八、消息流程之慢速查找

在Object-C中调方法在底层就是调objc_msgSend进行发送消息,消息发送时先在Class的cache中查找imp,这一步为快速查找,在cache中没有找到,就会在bits中查找,这一步为慢速查找。

1、慢速查找方法定位

结尾找到了MethodTableLookup,在搜索一下MethodTableLookup的定义:

image.png

这里只需要看bl _lookUpImpOrForward,这个跳转语句,这里是跳到_lookUpImpOrForward这个方法了,全局搜一下:

image.png

基本上都是跳转和调用,没看到调用。可以去掉下划线在搜索一下:

image.png

这就是由汇编调到C方法了,终于不用翻译汇编了。

2、慢速查找流程

先来浏览一下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();

    if (slowpath(!cls->isInitialized())) {
        // The first message sent to a class is often +new or +alloc, or +self
        // which goes through objc_opt_* or various optimized entry points.
        //
        // However, the class isn't realized/initialized yet at this point,
        // and the optimized entry points fall down through objc_msgSend,
        // which ends up here.
        //
        // We really want to avoid caching these, as it can cause IMP caches
        // to be made with a single entry forever.
        //
        // Note that this check is racy as several threads might try to
        // message a given class for the first time at the same time,
        // in which case we might cache anyway.
        behavior |= LOOKUP_NOCACHE;
    }

    // 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.
    checkIsKnownClass(cls);

    cls = realizeAndInitializeIfNeeded_locked(inst, cls, behavior & LOOKUP_INITIALIZE);
    // runtimeLock may have been dropped but is now locked again
    runtimeLock.assertLocked();
    curClass = cls;

    // The code used to lookup 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();;) {
        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;
}

代码的行数并不是很多,来逐行分析:
1、const IMP forward_imp = (IMP)_objc_msgForward_impcache;,初始化一个impforward_imp变量,看看_objc_msgForward_impcache是怎么定义的:

image.png

又跳到汇编里面来了,__objc_msgForward_impcache跳转到 __objc_msgForward,在__objc_msgForward里面x17变量调用__objc_forward_handler方法操作,看看__objc_forward_handler是怎么操作的:

image.png

看到这里熟不熟悉,如果不熟悉看下面的:

image.png

这个变量就是在经历慢速查找、消息动态决议等之后条用的方法。这里我看还可以看到,对底层而言是不存在类方法还是实例方法的,+-的判断是根据是不是元类来认为判断的。

2、cls = realizeAndInitializeIfNeeded_locked(inst, cls, behavior & LOOKUP_INITIALIZE);初始化当前的类,来看看怎么初始化的,一步步跟进来找到OC的初始化过程:

static Class realizeClassWithoutSwift(Class cls, Class previously)
{
    runtimeLock.assertLocked();

    class_rw_t *rw;
    Class supercls;
    Class metacls;

    if (!cls) return nil;
    if (cls->isRealized()) {
        validateAlreadyRealizedClass(cls);
        return cls;
    }
    ASSERT(cls == remapClass(cls));

    // fixme verify class is not in an un-dlopened part of the shared cache?

    auto ro = (const class_ro_t *)cls->data();
    auto isMeta = ro->flags & RO_META;
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro();
        ASSERT(!isMeta);
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        // Normal class. Allocate writeable class data.
        rw = objc::zalloc<class_rw_t>();
        rw->set_ro(ro);
        rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
        cls->setData(rw);
    }

    cls->cache.initializeToEmptyOrPreoptimizedInDisguise();

#if FAST_CACHE_META
    if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif

    // Choose an index for this class.
    // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
    cls->chooseClassArrayIndex();

    if (PrintConnecting) {
        _objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
                     cls->nameForLogging(), isMeta ? " (meta)" : "", 
                     (void*)cls, ro, cls->classArrayIndex(),
                     cls->isSwiftStable() ? "(swift)" : "",
                     cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
    }

    // Realize superclass and metaclass, if they aren't already.
    // This needs to be done after RW_REALIZED is set above, for root classes.
    // This needs to be done after class index is chosen, for root metaclasses.
    // This assumes that none of those classes have Swift contents,
    //   or that Swift's initializers have already been called.
    //   fixme that assumption will be wrong if we add support
    //   for ObjC subclasses of Swift classes.
    supercls = realizeClassWithoutSwift(remapClass(cls->getSuperclass()), nil);
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);

#if SUPPORT_NONPOINTER_ISA
    if (isMeta) {
        // Metaclasses do not need any features from non pointer ISA
        // This allows for a faspath for classes in objc_retain/objc_release.
        cls->setInstancesRequireRawIsa();
    } else {
        // Disable non-pointer isa for some classes and/or platforms.
        // Set instancesRequireRawIsa.
        bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
        bool rawIsaIsInherited = false;
        static bool hackedDispatch = false;

        if (DisableNonpointerIsa) {
            // Non-pointer isa disabled by environment or app SDK version
            instancesRequireRawIsa = true;
        }
        else if (!hackedDispatch  &&  0 == strcmp(ro->getName(), "OS_object"))
        {
            // hack for libdispatch et al - isa also acts as vtable pointer
            hackedDispatch = true;
            instancesRequireRawIsa = true;
        }
        else if (supercls  &&  supercls->getSuperclass()  &&
                 supercls->instancesRequireRawIsa())
        {
            // This is also propagated by addSubclass()
            // but nonpointer isa setup needs it earlier.
            // Special case: instancesRequireRawIsa does not propagate
            // from root class to root metaclass
            instancesRequireRawIsa = true;
            rawIsaIsInherited = true;
        }

        if (instancesRequireRawIsa) {
            cls->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
        }
    }
// SUPPORT_NONPOINTER_ISA
#endif

