lookUpImpOrForward方法

方法的快速查找流程，如果没有找到，最终进入_lookUpImpOrForward方法，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 (slowpath(!cls->isInitialized())) {
        behavior |= LOOKUP_NOCACHE;
    }
    runtimeLock.lock();
    
    checkIsKnownClass(cls);

    cls = realizeAndInitializeIfNeeded_locked(inst, cls, behavior & LOOKUP_INITIALIZE);
    runtimeLock.assertLocked();
    curClass = cls;

    for (unsigned attempts = unreasonableClassCount();;) {
        if (curClass->cache.isConstantOptimizedCache(/* strict */true)) {
#if CONFIG_USE_PREOPT_CACHES
            imp = cache_getImp(curClass, sel);
            if (imp) goto done_unlock;
            curClass = curClass->cache.preoptFallbackClass();
#endif
        } else {
            // curClass method list.
            Method meth = getMethodNoSuper_nolock(curClass, sel);
            if (meth) {
                imp = meth->imp(false);
                goto done;
            }

            if (slowpath((curClass = curClass->getSuperclass()) == 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);
        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:
    if (fastpath((behavior & LOOKUP_NOCACHE) == 0)) {
#if CONFIG_USE_PREOPT_CACHES
        while (cls->cache.isConstantOptimizedCache(/* strict */true)) {
            cls = cls->cache.preoptFallbackClass();
        }
#endif
        log_and_fill_cache(cls, imp, sel, inst, curClass);
    }
 done_unlock:
    runtimeLock.unlock();
    if (slowpath((behavior & LOOKUP_NIL) && imp == forward_imp)) {
        return nil;
    }
    return imp;
}

查看lookUpImpOrForward函数

• 是否注册类checkIsKnownClass，如果没有直接报错
• realizeAndInitializeIfNeeded_locked方法，实现相关类

supercls = realizeClassWithoutSwift(remapClass(cls->getSuperclass()), nil);
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);

进入查找流程，继续查看缓存，防止方法中途被插入缓存

        if (curClass->cache.isConstantOptimizedCache(/* strict */true)) {
#if CONFIG_USE_PREOPT_CACHES
            imp = cache_getImp(curClass, sel);
            if (imp) goto done_unlock;
            curClass = curClass->cache.preoptFallbackClass();
#endif
        }

缓存二次查找，仍然未查找到，进入Method meth = getMethodNoSuper_nolock(curClass, sel);方法查找，查看getMethodNoSuper_nolock函数。

getMethodNoSuper_nolock函数

从methods.beginLists()开始，到methods.endLists()，并且mlists != end，遍历查找search_method_list_inline

static method_t *getMethodNoSuper_nolock(Class cls, SEL sel)
{
    runtimeLock.assertLocked();

    ASSERT(cls->isRealized());
    // fixme nil cls? 
    // fixme nil sel?

    auto const methods = cls->data()->methods();
    for (auto mlists = methods.beginLists(),
              end = methods.endLists();
         mlists != end;
         ++mlists)
    {
        // <rdar://problem/46904873> getMethodNoSuper_nolock is the hottest
        // caller of search_method_list, inlining it turns
        // getMethodNoSuper_nolock into a frame-less function and eliminates
        // any store from this codepath.
        method_t *m = search_method_list_inline(*mlists, sel);
        if (m) return m;
    }

    return nil;
}

ALWAYS_INLINE static method_t *search_method_list_inline(const method_list_t *mlist, SEL sel)
{
    int methodListIsFixedUp = mlist->isFixedUp();
    int methodListHasExpectedSize = mlist->isExpectedSize();
    
    if (fastpath(methodListIsFixedUp && methodListHasExpectedSize)) {
        return findMethodInSortedMethodList(sel, mlist);
    } else {
        // Linear search of unsorted method list
        if (auto *m = findMethodInUnsortedMethodList(sel, mlist))
            return m;
    }

#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;
}

如果methodListIsFixedUp && methodListHasExpectedSize，进入有序查找findMethodInSortedMethodList，否则进入无序查找findMethodInUnsortedMethodList

二分查找法findMethodInSortedMethodList

有序查找，使用的是二分查找法

ALWAYS_INLINE static method_t *
findMethodInSortedMethodList(SEL key, const method_list_t *list, const getNameFunc &getName)
{
    ASSERT(list);

    auto first = list->begin();
    auto base = first;
    decltype(first) 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)getName(probe);
        
        if (keyValue == probeValue) {
            // `probe` is a match.
            // Rewind looking for the *first* occurrence of this value.
            // This is required for correct category overrides.
            while (probe > first && keyValue == (uintptr_t)getName((probe - 1))) {
                probe--;
            }
            return &*probe;
        }
        
        if (keyValue > probeValue) {
            base = probe + 1;
            count--;
        }
    }
    
    return nil;
}

例子：count = 8，需要查找为2

1. count = 8，base=0，probe = base + (count >> 1)为4，这时keyValue < probeValue，不做处理，进入流程2
2. count >>= 1，count = 4，base=0，probe = base + (count >> 1)为2，这时keyValue = probeValue，命中。

缓存方法

如果查找到方法，跳出循环，进入done，调用log_and_fill_cache函数将方法写入缓存，方便下次直接快速查找

    if (meth) {
         imp = meth->imp(false);
         goto done;
    }
            
done:
    if (fastpath((behavior & LOOKUP_NOCACHE) == 0)) {
#if CONFIG_USE_PREOPT_CACHES
        while (cls->cache.isConstantOptimizedCache(/* strict */true)) {
            cls = cls->cache.preoptFallbackClass();
        }
#endif
        log_and_fill_cache(cls, imp, sel, inst, curClass);
    }

如未找到，进入curClass = curClass->getSuperclass()父类查找，先查询父类缓存，imp = cache_getImp(curClass, sel);，找不到进入慢速查找，遍历，如最终未找到，返回forward_imp

实例方法查找流程 类->父类-NSObject->nil
类方法查找流程元类->元类的父类-根元类->NSObject->nil
由上面可以得知，直接用类调用NSObject里面的实例方法，可以成功。