前言
上章写了消息流程中,查找cache的过程,当cache中不存在需要的方法时,系统会调用lookUpImpOrForward进入慢速查找流程。
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();
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
runtimeLock.lock();
checkIsKnownClass(cls);
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
}
if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
}
runtimeLock.assertLocked();
curClass = cls;
for (unsigned attempts = unreasonableClassCount();;) {
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
imp = meth->imp;
goto done;
}
if (slowpath((curClass = curClass->superclass) == nil)) {
imp = forward_imp;
break;
}
if (slowpath(--attempts == 0)) {
_objc_fatal("Memory corruption in class list.");
}
imp = cache_getImp(curClass, sel);
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;
}
- 先再查找一遍
cache,避免异步造成的影响
if (fastpath(behavior & LOOKUP_CACHE)) {
imp = cache_getImp(cls, sel);
if (imp) goto done_nolock;
}
-
checkIsKnownClass(cls);检查当前类是否是已加载的类 -
realizeClassMaybeSwiftAndLeaveLocked确定继承链 -
initializeAndLeaveLocked确定全部初始化 - 慢速查找的关键流程
for (unsigned attempts = unreasonableClassCount();;)不断循环递归查找需要的方法 -
Method meth = getMethodNoSuper_nolock(curClass, sel);二分法查找方法,找到则进入done执行,没有继续后续步骤
//二分查找算法
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.
//往前查找 -- 分类的方法会覆盖类的方法
while (probe > first && keyValue == (uintptr_t)probe[-1].name) {
probe--;
}
return (method_t *)probe;
}
if (keyValue > probeValue) {
base = probe + 1;
count--;
}
}
return nil;
}
- 当前类没找到,会进入父类先查找
cache再查找父类方法列表,找到也跳出循环。 - 当循环结束后,跳出循环统一进行一次处理,也是OC的容错处理
if (slowpath(behavior & LOOKUP_RESOLVER)) {
behavior ^= LOOKUP_RESOLVER;
return resolveMethod_locked(inst, sel, cls, behavior);
}
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);
}
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));
}
}
}
- 消息会在
resolveInstanceMethod:进行第一次处理,处理完后会再走一遍lookUpImpOrNil,返回新的处理结果
加上上章cache查找,消息处理流程可以总结如图:(图片来源于iOS-底层原理 13：消息流程分析之慢速查找)
消息查找流程