iOS-底层原理 17：类的加载（上）

void
map_images(unsigned count, const char * const paths[],
           const struct mach_header * const mhdrs[])
{
    mutex_locker_t lock(runtimeLock);
    return map_images_nolock(count, paths, mhdrs);
}

void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
                  const struct mach_header * const mhdrs[])
{
    //...省略
    // Find all images with Objective-C metadata.查找所有带有Objective-C元数据的映像
    hCount = 0;
    // Count classes. Size various table based on the total.计算类的个数
    int totalClasses = 0;
    int unoptimizedTotalClasses = 0;
    //代码块：作用域，进行局部处理，即局部处理一些事件
    {
        //...省略
    }
    //...省略
    if (hCount > 0) {
        //加载镜像文件
        _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
    }
    firstTime = NO;
    // Call image load funcs after everything is set up.一切设置完成后，调用镜像加载功能。
    for (auto func : loadImageFuncs) {
        for (uint32_t i = 0; i < mhCount; i++) {
            func(mhdrs[i]);
        }
    }
}

if (!doneOnce) {
    //...省略
    // namedClasses
    // Preoptimized classes don't go in this table.
    // 4/3 is NXMapTable's load factor
    int namedClassesSize = 
        (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
//创建表（哈希表key-value），目的是查找快
    gdb_objc_realized_classes =
        NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
    ts.log("IMAGE TIMES: first time tasks");
}

// This is a misnomer: gdb_objc_realized_classes is actually a list of 
// named classes not in the dyld shared cache, whether realized or not.
//gdb_objc_realized_classes实际上是不在dyld共享缓存中的已命名类的列表，无论是否实现
NXMapTable *gdb_objc_realized_classes;  // exported for debuggers in objc-gdb.h

// Fix up @selector references 修复@selector引用
//sel 不是简单的字符串，而是带地址的字符串
static size_t UnfixedSelectors;
{
    mutex_locker_t lock(selLock);
    for (EACH_HEADER) {
        if (hi->hasPreoptimizedSelectors()) continue;
        bool isBundle = hi->isBundle();
        //通过_getObjc2SelectorRefs拿到Mach-O中的静态段__objc_selrefs
        SEL *sels = _getObjc2SelectorRefs(hi, &count);
        UnfixedSelectors += count;
        for (i = 0; i < count; i++) { //列表遍历
            const char *name = sel_cname(sels[i]);
            //注册sel操作，即将sel添加到
            SEL sel = sel_registerNameNoLock(name, isBundle);
            if (sels[i] != sel) {//当sel与sels[i]地址不一致时，需要调整为一致的
                sels[i] = sel;
            }
        }
    }
}

//      function name                 content type     section name
GETSECT(_getObjc2SelectorRefs,        SEL,             "__objc_selrefs"); 
GETSECT(_getObjc2MessageRefs,         message_ref_t,   "__objc_msgrefs"); 
GETSECT(_getObjc2ClassRefs,           Class,           "__objc_classrefs");
GETSECT(_getObjc2SuperRefs,           Class,           "__objc_superrefs");
GETSECT(_getObjc2ClassList,           classref_t const,      "__objc_classlist");
GETSECT(_getObjc2NonlazyClassList,    classref_t const,      "__objc_nlclslist");
GETSECT(_getObjc2CategoryList,        category_t * const,    "__objc_catlist");
GETSECT(_getObjc2CategoryList2,       category_t * const,    "__objc_catlist2");
GETSECT(_getObjc2NonlazyCategoryList, category_t * const,    "__objc_nlcatlist");
GETSECT(_getObjc2ProtocolList,        protocol_t * const,    "__objc_protolist");
GETSECT(_getObjc2ProtocolRefs,        protocol_t *,    "__objc_protorefs");
GETSECT(getLibobjcInitializers,       UnsignedInitializer, "__objc_init_func");

SEL sel_registerNameNoLock(const char *name, bool copy) {
    return __sel_registerName(name, 0, copy);  // NO lock, maybe copy
}
👇
static SEL __sel_registerName(const char *name, bool shouldLock, bool copy) 
{
    SEL result = 0;
    if (shouldLock) selLock.assertUnlocked();
    else selLock.assertLocked();
    if (!name) return (SEL)0;
    result = search_builtins(name);
    if (result) return result;
    conditional_mutex_locker_t lock(selLock, shouldLock);
    auto it = namedSelectors.get().insert(name);//sel插入表
    if (it.second) {
        // No match. Insert.
        *it.first = (const char *)sel_alloc(name, copy);
    }
    return (SEL)*it.first;
}

