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OC方法調(diào)用的本質(zhì):消息發(fā)送機(jī)制 - msg_Send.
OC方法調(diào)用的本質(zhì)就是給對象發(fā)送消息:objc_msgSend(),這個流程可以分為三個階段:
- 消息發(fā)送
- 動態(tài)方法解析
- 消息轉(zhuǎn)發(fā)
下面我們將從源碼上分析這三個過程的具體實現(xiàn).
一消息發(fā)送
消息發(fā)送咋們從[obj message]為出發(fā)點晾腔，從objc源碼里進(jìn)行一次正向梳理尝蠕。
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
MJPerson *person = ((MJPerson *(*)(id, SEL))(void *)objc_msgSend)((id)((MJPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("MJPerson"), sel_registerName("alloc")), sel_registerName("init"));
((void (*)(id, SEL))(void *)objc_msgSend)((id)person, sel_registerName("personTest"));
((void (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("MJPerson"), sel_registerName("initialize"));
}
return 0;
}
通過上面的命令行操作海铆，`[obj message]`編譯之后的底層表示是
((void (*)(id, SEL))(void *)objc_msgSend)((id)person, sel_registerName("personTest"));
```bash
objc_msgSend(person, sel_registerName("personTest"));
其中第一個參數(shù)person就是消息接受者，后面的sel_registerName逗载，可以在objc源碼中搜到它的函數(shù)聲明
SEL _Nonnull sel_registerName(const char * _Nonnull str)
同于OC里面的@selector()
消息轉(zhuǎn)發(fā)送碼分析:
查找源碼步驟:
打開runtime源碼 -> 搜索objc_msgSend-> 找到objc-msg-arm64.s文件 -> 找到ENTRY _objc_msgSend(方法入口):
找到ENTRY _objc_msgSend(方法入口)
匯編 使用
ENTRY + 函數(shù)名字作為一個函數(shù)的入口
END_ENTRY + 函數(shù)名字函數(shù)結(jié)束
從上圖中可以看到,如果reserver不為nil,就執(zhí)行CacheLookup從緩存中查找,所以我們搜索CacheLookup方法:
如果是命中緩存，找到了方法链烈，那就簡單了厉斟，直接返回并調(diào)用就好了，如果沒找强衡，就會進(jìn)入上圖中的__objc_msgSend_uncached
STATIC_ENTRY __objc_msgSend_uncached
UNWIND __objc_msgSend_uncached, FrameWithNoSaves
// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band x16 is the class to search
MethodTableLookup
br x17
END_ENTRY __objc_msgSend_uncached
STATIC_ENTRY __objc_msgLookup_uncached
UNWIND __objc_msgLookup_uncached, FrameWithNoSaves
__objc_msgSend_uncached中調(diào)用了MethodTableLookup
我們發(fā)現(xiàn)在MethodTableLookup里面擦秽，調(diào)用了__class_lookupMethodAndLoadCache3函數(shù)，而這個函數(shù)在當(dāng)前的匯編代碼里面是找不到實現(xiàn)的漩勤。你去objc源碼進(jìn)行全局搜索感挥，也搜不到.那會不會是用C語言實現(xiàn)的這個方法呢?我們?nèi)サ粢粋€下劃線_然后搜索(<font color='red'>為什么要去掉一個下劃線呢?因為C語言在編譯成匯編語言是,會默認(rèn)在方法前面加一個下劃線_,所以我們在C語言中搜索時要去掉一個</font>),發(fā)現(xiàn)還真搜索到了這個方法:
/***********************************************************************
* _class_lookupMethodAndLoadCache.
* Method lookup for dispatchers ONLY. OTHER CODE SHOULD USE lookUpImp().
* This lookup avoids optimistic cache scan because the dispatcher
* already tried that.
**********************************************************************/
IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{
return lookUpImpOrForward(cls, sel, obj,
YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}
進(jìn)入lookUpImpOrForward:
/***********************************************************************
* lookUpImpOrForward.
* The standard IMP lookup.
* initialize==NO tries to avoid +initialize (but sometimes fails)-------------------------------->??????標(biāo)準(zhǔn)的IMP查找流程
* cache==NO skips optimistic unlocked lookup (but uses cache elsewhere)
* Most callers should use initialize==YES and cache==YES.
