前言
iOS-OC对象原理_objc_msgSend（一）
在上篇文章中我们探索了objc_msgSend()的慢速查找流程，即先从cache_t cache中的buckets中查找是否有sel == _cmd,存在则返回；不存在，将进入慢速查找流程，即_lookUpImpOrForward流程，今天我们就详细的探索下它的具体实现。
回顾
我们第一次看到_lookUpImpOrForward是在objc-msg.arm64.s汇编文件中(这里以arm64为例):
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
// receiver and selector already in x0 and x1
mov x2, x16
mov x3, #3
bl _lookUpImpOrForward
// IMP in x0
mov x17, x0
但是我们在该文件下搜索，并没有找到该跳转指令的实现；然后通过全局搜索依然没有找到：
截屏2020-09-24 上午10.36.14.png
然后在汇编代码片段开始有两行醒目的注释：
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
// receiver and selector already in x0 and x1
全局搜索lookUpImpOrForward，豁然开朗，这一跳转就又来到了C++代码(骚的一批)，我们锁定了objc-runtime-new.mm:
截屏2020-09-24 上午10.43.29.png
lookUpImpOrForward实现源码：
IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)
{
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
IMP imp = nil;
Class curClass;
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
// 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.lock();
// We don't want people to be able to craft a binary blob that looks like
// a class but really isn't one and do a CFI attack.
//
// To make these harder we want to make sure this is a class that was
// either built into the binary or legitimately registered through
// objc_duplicateClass, objc_initializeClassPair or objc_allocateClassPair.
//
// TODO: this check is quite costly during process startup.
checkIsKnownClass(cls);
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
// runtimeLock may have been dropped but is now locked again
// 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
}
runtimeLock.assertLocked();
curClass = cls;
// The code used to lookpu the class's cache again right after
// we take the lock but for the vast majority of the cases
// evidence shows this is a miss most of the time, hence a time loss.
//
// The only codepath calling into this without having performed some
// kind of cache lookup is class_getInstanceMethod().
for (unsigned attempts = unreasonableClassCount();;) {
// curClass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
// Halt if there is a cycle in the superclass chain.
if (slowpath(--attempts == 0)) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel); // 有问题???? cache_getImp - lookup - lookUpImpOrForward
if (slowpath(imp == forward_imp)) {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
if (fastpath(imp)) {
// Found the method in a superclass. Cache it in this class.
goto done;
}
}
// No implementation found. Try method resolver once.
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
done:
log_and_fill_cache(cls, imp, sel, inst, curClass);
runtimeLock.unlock();
done_nolock:
if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
return nil;
}
return imp;
}
核心代码片段：
for (unsigned attempts = unreasonableClassCount();;) {
// curClass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
// Halt if there is a cycle in the superclass chain.
if (slowpath(--attempts == 0)) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel);
if (slowpath(imp == forward_imp)) {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
if (fastpath(imp)) {
// Found the method in a superclass. Cache it in this class.
goto done;
}
}
该片段会进入一个循环，开始从curClass->superClass->NSObject->nil，如果查询到执行goto done跳出循环，如果没有找到，即curClass == nil时，break跳出循环。如下图：
拓补图.006.jpeg
上面的流程图中就简单的展示了
lookUpImpOrForward方法实现的大致逻辑。这里的
log_and_fill_cacahe()会触发cache_t内部的fill_cache()方法，然后再到insert(),最终插入到缓存列表。
这里在更深入的看下getMethodNoSuper_nolock的内部实现：
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
// fixme nil cls?
// fixme nil sel?
auto const methods = cls->data()->methods();
for (auto mlists = methods.beginLists(),
end = methods.endLists();
mlists != end;
++mlists)
{
// <rdar://problem/46904873> getMethodNoSuper_nolock is the hottest
// caller of search_method_list, inlining it turns
// getMethodNoSuper_nolock into a frame-less function and eliminates
// any store from this codepath.