    // Update superclass and metaclass in case of remapping
    cls->setSuperclass(supercls);
    cls->initClassIsa(metacls);

    // Reconcile instance variable offsets / layout.
    // This may reallocate class_ro_t, updating our ro variable.
    if (supercls  &&  !isMeta) reconcileInstanceVariables(cls, supercls, ro);

    // Set fastInstanceSize if it wasn't set already.
    cls->setInstanceSize(ro->instanceSize);

    // Copy some flags from ro to rw
    if (ro->flags & RO_HAS_CXX_STRUCTORS) {
        cls->setHasCxxDtor();
        if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
            cls->setHasCxxCtor();
        }
    }
    
    // Propagate the associated objects forbidden flag from ro or from
    // the superclass.
    if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
        (supercls && supercls->forbidsAssociatedObjects()))
    {
        rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
    }

    // Connect this class to its superclass's subclass lists
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

    // Attach categories
    methodizeClass(cls, previously);

    return cls;
}

这个看起来有点复杂,简化一下流程,具体看注释:

static Class realizeClassWithoutSwift(Class cls, Class previously)
{

    class_rw_t *rw;
    Class supercls;
    Class metacls;
// 设置class的ro和rw
    auto ro = (const class_ro_t *)cls->data();
    auto isMeta = ro->flags & RO_META;
    if (ro->flags & RO_FUTURE) {
        // This was a future class. rw data is already allocated.
        rw = cls->data();
        ro = cls->data()->ro();
        cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
    } else {
        rw = objc::zalloc<class_rw_t>();
        rw->set_ro(ro);
        rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
        cls->setData(rw);
    }
// 递归初始化父类和元类

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

// 设置父类和元类
    cls->setSuperclass(supercls);
    cls->initClassIsa(metacls);

//设置子类
    if (supercls) {
        addSubclass(supercls, cls);
    } else {
        addRootClass(cls);
    }

    // Attach categories
    methodizeClass(cls, previously);

    return cls;
}

可以看出初始化类的作用就是:
(1)设置class的ro和rw
(2)递归初始化父类和元类
(3)设置父类和元类
(4)设置子类
经过以上4步,类的基本结构就已经完成了。另外,我们也可以看到一个类既有父类又有子类,可以说他就是双向链表结构。

3、接下来就是一个for结构的死循环:
3.1 第一次进来的时候现在又在自己的缓存中找一遍:

imp = cache_getImp(curClass, sel);
if (imp) goto done_unlock;

看苹果的注释:

image.png

curClass = curClass->cache.preoptFallbackClass();执行这一行码之后curClass->cache.isConstantOptimizedCache(/* strict */true)这个判断条件就为false了,下一次进来就会走到else里面了。

3.2 Method meth = getMethodNoSuper_nolock(curClass, sel);从当前类里面找,跟一下其实现方式:

template<class getNameFunc>
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;
}

这个方法介绍两点:
(1)这个查找方式是二分查找
(2)会优先查找category的方法
如果在当前类里面找到就会走到done

log_and_fill_cache(cls, imp, sel, inst, curClass);

static void
log_and_fill_cache(Class cls, IMP imp, SEL sel, id receiver, Class implementer)
{
#if SUPPORT_MESSAGE_LOGGING
    if (slowpath(objcMsgLogEnabled && implementer)) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    cls->cache.insert(sel, imp, receiver);
}

然后就走到cache.insert方法,这个方法在一篇中已经介绍过,就形成了一个完整的闭环。

3.3 如果在当前类没有找到,就会走到:

if (slowpath((curClass = curClass->getSuperclass()) == nil)) {
     // No implementation found, and method resolver didn't help.
     // Use forwarding.
     imp = forward_imp;
     break;
}

条件slowpath((curClass = curClass->getSuperclass()) == nil),有两个作用将curClass指向父类,判断父类是否为nil,如果父类为nil,将imp赋值为forward_imp,然后出循环,注意这里用的是break,如果父类不为nil,就继续查找父类的cache。

3.4

// 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;
}

查找父类的cache,如果找到imp == forward_imp也是break,如果不是goto done,都不满足接着下一个循环,一直到slowpath((curClass = curClass->getSuperclass()) == nil)这个条件满足跳出所有循环。

4

 if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }

在for的死循环中凡是break的都会走到这里来,说明类自己以及父类都没有找到,最后会调resolveMethod_locked动态方法决议来处理,这个过程下一篇来介绍。

  • objc_msgSend流程总结:
image.png
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