* inst is an instance of cls or a subclass thereof, or nil if none is known.
* If cls is an un-initialized metaclass then a non-nil inst is faster.
* May return _objc_msgForward_impcache. IMPs destined for external use
* must be converted to _objc_msgForward or _objc_msgForward_stret.
* If you don't want forwarding at all, use lookUpImpOrNil() instead.
**********************************************************************/
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) {//------------------------------>??????查詢當(dāng)前Class對象的緩存，如果找到方法越败，就返回該方法
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()) {//------------------------------>??????當(dāng)前Class如果沒有被realized触幼，就進(jìn)行realize操作
// 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()) {//-------------->??????當(dāng)前Class如果沒有初始化，就進(jìn)行初始化操作
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.//------------------------------>??????嘗試從該Class對象的緩存中查找究飞，如果找到置谦，就跳到done處返回該方法
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.//---------------->??????嘗試從該Class對象的方法列表中查找，找到的話亿傅，就緩存到該Class的cache_t里面媒峡，并跳到done處返回該方法
{
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.// Try superclass caches and method lists.------>??????進(jìn)入當(dāng)前Class對象的superclass對象
{
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;//------>??????該for循環(huán)每循環(huán)一次，就會進(jìn)入上一層的superclass對象袱蜡，進(jìn)行循環(huán)內(nèi)部方法查詢流程
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.------>??????在當(dāng)前superclass對象的緩存進(jìn)行查找
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;//------>??????如果在當(dāng)前superclass的緩存里找到了方法丝蹭，就調(diào)用log_and_fill_cache進(jìn)行方法緩存，注意這里傳入的參數(shù)是cls，也就是將方法緩存到消息接受對象所對應(yīng)的Class對象的cache_t中奔穿，然后跳到done處返回該方法
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;//---->??????如果緩存里找到的方法是_objc_msgForward_impcache镜沽，就跳出該輪循環(huán)，進(jìn)入上一層的superclass贱田，再次進(jìn)行查找
}
}
// Superclass method list.---->??????如過畫緩存里面沒有找到方法缅茉，則對當(dāng)前superclass的方法列表進(jìn)行查找
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
//------>??????如果在當(dāng)前superclass的方法列表里找到了方法，就調(diào)用log_and_fill_cache進(jìn)行方法緩存男摧，注意這里傳入的參數(shù)是cls蔬墩，也就是將方法緩存到消息接受對象所對應(yīng)的Class對象的cache_t中，然后跳到done處返回該方法
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
// No implementation found. Try method resolver once.------>??????如果到基類還沒有找到方法耗拓，就嘗試進(jìn)行方法解析
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. //------>??????如果方法解析不成功拇颅，就進(jìn)行消息轉(zhuǎn)發(fā)
// Use forwarding.
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlockRead();
return imp;
}
關(guān)于上面再方法列表查找的函數(shù)Method meth = getMethodNoSuper_nolock(cls, sel);還需要說明一下，進(jìn)入它的實現(xiàn)
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();
assert(cls->isRealized());
// fixme nil cls?
// fixme nil sel?
for (auto mlists = cls->data()->methods.beginLists(), //??遍歷class_rw_t 中的method_list
end = cls->data()->methods.endLists();
mlists != end;
++mlists)
{
method_t *m = search_method_list(*mlists, sel);//---??????核心函數(shù)
if (m) return m;
}
return nil;
}
進(jìn)入search_method_list(mlist, sel)看看是如何查找的:
/***********************************************************************
* getMethodNoSuper_nolock
* fixme
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static method_t *search_method_list(const method_list_t *mlist, SEL sel)
{
int methodListIsFixedUp = mlist->isFixedUp();
int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
if (__builtin_expect(methodListIsFixedUp && methodListHasExpectedSize, 1)) {
//---??????如果方法列表是經(jīng)過排序的乔询，則進(jìn)行二分查找
return findMethodInSortedMethodList(sel, mlist);
} else {
// Linear search of unsorted method list
//---??????如果方法列表沒有進(jìn)行排序樟插，則進(jìn)行線性遍歷查找
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;
}
ok,到現(xiàn)在我們就從源碼層面搞清楚了objc_msgSend()第一階段消息發(fā)送的步驟,我們用一張圖總結(jié)一下:
二:動態(tài)方法解析:
從lookUpImpOrForward方法實現(xiàn)中可以看到,如果在消息發(fā)送階段始終沒有找到方法,那么就會進(jìn)入動態(tài)方法解析:
// No implementation found. Try method resolver once.------>??????如果到基類還沒有找到方法，就嘗試進(jìn)行方法解析
if (resolver && !triedResolver) {//1????????判斷之前有沒有方法解析過
runtimeLock.unlockRead();
_class_resolveMethod(cls, sel, inst);//2????????如果沒有解析過竿刁，調(diào)用_class_resolveMethod
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;//3????????如果進(jìn)行過動態(tài)方法解析黄锤，就回到retry，再次進(jìn)行消息發(fā)送階段查找方法
}
// No implementation found, and method resolver didn't help. //------>??????如果方法解析不成功食拜，就進(jìn)行消息轉(zhuǎn)發(fā)
// Use forwarding.