method_t *m = search_method_list_inline(*mlists, sel);
if (m) return m;
}
return nil;
}
auto const methods = cls->data()->methods()，这里其实就是获取当前class的class_data_bits_t *bit下的bit->data()，也就是class_rw_t这个结构体的内容，并读取了该结构体的methods()，获取方法列表。
继续将获取到的methods传递给search_method_list_inline()方法:
ALWAYS_INLINE static method_t *
search_method_list_inline(const method_list_t *mlist, SEL sel)
{
int methodListIsFixedUp = mlist->isFixedUp();
int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
return findMethodInSortedMethodList(sel, mlist);
} else {
// Linear search of unsorted method list
for (auto& meth : *mlist) {
if (meth.name == sel) return &meth;
}
}
#if DEBUG
// sanity-check negative results
if (mlist->isFixedUp()) {
for (auto& meth : *mlist) {
if (meth.name == sel) {
_objc_fatal("linear search worked when binary search did not");
}
}
}
#endif
return nil;
}
经过一个判断后，又将方法列表mlist和目标sel,传递到findMethodInSortedMethodList()方法中：
ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list)
{
ASSERT(list);
const method_t * const first = &list->first;
const method_t *base = first;
const method_t *probe;
uintptr_t keyValue = (uintptr_t)key;
uint32_t count;
for (count = list->count; count != 0; count >>= 1) {
probe = base + (count >> 1);
uintptr_t probeValue = (uintptr_t)probe->name;
if (keyValue == probeValue) {
// `probe` is a match.
// Rewind looking for the *first* occurrence of this value.
// This is required for correct category overrides.
while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
probe--;
}
return (method_t *)probe;
}
if (keyValue > probeValue) {
base = probe + 1;
count--;
}
}
return nil;
}
这里是真正的开始查找流程，这里采用了二分法查询，当查询到就返回method_t,否则返回nil。
大致流程图如下：
拓补图.007.jpeg
补充
forward_imp
const IMP forward_imp = (IMP)_objc_msgForward_impcache;
在lookUpImpOrForward()方法中，在某种情况下会imp = forward_imp,当返回forward_imp后，又做了哪些处理呐？我们来探索下：
#if !OBJC_OLD_DISPATCH_PROTOTYPES
extern void _objc_msgForward_impcache(void);
#else
extern id _objc_msgForward_impcache(id, SEL, ...);
#endif
又看不到实现了，经验告诉我又要跳转到汇编：
STATIC_ENTRY __objc_msgForward_impcache
// No stret specialization.
b __objc_msgForward
END_ENTRY __objc_msgForward_impcache
ENTRY __objc_msgForward
adrp x17, __objc_forward_handler@PAGE
ldr p17, [x17, __objc_forward_handler@PAGEOFF]
TailCallFunctionPointer x17
END_ENTRY __objc_msgForward
这里面的流程：__objc_msgForward_impcache->__objc_msgForward->__objc_forward_handler，在汇编中又找不到__objc_forward_handler的实现了，妈妈的~，回到C++:
// Default forward handler halts the process.
__attribute__((noreturn, cold)) void
objc_defaultForwardHandler(id self, SEL sel)
{
_objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
"(no message forward handler is installed)",
class_isMetaClass(object_getClass(self)) ? '+' : '-',
object_getClassName(self), sel_getName(sel), self);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
好熟悉的味道 😃😀,unrecognized selector sent to instance xxxx,看到了让你大脑瞬间崩溃的东西，原来是来自这里。
这也就意味着，当imp = forward_imp,系统也就放弃治疗了，直接进入Crash流程。
总结
我们以 [person sayHello]为例：
该方法在编译后变为objc_msgSend(person,sel_registerName("sayHello"))；
首先会进入快速查找流程，该流程的实现在汇编指令中完成：
1.查到person的类对象，person->isa->Class;
2.查找缓存列表，Class->offset(16)->cache->_maskAndBuckets->buckets;
3.在buckets列表中查询是否有sel == _cmd，存在则返回，否则进入CheckMiss->__objc_msgSend_uncached->MethodTableLookup->_lookUpImpOrForward,由此进入慢速查找流程；
开始慢速查找流程：
1.查找curClass类对象的方法列表,curCls->data()->methods()(objc_class->class_rw_t->methods());
2.通过二分遍历查询当前方法列表中是有sel==_cmd,如果存在则返回并执行4，否则执行3；
3.将当前的curCls= curCls->superclass,首先在父类中的cache->_maskAndBuckets->buckets缓存列表查询(imp = cache_getImp(curCls,sel))，如果存在，执行4，否则执行1；直到curClass = nil时，依然没有匹配，跳出循环，执行5；
4.找到目标imp，添加到缓存列表，并返回；
5.没有找到对应的imp，进入消息转发流程；消息转发流程
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