//3、错误混乱的类处理
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();
//读取类：readClass
for (EACH_HEADER) {
    if (! mustReadClasses(hi, hasDyldRoots)) {
        // Image is sufficiently optimized that we need not call readClass()
        continue;
    }
    //从编译后的类列表中取出所有类，即从Mach-O中获取静态段__objc_classlist，是一个classref_t类型的指针
    classref_t const *classlist = _getObjc2ClassList(hi, &count);
    bool headerIsBundle = hi->isBundle();
    bool headerIsPreoptimized = hi->hasPreoptimizedClasses();
    for (i = 0; i < count; i++) {
        Class cls = (Class)classlist[i];//此时获取的cls只是一个地址
        Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized); //读取类，经过这步后，cls获取的值才是一个名字
        //经过调试，并未执行if里面的流程
        //初始化所有懒加载的类需要的内存空间，但是懒加载类的数据现在是没有加载到的，连类都没有初始化
        if (newCls != cls  &&  newCls) {
            // Class was moved but not deleted. Currently this occurs 
            // only when the new class resolved a future class.
            // Non-lazily realize the class below.
            //将懒加载的类添加到数组中
            resolvedFutureClasses = (Class *)
                realloc(resolvedFutureClasses, 
                        (resolvedFutureClassCount+1) * sizeof(Class));
            resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
        }
    }
}
ts.log("IMAGE TIMES: discover classes");

//4、修复重映射一些没有被镜像文件加载进来的类
// Fix up remapped classes 修正重新映射的类
// Class list and nonlazy class list remain unremapped.类列表和非惰性类列表保持未映射
// Class refs and super refs are remapped for message dispatching.类引用和超级引用将重新映射以进行消息分发
//经过调试，并未执行if里面的流程
//将未映射的Class 和 Super Class重映射，被remap的类都是懒加载的类
if (!noClassesRemapped()) {
    for (EACH_HEADER) {
        Class *classrefs = _getObjc2ClassRefs(hi, &count);//Mach-O的静态段 __objc_classrefs
        for (i = 0; i < count; i++) {
            remapClassRef(&classrefs[i]);
        }
        // fixme why doesn't test future1 catch the absence of this?
        classrefs = _getObjc2SuperRefs(hi, &count);//Mach_O中的静态段 __objc_superrefs
        for (i = 0; i < count; i++) {
            remapClassRef(&classrefs[i]);
        }
    }
}
ts.log("IMAGE TIMES: remap classes");

#if SUPPORT_FIXUP
//5、修复一些消息
    // Fix up old objc_msgSend_fixup call sites
    for (EACH_HEADER) {
        // _getObjc2MessageRefs 获取Mach-O的静态段 __objc_msgrefs
        message_ref_t *refs = _getObjc2MessageRefs(hi, &count);
        if (count == 0) continue;
        if (PrintVtables) {
            _objc_inform("VTABLES: repairing %zu unsupported vtable dispatch "
                         "call sites in %s", count, hi->fname());
        }
        //经过调试，并未执行for里面的流程
        //遍历将函数指针进行注册，并fix为新的函数指针
        for (i = 0; i < count; i++) {
            fixupMessageRef(refs+i);
        }
    }
    ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif

//6、当类里面有协议时：readProtocol 读取协议
// Discover protocols. Fix up protocol refs. 发现协议。修正协议参考
//遍历所有协议列表，并且将协议列表加载到Protocol的哈希表中
for (EACH_HEADER) {
    extern objc_class OBJC_CLASS_$_Protocol;
    //cls = Protocol类，所有协议和对象的结构体都类似，isa都对应Protocol类
    Class cls = (Class)&OBJC_CLASS_$_Protocol;
    ASSERT(cls);
    //获取protocol哈希表 -- protocol_map
    NXMapTable *protocol_map = protocols();
    bool isPreoptimized = hi->hasPreoptimizedProtocols();
    // Skip reading protocols if this is an image from the shared cache
    // and we support roots
    // Note, after launch we do need to walk the protocol as the protocol
    // in the shared cache is marked with isCanonical() and that may not
    // be true if some non-shared cache binary was chosen as the canonical
    // definition
    if (launchTime && isPreoptimized && cacheSupportsProtocolRoots) {
        if (PrintProtocols) {
            _objc_inform("PROTOCOLS: Skipping reading protocols in image: %s",
                         hi->fname());
        }
        continue;
    }
    bool isBundle = hi->isBundle();
    //通过_getObjc2ProtocolList 获取到Mach-O中的静态段__objc_protolist协议列表，
    //即从编译器中读取并初始化protocol
    protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
    for (i = 0; i < count; i++) {
        //通过添加protocol到protocol_map哈希表中
        readProtocol(protolist[i], cls, protocol_map, 
                     isPreoptimized, isBundle);
    }
}
ts.log("IMAGE TIMES: discover protocols");