//??????消息轉(zhuǎn)發(fā)階段
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlockRead();
return imp;
下面再繼續(xù)看一下方法動態(tài)解析里面的核心函數(shù)_class_resolveMethod(cls, sel, inst);
/***********************************************************************
* _class_resolveMethod
* Call +resolveClassMethod or +resolveInstanceMethod.
* Returns nothing; any result would be potentially out-of-date already.
* Does not check if the method already exists.
**********************************************************************/
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
if (! cls->isMetaClass()) {//??????判斷當(dāng)前的參數(shù)cls是否是一個meta-class對象
// try [cls resolveInstanceMethod:sel]
_class_resolveInstanceMethod(cls, sel, inst);//-->??????
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
_class_resolveClassMethod(cls, sel, inst);//-->??????
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
_class_resolveInstanceMethod(cls, sel, inst);
}
}
}
_class_resolveInstanceMethod(cls, sel, inst);鸵熟，進(jìn)入它的函數(shù)實現(xiàn)如下
/***********************************************************************
* _class_resolveInstanceMethod
* Call +resolveInstanceMethod, looking for a method to be added to class cls.
* cls may be a metaclass or a non-meta class.
* Does not check if the method already exists.
**********************************************************************/
static void _class_resolveInstanceMethod(Class cls, SEL sel, id inst)
{
//---??????查看cls的meta-class對象的方法列表里面是否有SEL_resolveInstanceMethod函數(shù)，
//---??????也就是看是否實現(xiàn)了+(BOOL)resolveInstanceMethod:(SEL)sel方法
if (! lookUpImpOrNil(cls->ISA(), SEL_resolveInstanceMethod, cls,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
// Resolver not implemented.---??????如果沒找到负甸，直接返回流强，
return;
}
//---??????如果找到，則通過objc_msgSend調(diào)用一下+(BOOL)resolveInstanceMethod:(SEL)sel方法
//---??????完成里面的動態(tài)增加方法的步驟
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
bool resolved = msg(cls, SEL_resolveInstanceMethod, 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(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
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));
}
}
}
動態(tài)方法解析的核心步驟完成之后惑惶，會一層一層往上返回到lookUpImpOrForward函數(shù)煮盼，跳到retry標(biāo)記處，重新查詢方法带污，因為在方法解析這一步僵控，如果對某個目標(biāo)方法名xxx有過處理，為其動態(tài)增加了方法實現(xiàn)鱼冀，那么再次查詢該方法报破，則一定可以在消息發(fā)送階段被找到并調(diào)用。 對于類方法（+方法）的動態(tài)解析其實跟上面的過程大致相同千绪，只不過解析的時候調(diào)用的+(BOOL)resolveClassMethod:(SEL)sel方法充易，來完成類方法的動態(tài)添加綁定。
我們用代碼驗證一下
//--------------------MJPerson.h-------
@interface MJPerson : NSObject
- (void)test;
@end
//--------------------MJPerson.m-------
#import "MJPerson.h"
#import <objc/runtime.h>
@implementation MJPerson
- (void)other
{
NSLog(@"%s", __func__);
}
struct method_t {
SEL sel;
char *types;
IMP imp;
};
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
if (sel == @selector(test)) {
// 獲取其他方法
struct method_t *method = (struct method_t *)class_getInstanceMethod(self, @selector(other));
