Object-c 反射技术

C++
Class objc_getClass(const char *aClassName)
{
    if (!aClassName) return Nil;
 
    // NO unconnected, YES class handler
    return look_up_class(aClassName, NO, YES);
}
 
 。。。。
 
 
/***********************************************************************
* look_up_class
* Look up a class by name, and realize it.
* Locking: acquires runtimeLock
**********************************************************************/
static BOOL empty_getClass(const char *name, Class *outClass)
{
    *outClass = nil;
    return NO;
}
 
static ChainedHookFunction GetClassHook{empty_getClass};
 
void objc_setHook_getClass(objc_hook_getClass newValue,
                           objc_hook_getClass *outOldValue)
{
    GetClassHook.set(newValue, outOldValue);
}
 
Class 
look_up_class(const char *name, 
              bool includeUnconnected __attribute__((unused)), 
              bool includeClassHandler __attribute__((unused)))
{
    if (!name) return nil;
 
    Class result;
    bool unrealized;
    {
        runtimeLock.lock();
        result = getClassExceptSomeSwift(name);
        unrealized = result  &&  !result->isRealized();
        if (unrealized) {
            result = realizeClassMaybeSwiftAndUnlock(result, runtimeLock);
            // runtimeLock is now unlocked
        } else {
            runtimeLock.unlock();
        }
    }
 
    if (!result) {
        // Ask Swift about its un-instantiated classes.
 
        // We use thread-local storage to prevent infinite recursion
        // if the hook function provokes another lookup of the same name
        // (for example, if the hook calls objc_allocateClassPair)
 
        auto *tls = _objc_fetch_pthread_data(true);
 
        // Stop if this thread is already looking up this name.
        for (unsigned i = 0; i < tls->classNameLookupsUsed; i++) {
            if (0 == strcmp(name, tls->classNameLookups[i])) {
                return nil;
            }
        }
 
        // Save this lookup in tls.
        if (tls->classNameLookupsUsed == tls->classNameLookupsAllocated) {
            tls->classNameLookupsAllocated =
                (tls->classNameLookupsAllocated * 2 ?: 1);
            size_t size = tls->classNameLookupsAllocated *
                sizeof(tls->classNameLookups[0]);
            tls->classNameLookups = (const char **)
                realloc(tls->classNameLookups, size);
        }
        tls->classNameLookups[tls->classNameLookupsUsed++] = name;
 
        // Call the hook.
        Class swiftcls = nil;
        if (GetClassHook.get()(name, &swiftcls)) {
            ASSERT(swiftcls->isRealized());
            result = swiftcls;
        }
 
        // Erase the name from tls.
        unsigned slot = --tls->classNameLookupsUsed;
        ASSERT(slot >= 0  &&  slot < tls->classNameLookupsAllocated);
        ASSERT(name == tls->classNameLookups[slot]);
        tls->classNameLookups[slot] = nil;
    }
 
    return result;
}
 

C++
 OBJC_EXPORT Method _Nullable
class_getClassMethod(Class _Nullable cls, SEL _Nonnull name)

 。。。。。
 NEVER_INLINE
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;
}

C++
 {
            Class cls = objc_getClass("NSString");
            NSObject *pObject = [cls alloc];
            Method thmod=  class_getInstanceMethod(cls, @selector(init));
            IMP funPtr =   method_getImplementation(thmod);
            NSString *pS =  ((id(*)(id, SEL))funPtr)(pObject, @selector(init));
            
  }

C++
 Class metaCls = objc_getMetaClass("NSString");
if (metaCls)
{
   Method thmod =  class_getClassMethod(cls, @selector(string));
   IMP funPtr =   method_getImplementation(thmod);
   NSString *pS =  ((id(*)(id, SEL))funPtr)(cls, @selector(string));

}

C++
Method class_getClassMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return nil;
 
    return class_getInstanceMethod(cls->getMeta(), sel);
}
     Class getMeta() {
        if (isMetaClassMaybeUnrealized()) return (Class)this;
        else return this->ISA();
    }