/***********************************************************************
* protocols
* Returns the protocol name => protocol map for protocols.
* Locking: runtimeLock must read- or write-locked by the caller
**********************************************************************/
static NXMapTable *protocols(void)
{
    static NXMapTable *protocol_map = nil;
    runtimeLock.assertLocked();
    INIT_ONCE_PTR(protocol_map, 
                  NXCreateMapTable(NXStrValueMapPrototype, 16), 
                  NXFreeMapTable(v) );
    return protocol_map;
}

protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);

readProtocol(protolist[i], cls, protocol_map, 
                         isPreoptimized, isBundle);

//7、修复没有被加载的协议
// Fix up @protocol references
// Preoptimized images may have the right 
// answer already but we don't know for sure.
for (EACH_HEADER) {
    // At launch time, we know preoptimized image refs are pointing at the
    // shared cache definition of a protocol.  We can skip the check on
    // launch, but have to visit @protocol refs for shared cache images
    // loaded later.
    if (launchTime && cacheSupportsProtocolRoots && hi->isPreoptimized())
        continue;
    //_getObjc2ProtocolRefs 获取到Mach-O的静态段 __objc_protorefs
    protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
    for (i = 0; i < count; i++) {//遍历
        //比较当前协议和协议列表中的同一个内存地址的协议是否相同，如果不同则替换
        remapProtocolRef(&protolist[i]);//经过代码调试，并未执行
    }
}
ts.log("IMAGE TIMES: fix up @protocol references");

//8、分类处理
// Discover categories. Only do this after the initial category 发现分类
// attachment has been done. For categories present at startup,
// discovery is deferred until the first load_images call after
// the call to _dyld_objc_notify_register completes. rdar://problem/53119145
if (didInitialAttachCategories) {
    for (EACH_HEADER) {
        load_categories_nolock(hi);
    }
}
ts.log("IMAGE TIMES: discover categories");

// Realize non-lazy classes (for +load methods and static instances) 初始化非懒加载类，进行rw、ro等操作：realizeClassWithoutSwift
    //懒加载类 -- 别人不动我，我就不动
    //实现非懒加载的类，对于load方法和静态实例变量
    for (EACH_HEADER) {
        //通过_getObjc2NonlazyClassList获取Mach-O的静态段__objc_nlclslist非懒加载类表
        classref_t const *classlist = 
            _getObjc2NonlazyClassList(hi, &count);
        for (i = 0; i < count; i++) {
            Class cls = remapClass(classlist[i]);
            const char *mangledName  = cls->mangledName();
             const char *LGPersonName = "LGPerson";
             if (strcmp(mangledName, LGPersonName) == 0) {
                 auto kc_ro = (const class_ro_t *)cls->data();
                 printf("_getObjc2NonlazyClassList: 这个是我要研究的 %s \n",LGPersonName);
             }
            if (!cls) continue;
            addClassTableEntry(cls);//插入表，但是前面已经插入过了，所以不会重新插入
            if (cls->isSwiftStable()) {
                if (cls->swiftMetadataInitializer()) {
                    _objc_fatal("Swift class %s with a metadata initializer "
                                "is not allowed to be non-lazy",
                                cls->nameForLogging());
                }
                // fixme also disallow relocatable classes
                // We can't disallow all Swift classes because of
                // classes like Swift.__EmptyArrayStorage
            }
            //实现当前的类，因为前面readClass读取到内存的仅仅只有地址+名称，类的data数据并没有加载出来
            //实现所有非懒加载的类(实例化类对象的一些信息，例如rw)
            realizeClassWithoutSwift(cls, nil);
        }
    }
    ts.log("IMAGE TIMES: realize non-lazy classes");

