在objc_msgSend-快速查找流程中我們講到摹闽,objc_msgSend首先通過匯編快速查找方法緩存粘驰,如果找到簇捍,調(diào)用TailCallCachedImp直接將方法緩存起來然后進行調(diào)用就可以了周荐,如果查找不到就跳到CheckMiss稳摄,然后走慢速查找流程公般。接下來我們分析一下objc_msgSend慢速查找流程忍啸。
objc_msgSend查找流程:
- 獲取傳入對象所屬的類
- 獲取該類的方法緩存表
- 使用傳入的sel選擇器在緩存中查詢
- 如果緩存中不存在砰琢,則開始慢速查找流程
- 慢速查找流程蘸嘶,找到后,跳轉(zhuǎn)到IMP映射位置的方法
在objc_msgSend-快速查找流程中我們分析了陪汽,先通過GetClassFromIsa_p16獲取到傳入對象所屬的類训唱,然后通過CacheLookup在方法緩存表中查找,如果緩存命中則走CacheHit方法挚冤,緩存沒命中走CheckMiss方法况增。
一、CheckMiss 方法
.macro CheckMiss
// miss if bucket->sel == 0
.if $0 == GETIMP
cbz p9, LGetImpMiss
.elseif $0 == NORMAL //傳進來的是NORMAL,所以走這里
cbz p9, __objc_msgSend_uncached
.elseif $0 == LOOKUP
cbz p9, __objc_msgLookup_uncached
.else
.abort oops
.endif
.endmacro
傳進來的是NORMAL训挡,所以會走到 __objc_msgSend_uncached 方法
二澳骤、__objc_msgSend_uncached 方法
STATIC_ENTRY __objc_msgSend_uncached
UNWIND __objc_msgSend_uncached, FrameWithNoSaves
// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band p16 is the class to search
MethodTableLookup
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
緊接著又會來到 MethodTableLookup 方法
三、MethodTableLookup 方法
.macro MethodTableLookup
// push frame
SignLR
stp fp, lr, [sp, #-16]!
mov fp, sp
// save parameter registers: x0..x8, q0..q7
sub sp, sp, #(10*8 + 8*16)
stp q0, q1, [sp, #(0*16)]
stp q2, q3, [sp, #(2*16)]
stp q4, q5, [sp, #(4*16)]
stp q6, q7, [sp, #(6*16)]
stp x0, x1, [sp, #(8*16+0*8)]
stp x2, x3, [sp, #(8*16+2*8)]
stp x4, x5, [sp, #(8*16+4*8)]
stp x6, x7, [sp, #(8*16+6*8)]
str x8, [sp, #(8*16+8*8)]
// 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
// restore registers and return
ldp q0, q1, [sp, #(0*16)]
ldp q2, q3, [sp, #(2*16)]
ldp q4, q5, [sp, #(4*16)]
ldp q6, q7, [sp, #(6*16)]
ldp x0, x1, [sp, #(8*16+0*8)]
ldp x2, x3, [sp, #(8*16+2*8)]
ldp x4, x5, [sp, #(8*16+4*8)]
ldp x6, x7, [sp, #(8*16+6*8)]
ldr x8, [sp, #(8*16+8*8)]
mov sp, fp
ldp fp, lr, [sp], #16
AuthenticateLR
.endmacro
接著又會來到 lookUpImpOrForward 方法
四澜薄、lookUpImpOrForward 方法
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.lock();
// 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.assertLocked();
curClass = cls;
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)) {
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); // 有問題???? cache_getImp - lookup - lookUpImpOrForward
if (slowpath(imp == forward_imp)) {
break;
}
if (fastpath(imp)) {
goto done;
}
}
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;
}
4.1 判斷緩存是否存在为肮,存在則直接通過cls和sel直接獲取imp,并返回表悬。
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
4.2 相關類信息判斷
根據(jù)所有已知類的列表檢查給定的類弥锄,有問題直接內(nèi)部拋出異常。
判斷類是否已經(jīng)被實現(xiàn)蟆沫,未實現(xiàn)則去實現(xiàn)籽暇,這部分后面類的加載篇章會詳細分析,主要是按照superclass和isa走向去遞歸實現(xiàn)父類和元類饭庞,同時準備好對象方法和類方法的查找鏈戒悠。
-
判斷類是否被初始化,未初始化則去初始化舟山。
checkIsKnownClass(cls); if (slowpath(!cls->isRealized())) { cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock); } if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) { cls = initializeAndLeaveLocked(cls, inst, runtimeLock); }
