前言
在之前的的objc_msgSend()探索中，當(dāng)調(diào)用一個(gè)方法后乍桂，首先會(huì)進(jìn)入快速查找來匹配sel對(duì)應(yīng)的imp冲杀；當(dāng)沒有找到時(shí)，會(huì)進(jìn)入慢速查找模蜡，開始匹配漠趁；當(dāng)依然無法匹配時(shí)，蘋果給出了一次轉(zhuǎn)發(fā)的機(jī)會(huì)(動(dòng)態(tài)方法決議)：
// No implementation found. Try method resolver once.
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
從注釋：// No implementation found. Try method resolver once.也是一目了然忍疾。
這篇文章闯传，我們就一起探索下resolveMethod_locked()的內(nèi)部實(shí)現(xiàn)。
開始
直接看下resolveMethod_locked()的源碼：
static NEVER_INLINE IMP
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(bào)錯(cuò)
// 給你一次機(jī)會(huì)
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);
}
首先判斷當(dāng)前的cls是否為元類,當(dāng)不是元類則調(diào)用resolveInstanceMethod(),否則調(diào)用resolveClassMethod()卤妒。接下來分別看下兩個(gè)方法的具體實(shí)現(xiàn)：
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
/***********************************************************************
* 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 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));
}
}
}
該方法一開始的注釋寫的就很清楚：
- Call +resolveInstanceMethod, looking for a method to be added to class cls.
意味著甥绿，resolveInstanceMethod執(zhí)行會(huì)響應(yīng)+resolveInstanceMethod,我們可以通過在類中實(shí)現(xiàn)該方法字币，來接收響應(yīng)，并做處理共缕。同時(shí)我們可以看到發(fā)起響應(yīng)的關(guān)鍵代碼：
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);
即通過objc_msgSend向cls發(fā)送了一個(gè)resolveInstanceMethod消息洗出。
static void resolveClassMethod(id inst, SEL sel, Class cls)
//在cls中添加并實(shí)現(xiàn)
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
//自定義動(dòng)態(tài)處理
return [super resolveInstanceMethod:sel];
}
再來看下 resolveClassMethod的具體實(shí)現(xiàn)
/***********************************************************************
* resolveClassMethod
* Call +resolveClassMethod, looking for a method to be added to class cls.
* cls should be a metaclass.
* Does not check if the method already exists.
**********************************************************************/
static void resolveClassMethod(id inst, SEL sel, Class cls)
{
runtimeLock.assertUnlocked();
ASSERT(cls->isRealized());
ASSERT(cls->isMetaClass());
if (!lookUpImpOrNil(inst, @selector(resolveClassMethod:), cls)) {
// Resolver not implemented.
return;
}
Class nonmeta;
{
mutex_locker_t lock(runtimeLock);
nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);
// +initialize path should have realized nonmeta already
if (!nonmeta->isRealized()) {
_objc_fatal("nonmeta class %s (%p) unexpectedly not realized",
nonmeta->nameForLogging(), nonmeta);
}
}
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
bool resolved = msg(nonmeta, @selector(resolveClassMethod:), sel);
// Cache the result (good or bad) so the resolver doesn't fire next time.
// +resolveClassMethod adds to self->ISA() 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 resolveClassMethod:%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));
}
}
}
同樣在方法的注釋里面也寫的很清楚，在cls中添加并實(shí)現(xiàn)該方法來接收響應(yīng)图谷，關(guān)鍵的觸發(fā)響應(yīng)代碼翩活，同樣是通過objc_msgSend發(fā)送消息，跟resolveInstanceMethod的邏輯如出一轍：
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
bool resolved = msg(nonmeta, @selector(resolveClassMethod:), sel);
回到我們的類：
+ (BOOL)resolveInstanceMethod:(SEL)sel
{
//添加自定義處理
return [super resolveInstanceMethod:sel];
}
+ (BOOL)resolveClassMethod:(SEL)sel
{
//添加自定義處理
return [super resolveClassMethod:sel];
}
消息轉(zhuǎn)發(fā)
當(dāng)方法決議后依然沒有匹配到imp時(shí)便贵，會(huì)觸發(fā)消息轉(zhuǎn)發(fā)機(jī)制菠镇，由libObjC轉(zhuǎn)向CoreFoundation處理并觸發(fā)轉(zhuǎn)發(fā)回調(diào)，這個(gè)可以在Crash時(shí)的堆棧信息中看到：
截屏2020-10-15 下午2.54.31.png
關(guān)于
CoreFoundation的源碼這里就不展開驗(yàn)證了(需要借助一些反編譯工具查看一些偽代碼)承璃。此時(shí)會(huì)觸發(fā)以下幾個(gè)熟悉的回調(diào)函數(shù)：
//快速轉(zhuǎn)發(fā)流程
- (id)forwardingTargetForSelector:(SEL)aSelector
{
return nil;
}
//慢速轉(zhuǎn)發(fā)流程
//1.方法簽名
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector
{
return [NSMethodSignature signatureWithObjCTypes:"v@:"];
}
//2.轉(zhuǎn)發(fā)處理
- (void)forwardInvocation:(NSInvocation *)anInvocation
{
// [anInvocation invoke];
}
//當(dāng)沒有實(shí)現(xiàn)任何處理時(shí)
- (void)doesNotRecognizeSelector:(SEL)aSelector
{
}
在日常的開發(fā)中利耍，我們可以在適當(dāng)?shù)奈恢茫鲰憫?yīng)的容錯(cuò)處理代碼盔粹。
消息轉(zhuǎn)發(fā)簡(jiǎn)單流程圖
消息轉(zhuǎn)發(fā)流程.008.jpeg
這里解釋下
resolveClassMethod->resolveInstanceMethod:當(dāng)調(diào)用類方法的時(shí)候隘梨，
Objc會(huì)去該類的元類中去查找對(duì)應(yīng)的IMP,沒有找到會(huì)去他的父元類中查找，最終會(huì)查詢到根元類舷嗡，根元類依然沒有找到轴猎，會(huì)繼續(xù)去根元類的父類，即根類(NSObject)中進(jìn)行查找咬崔，而類中只有實(shí)例方法税稼，此時(shí)objc內(nèi)部做了一些處理，當(dāng)在根元類無法找到IMP時(shí)垮斯，直接響應(yīng)父類(NSObject類)的resolveInstanceMethod方法郎仆，可以在如下的源碼中體現(xiàn)：
static NEVER_INLINE IMP
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
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);
}
總結(jié)
至此，我們能看到兜蠕，在我們程序?qū)⒁l(fā)生崩潰時(shí)扰肌，我們有兩次避免崩潰的機(jī)會(huì)，即動(dòng)態(tài)方法決議和消息轉(zhuǎn)發(fā)處理熊杨，前者是在libObjC下觸發(fā)曙旭，或者在CoreFoundation下觸發(fā)。在實(shí)際開發(fā)中我們應(yīng)該盡量避免在方法決議層做邏輯處理晶府，而是在更底層的消息轉(zhuǎn)發(fā)層做相應(yīng)的容錯(cuò)處理桂躏。