// Realize newly-resolved future classes, in case CF manipulates them
    if (resolvedFutureClasses) {
        for (i = 0; i < resolvedFutureClassCount; i++) {
            Class cls = resolvedFutureClasses[i];
            if (cls->isSwiftStable()) {
                _objc_fatal("Swift class is not allowed to be future");
            }
            //实现类
            realizeClassWithoutSwift(cls, nil);
            cls->setInstancesRequireRawIsaRecursively(false/*inherited*/);
        }
        free(resolvedFutureClasses);
    }
    ts.log("IMAGE TIMES: realize future classes");
    if (DebugNonFragileIvars) {
        //实现所有类
        realizeAllClasses();
    }

/***********************************************************************
* readClass
* Read a class and metaclass as written by a compiler. 读取编译器编写的类和元类
* Returns the new class pointer. This could be:  返回新的类指针，可能是：
* - cls
* - nil  (cls has a missing weak-linked superclass)
* - something else (space for this class was reserved by a future class)
*
* Note that all work performed by this function is preflighted by 
* mustReadClasses(). Do not change this function without updating that one.
*
* Locking: runtimeLock acquired by map_images or objc_readClassPair
**********************************************************************/
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();//名字
    // **CJL写的** ----如果想进入自定义，自己加一个判断
    const char *LGPersonName = "LGPerson";
    if (strcmp(mangledName, LGPersonName) == 0) {
        auto kc_ro = (const class_ro_t *)cls->data();
        printf("%s -- 研究重点--%s\n", __func__,mangledName);
    }
    //当前类的父类中若有丢失的weak-linked类，则返回nil
    if (missingWeakSuperclass(cls)) {
        // No superclass (probably weak-linked). 
        // Disavow any knowledge of this subclass.
        if (PrintConnecting) {
            _objc_inform("CLASS: IGNORING class '%s' with "
                         "missing weak-linked superclass", 
                         cls->nameForLogging());
        }
        addRemappedClass(cls, nil);
        cls->superclass = nil;
        return nil;
    }
    cls->fixupBackwardDeployingStableSwift();
//判断是不是后期要处理的类
    //正常情况下，不会走到popFutureNamedClass，因为这是专门针对未来待处理的类的操作
    //通过断点调试，不会走到if流程里面，因此也不会对ro、rw进行操作
    Class replacing = nil;
    if (Class newCls = popFutureNamedClass(mangledName)) {
        // This name was previously allocated as a future class.
        // Copy objc_class to future class's struct.
        // Preserve future's rw data block.
        if (newCls->isAnySwift()) {
            _objc_fatal("Can't complete future class request for '%s' "
                        "because the real class is too big.", 
                        cls->nameForLogging());
        }
        //读取class的data，设置ro、rw
        //经过调试，并不会走到这里
        class_rw_t *rw = newCls->data();
        const class_ro_t *old_ro = rw->ro();
        memcpy(newCls, cls, sizeof(objc_class));
        rw->set_ro((class_ro_t *)newCls->data());
        newCls->setData(rw);
        freeIfMutable((char *)old_ro->name);
        free((void *)old_ro);
        addRemappedClass(cls, newCls);
        replacing = cls;
        cls = newCls;
    }
    //判断是否类是否已经加载到内存
    if (headerIsPreoptimized  &&  !replacing) {
        // class list built in shared cache
        // fixme strict assert doesn't work because of duplicates
        // ASSERT(cls == getClass(name));
        ASSERT(getClassExceptSomeSwift(mangledName));
    } else {
        addNamedClass(cls, mangledName, replacing);//加载共享缓存中的类
        addClassTableEntry(cls);//插入表，即相当于从mach-O文件 读取到 内存 中
    }
    // for future reference: shared cache never contains MH_BUNDLEs
    if (headerIsBundle) {
        cls->data()->flags |= RO_FROM_BUNDLE;
        cls->ISA()->data()->flags |= RO_FROM_BUNDLE;
    }
    return cls;
}

const char *mangledName() { 
        // fixme can't assert locks here
        ASSERT(this);
        if (isRealized()  ||  isFuture()) { //这个初始化判断在lookupImp也有类似的
            return data()->ro()->name;//如果已经实例化，则从ro中获取name
        } else {
            return ((const class_ro_t *)data())->name;//反之，从mach-O的数据data中获取name
        }
    }