// 動態(tài)添加test方法的實現(xiàn)
class_addMethod(self, sel, method->imp, method->types);
// 返回YES代表有動態(tài)添加方法
return YES;
}
return [super resolveInstanceMethod:sel];
}
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJPerson *person = [[MJPerson alloc] init];
[person test];
}
return 0;
}
RUN>
*********************** 運(yùn)行結(jié)果 ************************** 2021-05-06 18:17:40.215622+0800 Interview03-動態(tài)方法解析[5697:221883] -[MJPerson other]
動態(tài)方法解析的步驟
關(guān)于動態(tài)方法解析的注意點:
- 1:通過
class_addMethod動態(tài)添加的方法是添加到class_rw_t中的method_list_t中的,我們從源碼中也可以看到,動態(tài)添加方法的實現(xiàn)后,進(jìn)入goto retry,重新進(jìn)入消息發(fā)送階段,從類的cache_t或者class_rw_t中查找 - 2:動態(tài)添加方法的實現(xiàn)后,會重新進(jìn)入消息發(fā)送階段,重新查找方法
三:消息轉(zhuǎn)發(fā)
經(jīng)過前兩個流程之后荸型，如果還沒能找到方法對應(yīng)的函數(shù)盹靴，說明當(dāng)前類已經(jīng)盡力了，但是確實沒有能力處理目標(biāo)方法，因子只能把方法拋給別人稿静，也就丟給其他的類去處理梭冠，因此最后一個流程為什么叫消息轉(zhuǎn)發(fā)，顧名思義改备。
下面控漠，我們來搞定消息轉(zhuǎn)發(fā)，入口如下悬钳，位于lookUpImpOrForward函數(shù)的尾部
// No implementation found, and method resolver didn't help. //------>??????如果方法解析不成功盐捷，就進(jìn)行消息轉(zhuǎn)發(fā)
// Use forwarding.
//??????消息轉(zhuǎn)發(fā)階段
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
我們進(jìn)入_objc_msgForward_impcache內(nèi)部會發(fā)現(xiàn)沒有別的線索了,只是一個聲明:
#if !OBJC_OLD_DISPATCH_PROTOTYPES
extern void _objc_msgForward_impcache(void);
#else
extern id _objc_msgForward_impcache(id, SEL, ...);
#endif
消息轉(zhuǎn)發(fā)階段后,就會進(jìn)入__forwarding__方法處理.為什么是__forwarding__這個方法呢?我們把resolveInstanceMethod方法注釋掉,看看系統(tǒng)會報什么錯誤:
可以看到從底層上來，調(diào)用了CF框架的_CF_forwarding_prep_0默勾，然后就調(diào)用了___forwarding___碉渡。該函數(shù)就屬于蘋果未開源部分，騰訊課堂iOS底層原理班的分享灾测，國外的大神把消息轉(zhuǎn)發(fā)的流程通過偽代碼寫了出來.
int __forwarding__(void *frameStackPointer, int isStret) {
id receiver = *(id *)frameStackPointer;
SEL sel = *(SEL *)(frameStackPointer + 8);
const char *selName = sel_getName(sel);
Class receiverClass = object_getClass(receiver);
// 調(diào)用 forwardingTargetForSelector:
if (class_respondsToSelector(receiverClass, @selector(forwardingTargetForSelector:))) {
id forwardingTarget = [receiver forwardingTargetForSelector:sel];
if (forwardingTarget && forwardingTarget != receiver) {
return objc_msgSend(forwardingTarget, sel, ...);
}
}
// 調(diào)用 methodSignatureForSelector 獲取方法簽名后再調(diào)用 forwardInvocation
if (class_respondsToSelector(receiverClass, @selector(methodSignatureForSelector:))) {
NSMethodSignature *methodSignature = [receiver methodSignatureForSelector:sel];
if (methodSignature && class_respondsToSelector(receiverClass, @selector(forwardInvocation:))) {
NSInvocation *invocation = [NSInvocation _invocationWithMethodSignature:methodSignature frame:frameStackPointer];
[receiver forwardInvocation:invocation];
void *returnValue = NULL;
[invocation getReturnValue:&value];
return returnValue;
}
}
if (class_respondsToSelector(receiverClass,@selector(doesNotRecognizeSelector:))) {
[receiver doesNotRecognizeSelector:sel];
}
// The point of no return.
