;
}
// Send the +initialize message.
// Note that +initialize is sent to the superclass (again) if
// this class doesn't implement +initialize. 2157218
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
// Exceptions: A +initialize call that throws an exception
// is deemed to be a complete and successful +initialize.
//
// Only __OBJC2__ adds these handlers. !__OBJC2__ has a
// bootstrapping problem of this versus CF's call to
// objc_exception_set_functions().
#if __OBJC2__
@try
#endif
{
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
}
#if __OBJC2__
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: +[%s initialize] "
"threw an exception",
pthread_self(), cls->nameForLogging());
}
@throw;
}
@finally
#endif
{
// Done initializing.
lockAndFinishInitializing(cls, supercls);
}
return;
}
else if (cls->isInitializing()) {
// We couldn't set INITIALIZING because INITIALIZING was already set.
// If this thread set it earlier, continue normally.
// If some other thread set it, block until initialize is done.
// It's ok if INITIALIZING changes to INITIALIZED while we're here,
// because we safely check for INITIALIZED inside the lock
// before blocking.
if (_thisThreadIsInitializingClass(cls)) {
return;
} else if (!MultithreadedForkChild) {
waitForInitializeToComplete(cls);
return;
} else {
// We're on the child side of fork(), facing a class that
// was initializing by some other thread when fork() was called.
_setThisThreadIsInitializingClass(cls);
performForkChildInitialize(cls, supercls);
}
}
else if (cls->isInitialized()) {
// Set CLS_INITIALIZING failed because someone else already
// initialized the class. Continue normally.
// NOTE this check must come AFTER the ISINITIALIZING case.
// Otherwise: Another thread is initializing this class. ISINITIALIZED
// is false. Skip this clause. Then the other thread finishes
// initialization and sets INITIALIZING=no and INITIALIZED=yes.
// Skip the ISINITIALIZING clause. Die horribly.
return;
}
else {
// We shouldn't be here.
_objc_fatal("thread-safe class init in objc runtime is buggy!");
}
}
下面代码是一部分可以看到，初始化当前类时会先判断父类是否为真，父类是否初始化，如果未初始化去初始化父类，递归调用。
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
- 父类初始化完成在初始化当前类调用callInitialize，如下面代码所示,初始化完成调用lockAndFinishInitializing
callInitialize源码
- 如下，可以看到是掉用的objc_msgSend()
void callInitialize(Class cls)
{
((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
asm("");
}
#if __OBJC2__
@try
#endif
{
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
}
#if __OBJC2__
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: +[%s initialize] "
"threw an exception",
pthread_self(), cls->nameForLogging());
}
@throw;
}
@finally
#endif
{
// Done initializing.
lockAndFinishInitializing(cls, supercls);
}
return;
}
lockAndFinishInitializing源码,将类标识为已初始化将flags= RW_INITIALIZED，然后判断是否初始化代码：
bool isInitialized() {
return getMeta()->data()->flags & RW_INITIALIZED;
}
static void lockAndFinishInitializing(Class cls, Class supercls)
{
monitor_locker_t lock(classInitLock);
if (!supercls || supercls->isInitialized()) {
_finishInitializing(cls, supercls);
} else {
_finishInitializingAfter(cls, supercls);
}
}
如果你看明白了上面的原理，看看下面这种情况会打印什么，如果你还能猜到打印什么，那证明你真的看懂了原理，
@interface LSPerson : NSObject
@end
@implementation LSPerson
+ (void)initialize{
NSLog(@"LSPerson +initialize");
}
@end
@interface LSStudent : LSPerson
@end
@implementation LSStudent
+ (void)initialize{
NSLog(@"LSStudent +initialize");
}
@end
@interface LSTeacher : LSPerson
@end
@implementation LSTeacher
+ (void)initialize{
NSLog(@"LSTeacher +initialize");
}
@end
- 1.第一种情况 正常运行，结果在最下面
int main(int argc, const char * argv[]) {
@autoreleasepool {
[LSStudent alloc];
[LSTeacher alloc];
}
return 0;
}
- 2.第二种情况 将 LSStudent里的load方法注释掉，然后运行
int main(int argc, const char * argv[]) {
@autoreleasepool {
[LSStudent alloc];
[LSTeacher alloc];
}
return 0;
}
- 3.第三种情况 将 LSStudent里的load方法注释掉，将 LSTeacher里的load方法也注释掉，然后运行
int main(int argc, const char * argv[]) {
@autoreleasepool {
[LSStudent alloc];
[LSTeacher alloc];
}
return 0;
}
- 4.第四种情况 将 LSStudent里的load方法注释掉，将 LSTeacher里的load方法也注释掉，然后运行
int main(int argc, const char * argv[]) {
@autoreleasepool {
[LSStudent initialize];
[LSTeacher initialize];
}
return 0;
}
-
第一种情况打印结果:都不注释
LSPerson +initialize
LSStudent +initialize
LSTeacher +initialize
-
第二种情况打印结果:注释LSStudent的initialize方法
LSPerson +initialize
LSPerson +initialize
LSTeacher +initialize
-
第三种情况打印结果:注释LSStudent的initialize方法，注释LSTeacher的initialize方法
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
-
第四种情况打印结果:
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
LSPerson +initialize
以下是调用
[LSStudent alloc];
[LSTeacher alloc];
的伪代码，如果是objc_msgSend([LSStudent class],@selector(initialize)),而student类没有就会从父类找，所以掉了父类的initialize方法，并不是初始化了多次，teacher也是类似，而第四种情况是，类第一次收到消息触发initialize方法，然后所有逻辑都走完之后再去调用你真正想调用的initialize方法，所以比第三种情况多2打印了遍
BOOL sutdentInitialized = NO;
BOOL personInitialized = NO;
BOOL teacherInitialized = NO;
if (!sutdentInitialized) {
if (!personInitialized) {
objc_msgSend(LSStudent.superclass, @selector(initialize));
personInitialized = YES;
}
objc_msgSend([LSStudent class], @selector(initialize));
sutdentInitialized = YES;
}
if (!teacherInitialized) {
if (!personInitialized) {
objc_msgSend(LSTeacher.superclass, @selector(initialize));
personInitialized = YES;
}
objc_msgSend([LSTeacher class], @selector(initialize));
teacherInitialized = YES;
}
最后附上两个方法的分析结论和步骤图
load方法
屏幕快照 2018-11-21 下午2.19.21.png
屏幕快照 2018-11-21 下午2.19.27.png
