下边的图是查看Runtime源码（objc-756），概括出来的大概的OC 方法查找流程。
sel-IMP查找流程
首先你需要知道一些基本的知识，了解一个alloc 出来的对象，它都包含什么？
你要了解 ISA走位图，superclass的走位图。
知道：对象分为： 实例对象（Instance Object），类对象（Class Object），元类对象（Meta Class）
-实例方法存放在类对象里，+类方法存放在元类对象里
知道：Runtime是由汇编语言C C++语言编成的，提供运行时功能的库。
知道：OC对象的本质是在C层面，它是一个Struct，方法调用，是发送消息。
知道：将下面代码转译成C++代码，clang -rewrite-objc main.m，转译生成main.cpp文件后，方法是objc_msgSend
LGTeacher *p = [LGTeacher alloc];
[p sayHello];
OC方法查找可大概分为慢速查找和快速查找。
开启Xcode debug 汇编模式，断点方法调用，会找到_objc_msgSend汇编代码。
1:在objc-msg-arm64.s中找到 ENTRY _objc_msgSend,
ENTRY _objc_msgSend
UNWIND _objc_msgSend, NoFrame
cmp p0, #0 // nil check and tagged pointer check
#if SUPPORT_TAGGED_POINTERS
b.le LNilOrTagged // (MSB tagged pointer looks negative)
#else
b.eq LReturnZero
#endif
ldr p13, [x0] // p13 = isa
GetClassFromIsa_p16 p13 // p16 = class
LGetIsaDone:
// calls imp or objc_msgSend_uncached
CacheLookup NORMAL, _objc_msgSend
......
其上，会发现，进入_objc_msgSend，会进行
1：Tagged Pointer 判定
2：isa 与 class 绑定
3：然后CacheLookup NORMAL（// calls imp or objc_msgSend_uncached）
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//方法表查找
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
.macro MethodTableLookup
....
// lookUpImpOrForward(obj, sel, cls, LOOKUP_INITIALIZE | LOOKUP_RESOLVER)
// receiver and selector already in x0 and x1
mov x2, x16
mov x3, #3
bl _lookUpImpOrForward
.....
.endmacro
CacheLookup or objc_msgSend_uncached，若找到imp 则返回IMP，否则进入慢速查找lookUpImpOrForward
2:在objc-runtime-new.mm找到lookUpImpOrForward
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) {//缓存返回imp
imp = cache_getImp(cls, sel);
if (imp) return imp;
}
runtimeLock.lock();// 多个sel 查找，需要加锁
checkIsKnownClass(cls);// check unKnown cls
if (!cls->isRealized()) {//检测类是否 已实现（构建）
realizeClass(cls);//实现类（构建类）
}
if (initialize && !cls->isInitialized()) {//初始化 cls
runtimeLock.unlock();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.lock();
// 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.assertLocked();
// Try this class's cache.
imp = cache_getImp(cls, sel);//缓存查找
if (imp) goto done;
// 1: 查找当前cls的方法list
// Try this class's method lists.
{
Method meth = getMethodNoSuper_nolock(cls, sel);//方法查找
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, cls);//缓存
imp = meth->imp;
goto done;
}
}
// 2:查找父类的缓存和方法list
// 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;
}
}
}
// 3: 没有找到实现，尝试方法 resolver once
// No implementation found. Try method resolver once.
if (resolver && !triedResolver) {
runtimeLock.unlock();
_class_resolveMethod(cls, sel, inst);
runtimeLock.lock();
// 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.
// 4: 实现没有找到，resolver method 没实现，进行消息转发
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);//进行缓存
done:
runtimeLock.unlock();
return imp;
}
慢速查找流程大体如下：
1：cache_getImp，缓存确认，返回imp
2：try 当前cls的 method list
3：try父类cache和method list
4：没找到 --> try method resolver one
5： no resolver -> try Use forwarding（消息转发）
6：依然没找到，cache_fill()