kill(getpid(), 9);
}
-(id)forwardingTargetForSelector:(SEL)aSelector——__forwarding__首先會看類有沒有實現(xiàn)這個方法爆价，這個方法返回的是一個id類型的轉(zhuǎn)發(fā)對象forwardingTarget，如果其不為空媳搪，則會通過objc_msgSend函數(shù)對其直接發(fā)送消息objc_msgSend(forwardingTarget, sel, ...);，也就是說讓轉(zhuǎn)發(fā)對象forwardingTarget去處理當(dāng)前的方法SEL骤宣。如果forwardingTarget為nil秦爆，則進(jìn)入下面的方法
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector——這個方法是讓我們根據(jù)方法選擇器SEL生成一個NSMethodSignature方法簽名并返回，這個方法簽名里面其實就是封裝了返回值類型憔披，參數(shù)類型的信息等限。
__forwarding__會利用這個方法簽名，生成一個NSInvocation芬膝，將其作為參數(shù)望门，調(diào)用- (void)forwardInvocation:(NSInvocation *)anInvocation方法。如果我們在這里沒有返回方法簽名锰霜，系統(tǒng)則認(rèn)為我們徹底不想處理這個方法了筹误，就會調(diào)用doesNotRecognizeSelector:方法拋出經(jīng)典的報錯報錯unrecognized selector sent to instance 0xXXXXXXXX，結(jié)束消息機(jī)制的全部流程癣缅。
- (void)forwardInvocation:(NSInvocation *)anInvocation——如果我們在上面提供了方法簽名厨剪，__forwarding__則會最終調(diào)用這個方法。在這個方法里面友存，我們會拿到一個參數(shù)(NSInvocation *)anInvocation祷膳，這個anInvocation其實是__forwarding__對如下三個信息的封裝：
anInvocation.target-- 方法調(diào)用者anInvocation.selector-- 方法名- (void)getArgument:(void *)argumentLocation atIndex:(NSInteger)idx;-- 方法參數(shù)
因此在此方法里面屡立，我們可以決定將消息轉(zhuǎn)發(fā)給誰（target）直晨，甚至還可以修改消息的參數(shù)，由于anInvocation會存儲消息selector里面帶來的參數(shù)，并且可以根據(jù)消息所對應(yīng)的方法簽名確定消息參數(shù)的個數(shù)勇皇，所以我們通過- (void)setArgument:(void *)argumentLocation atIndex:(NSInteger)idx;可以對參數(shù)進(jìn)行修改奕巍。總之你可以按照你的意愿，配置好anInvocation儒士，然后簡單一句[anInvocation invoke];即可完成消息的轉(zhuǎn)發(fā)調(diào)用的止，也可以不做任何處理，輕輕地來着撩，輕輕地走诅福，但是不會導(dǎo)致程序報錯。
我們用代碼驗證一下:
***********************??MJPerson.h ??**************************
#import <Foundation/Foundation.h>
@interface MJPerson : NSObject
- (void)test;
@end
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
- (id)forwardingTargetForSelector:(SEL)aSelector
{
if (aSelector == @selector(test)) {
// objc_msgSend([[MJCat alloc] init], aSelector)
return [[MJCat alloc] init];
}
return [super forwardingTargetForSelector:aSelector];
}
@end
***********************??MJCat.h ??**************************
#import <Foundation/Foundation.h>
@interface MJCat : NSObject
- (void)test;
@end
***********************??MJCat.m ??**************************
#import "MJCat.h"
@implementation MJCat
- (void)test
{
NSLog(@"%s", __func__);
}
@end
#import <Foundation/Foundation.h>
#import "MJPerson.h"
// 消息轉(zhuǎn)發(fā)：將消息轉(zhuǎn)發(fā)給別人
int main(int argc, const char * argv[]) {
@autoreleasepool {
MJPerson *person = [[MJPerson alloc] init];
[person test];
}
return 0;
}
RUN>
*********************** 運(yùn)行結(jié)果 ************************** 2021-05-06 19:59:53.504938+0800 Interview01-消息轉(zhuǎn)發(fā)[5988:243172] -[MJCat test]
修改??MJPerson.m ??
