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前言
本文主要是为了探索方法调用在iOS底层的实现。
正文
一、回顾
在上一篇文章——iOS底层探索：objc_msgSend快速查找中，我们讲到了方法的快速查找流程，如果快速查找流程缓存命中，那么方法查找过程就到此结束了，但是如果缓存中没有找到对应的方法，则会跳转到JumpMiss流程中的__objc_msgSend_uncached。
二、慢速查找流程探索
1、__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
TailCallFunctionPointer x17
END_ENTRY __objc_msgSend_uncached
2、MethodTableLookup
.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
经过1、2两个过程后，就进到慢速查找流程的关键函数IMP lookUpImpOrForward(id inst, SEL sel, Class cls, int behavior)；
3、lookUpImpOrForward
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;
}
大致过程分析：
a.判断是否是已知的类：checkIsKnownClass(cls);
b.判断类是否实现isRealized()，如果没有，则需要先实现，确定类和元类的继承链：cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
c.判断类是否初始化isInitialized()，如果没有，则初始化
d.for (unsigned attempts = unreasonableClassCount();;)循环遍历继承链：
如果是实例方法，则首先查找当前类的方法列表，如果没有找到则对当前类的父类进行快速查找、慢速查找（当前类->父类->NSObject->nil），以此类推；
如果是类方法，则首先查找当前类的元类的方法列表，如果没有找到则对当前类的父类的元类进行快速查找、慢速查找（当前类的元类->父类的元类->NSObject元类->NSObject->nil），以此类推；
Method meth = getMethodNoSuper_nolock(curClass, sel);内部主要是通过二分查找的方式，对方法列表进行查找：
f.如果慢速查找找到了对应的方法，则会进行缓存操作log_and_fill_cache(cls, imp, sel, inst, curClass);，将方法存储带类的cache_t信息中，方便下次调用时快速查找使用。
g.如果慢速查找未找到了对应的方法，首先会走到动态方法决议，这部分我们下篇文章在进行讲解；
h.如果最终都没有找到方法，那么就会走到IMP forward_imp = (IMP)_objc_msgForward_impcache;这个方法实现中，我们进一步查看，就会发现这个地方就是我们日常会看到的报错信息。