4.3 查找本類的方法列表
4.3.1 利用 getMethodNoSuper_nolock 查找本類的方法列表绸狐,如果找到了卤恳,進入 goto done;
4.3.2 getMethodNoSuper_nolock 方法
調(diào)用調(diào)用 search_method_list_inline 方法 對本類方法列表進行查找
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;
}
4.3.3 search_method_list_inline 方法
調(diào)用 findMethodInSortedMethodList 方法 對本類方法列表進行二分查找
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;
}
4.3.4 findMethodInSortedMethodList 方法
對本類方法列表進行二分查找
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;
}
4.4 done 方法
-
如果找到了,進入本方法寒矿,調(diào)用 log_and_fill_cache 方法
done: log_and_fill_cache(cls, imp, sel, inst, curClass); runtimeLock.unlock();
4.5 log_and_fill_cache 方法
-
利用 cache_fill 方法 寫入到緩存里面突琳,為了下次直接從緩存里面快速查找到。
static void log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer) { #if SUPPORT_MESSAGE_LOGGING if (slowpath(objcMsgLogEnabled && implementer)) { bool cacheIt = logMessageSend(implementer->isMetaClass(), cls->nameForLogging(), implementer->nameForLogging(), sel); if (!cacheIt) return; } #endif // objc_msgSend -> 二分查找自己 -> cache_fill -> objc_msgSend // cache_fill(cls, sel, imp, receiver); }
4.6 遞歸查找父類的緩存
4.6.1 查找本類的方法列表 如果找不到符相,就遞歸查找父類的緩存
-
調(diào)用 cache_getImp 方法 找到父類
// Superclass cache. imp = cache_getImp(curClass, sel); // 有問題???? cache_getImp - lookUpImpOrForward
-
cache_getImp 方法
STATIC_ENTRY _cache_getImp GetClassFromIsa_p16 p0 CacheLookup GETIMP, _cache_getImp LGetImpMiss: mov p0, #0 ret END_ENTRY _cache_getImp
4.7 遞歸父類緩存查找不到拆融,利用 imp = forward_imp
if (slowpath((curClass = curClass->superclass) == nil)) {
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = forward_imp;
break;
}
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;
}
4.7.1 forward_imp
- const IMP forward_imp = (IMP)_objc_msgForward_impcache;
4.7.2 _objc_msgForward_impcache
_objc_msgForward_impcache 方法 調(diào)用 __objc_msgForward 方法
-
__objc_msgForward 方法 調(diào)用 TailCallFunctionPointer x17
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
4.7.3 TailCallFunctionPointer 方法
TailCallFunctionPointer 方法 就是返回指針的值,返回 x17 的值啊终,x17 的值是 __objc_forward_handler 方法 確定的
.macro TailCallFunctionPointer
// $0 = function pointer value
braaz $0
.endmacro
4.7.4 __objc_forward_handler 方法
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;
如果方法沒有實現(xiàn)镜豹,imp 會置換成 forward_imp , forward_imp 最終會走到 __objc_forward_handler 方法 返回 unrecognized selector sent to instance ... 信息,我們查看一下方法沒有實現(xiàn)的報錯信息會發(fā)現(xiàn)蓝牲,報錯信息的模板原來在這趟脂。
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[LGPerson say666]: unrecognized selector sent to instance 0x1007738f0'
4.8 動態(tài)方法決議
在4.7中將 imp 置換成 forward_imp 后,會 break 跳出循環(huán)例衍,走到動態(tài)方法決議這里:
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
4.8.1 resolveMethod_locked 方法
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
// 方法沒有你怎么不知道