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
// 方法簽名：返回值類型拖叙、參數(shù)類型
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
NSLog(@"methodSignatureForSelector");
if (aSelector == @selector(test)) {
return [NSMethodSignature signatureWithObjCTypes:"v16@0:8"];
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
NSLog(@"forwardInvocation");
}
@end
RUN>
*********************** 運(yùn)行結(jié)果 ************************** 2021-05-06 20:05:39.826862+0800 Interview01-消息轉(zhuǎn)發(fā)[6060:246757] methodSignatureForSelector 2021-05-06 20:05:39.827236+0800 Interview01-消息轉(zhuǎn)發(fā)[6060:246757] forwardInvocation
繼續(xù)修改??MJPerson.m ??
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
// 方法簽名：返回值類型氓润、參數(shù)類型
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
NSLog(@"methodSignatureForSelector");
if (aSelector == @selector(test)) {
return [NSMethodSignature signatureWithObjCTypes:"v16@0:8"];
}
return [super methodSignatureForSelector:aSelector];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
NSLog(@"forwardInvocation");
[anInvocation invokeWithTarget:[[MJCat alloc] init]];
}
@end
RUN>
*********************** ??運(yùn)行結(jié)果?? ************************** 2021-05-06 20:11:46.271551+0800 Interview01-消息轉(zhuǎn)發(fā)[6159:252454] methodSignatureForSelector 2021-05-06 20:11:46.271920+0800 Interview01-消息轉(zhuǎn)發(fā)[6159:252454] forwardInvocation 2021-05-06 20:11:46.271998+0800 Interview01-消息轉(zhuǎn)發(fā)[6159:252454] -[MJCat test]
以上都是以實力方法為例,下面我們把-(void)test;更改為+(void)test:
***********************??MJPerson.h ??**************************
#import <Foundation/Foundation.h>
@interface MJPerson : NSObject
+ (void)test;
@end
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
- (id)forwardingTargetForSelector:(SEL)aSelector
{
if (aSelector == @selector(test)) {
return [[MJCat alloc] init];
}
return [super forwardingTargetForSelector:aSelector];
}
@end
***********************??MJCat.h ??**************************
#import <Foundation/Foundation.h>
@interface MJCat : NSObject
+ (void)test;
- (void)test;
@end
***********************??MJCat.m ??**************************
#import "MJCat.h"
@implementation MJCat
+ (void)test
{
NSLog(@"%s", __func__);
}
- (void)test
{
NSLog(@"%s", __func__);
}
@end
修改??MJPerson.m ??
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
+ (id)forwardingTargetForSelector:(SEL)aSelector
{
if (aSelector == @selector(test)) {
return [[MJCat alloc] init];
}
return [super forwardingTargetForSelector:aSelector];
}
@end
繼續(xù)修改??MJPerson.m ??
***********************??MJPerson.m ??**************************
#import "MJPerson.h"
#import <objc/runtime.h>
#import "MJCat.h"
@implementation MJPerson
+ (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
if (aSelector == @selector(test)) return [NSMethodSignature signatureWithObjCTypes:"v@:"];
return [super methodSignatureForSelector:aSelector];
}
+ (void)forwardInvocation:(NSInvocation *)anInvocation
{
NSLog(@"1123");
}
@end
RUN>
*********************** ??運(yùn)行結(jié)果?? ************************** 2021-05-06 20:21:55.074676+0800 Interview01-消息轉(zhuǎn)發(fā)[6264:259359] 1123
到這里我們就搞清楚了消息轉(zhuǎn)發(fā)的所有流程和細(xì)節(jié)
image-20210506202309515
特別備注
本系列文章總結(jié)自MJ老師在騰訊課堂iOS底層原理班（下）/OC對象/關(guān)聯(lián)對象/多線程/內(nèi)存管理/性能優(yōu)化，相關(guān)圖片素材均取自課程中的課件薯鳍。如有侵權(quán)咖气，請聯(lián)系我刪除，謝謝挖滤！