// 報錯
// 給你一次機會
runtimeLock.unlock();
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
resolveInstanceMethod(inst, sel, cls);
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
resolveClassMethod(inst, sel, cls);
if (!lookUpImpOrNil(inst, sel, cls)) {
resolveInstanceMethod(inst, sel, cls);
}
}
// chances are that calling the resolver have populated the cache
// so attempt using it
return lookUpImpOrForward(inst, sel, cls, behavior | LOOKUP_CACHE);
}
4.8.2 resolveInstanceMethod 方法`
我們發(fā)現(xiàn)在 resolveInstanceMethod 方法 中將 IMP imp = lookUpImpOrNil(inst, sel, cls); 昔期,所以我們跳進 lookUpImpOrNil 方法 看一下會發(fā)現(xiàn)又回到了 lookUpImpOrForward 方法 ,那對之前做了什么產(chǎn)生了好奇佛玄。
往上走我們發(fā)現(xiàn)有下面兩行代碼
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend; bool resolved = msg(cls, resolve_sel, sel);-
如果我們實現(xiàn) resolveInstanceMethod 方法 將方法的 imp 進行賦值镇眷,然后再回到 lookUpImpOrForward 方法 之后 imp 有值,就不會報錯了翎嫡。
static void resolveInstanceMethod(id inst, SEL sel, Class cls) { runtimeLock.assertUnlocked(); ASSERT(cls->isRealized()); SEL resolve_sel = @selector(resolveInstanceMethod:); if (!lookUpImpOrNil(cls, resolve_sel, cls->ISA())) { // Resolver not implemented. return; } BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend; bool resolved = msg(cls, resolve_sel, sel); // Cache the result (good or bad) so the resolver doesn't fire next time. // +resolveInstanceMethod adds to self a.k.a. cls IMP imp = lookUpImpOrNil(inst, sel, cls); if (resolved && PrintResolving) { if (imp) { _objc_inform("RESOLVE: method %c[%s %s] " "dynamically resolved to %p", cls->isMetaClass() ? '+' : '-', cls->nameForLogging(), sel_getName(sel), imp); } else { // Method resolver didn't add anything? _objc_inform("RESOLVE: +[%s resolveInstanceMethod:%s] returned YES" ", but no new implementation of %c[%s %s] was found", cls->nameForLogging(), sel_getName(sel), cls->isMetaClass() ? '+' : '-', cls->nameForLogging(), sel_getName(sel)); } } }
4.8.3 動態(tài)方法決議實現(xiàn)
#import "LGPerson.h"
#import <objc/message.h>
@implementation LGPerson
- (void)askHelp{
NSLog(@"%s",__func__);
}
+ (BOOL)resolveInstanceMethod:(SEL)sel{
if (sel == @selector(say666)) {
NSLog(@"%@ 來了老弟",NSStringFromSelector(sel));
IMP imp = class_getMethodImplementation(self, @selector(askHelp));
Method sayAskHelpMethod = class_getInstanceMethod(self, @selector(askHelp));
const char *type = method_getTypeEncoding(sayAskHelpMethod);
return class_addMethod(self, sel, imp, type);
}
return [super resolveInstanceMethod:sel];
}
5. 總結
- 當在 objc_msgSend 緩存中沒有找到方法,就會來到 CheckMiss -> __objc_msgSend_uncached -> MethodTableLookup -> lookUpImpOrForward 進行慢速查找流程永乌。
- 在 lookUpImpOrForward 里面會先去本類當中查找方法 getMethodNoSuper_nolock 惑申,本類沒有找到就會去遞歸的去父類當中查找。
- 如果本類和父類都沒有找到翅雏,就會進行動態(tài)方法決議 _class_resolveMethod 圈驼,這是蘋果爸爸給我們的最后的機會。
最終到 _objc_forward_handler 方法 崩潰報錯 unrecognized selector sent to instance ...望几。