/***********************************************************************
* addNamedClass 加载共享缓存中的类 插入表
* Adds name => cls to the named non-meta class map. 将name=> cls添加到命名的非元类映射
* Warns about duplicate class names and keeps the old mapping.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
    runtimeLock.assertLocked();
    Class old;
    if ((old = getClassExceptSomeSwift(name))  &&  old != replacing) {
        inform_duplicate(name, old, cls);
        // getMaybeUnrealizedNonMetaClass uses name lookups.
        // Classes not found by name lookup must be in the
        // secondary meta->nonmeta table.
        addNonMetaClass(cls);
    } else {
        //添加到gdb_objc_realized_classes哈希表
        NXMapInsert(gdb_objc_realized_classes, name, cls);
    }
    ASSERT(!(cls->data()->flags & RO_META));
    // wrong: constructed classes are already realized when they get here
    // ASSERT(!cls->isRealized());
}

/***********************************************************************
* addClassTableEntry 将一个类添加到所有类的表中
* Add a class to the table of all classes. If addMeta is true,
* automatically adds the metaclass of the class as well.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void
addClassTableEntry(Class cls, bool addMeta = true)
{
    runtimeLock.assertLocked();
    // This class is allowed to be a known class via the shared cache or via
    // data segments, but it is not allowed to be in the dynamic table already.
    auto &set = objc::allocatedClasses.get();//开辟的类的表，在objc_init中的runtime_init就创建了表
    ASSERT(set.find(cls) == set.end());
    if (!isKnownClass(cls))
        set.insert(cls);
    if (addMeta)
        //添加到allocatedClasses哈希表
        addClassTableEntry(cls->ISA(), false);
}

// fixme verify class is not in an un-dlopened part of the shared cache?
//读取class的data()，以及ro/rw创建
auto ro = (const class_ro_t *)cls->data(); //读取类结构的bits属性、//ro -- clean memory，在编译时就已经确定了内存
auto isMeta = ro->flags & RO_META; //判断元类
if (ro->flags & RO_FUTURE) {
    // This was a future class. rw data is already allocated.
    rw = cls->data(); //dirty memory 进行赋值
    ro = cls->data()->ro();
    ASSERT(!isMeta);
    cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
} else { //此时将数据读取进来了，也赋值完毕了
    // Normal class. Allocate writeable class data.
    rw = objc::zalloc<class_rw_t>(); //申请开辟zalloc -- rw
    rw->set_ro(ro);//rw中的ro设置为临时变量ro
    rw->flags = RW_REALIZED|RW_REALIZING|isMeta;
    cls->setData(rw);//将cls的data赋值为rw形式
}

 // 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. --
    //递归调用realizeClassWithoutSwift完善继承链,并处理当前类的父类、元类
    //递归实现 设置当前类、父类、元类的 rw，主要目的是确定继承链 （类继承链、元类继承链）
    //实现元类、父类
    //当isa找到根元类之后，根元类的isa是指向自己的，不会返回nil从而导致死循环——remapClass中对类在表中进行查找的操作，如果表中已有该类，则返回一个空值；如果没有则返回当前类，这样保证了类只加载一次并结束递归
    supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);
    metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);
...
// Update superclass and metaclass in case of remapping -- class 是 双向链表结构 即父子关系都确认了
// 将父类和元类给我们的类 分别是isa和父类的对应值
cls->superclass = supercls;
cls->initClassIsa(metacls);
...
// Connect this class to its superclass's subclass lists
//双向链表指向关系 父类中可以找到子类 子类中也可以找到父类
//通过addSubclass把当前类放到父类的子类列表中去
if (supercls) {
    addSubclass(supercls, cls);
} else {
    addRootClass(cls);
}

static Class realizeClassWithoutSwift(Class cls, Class previously)
{
    runtimeLock.assertLocked();
    //如果类不存在，则返回nil
    if (!cls) return nil;
    如果类已经实现，则直接返回cls
    if (cls->isRealized()) return cls;
    ASSERT(cls == remapClass(cls));
    ...
}

/***********************************************************************
* remapClass
* Returns the live class pointer for cls, which may be pointing to 
* a class struct that has been reallocated.
* Returns nil if cls is ignored because of weak linking.
* Locking: runtimeLock must be read- or write-locked by the caller
**********************************************************************/
static Class remapClass(Class cls)
{
    runtimeLock.assertLocked();
    if (!cls) return nil;//如果cls不存在，则返回nil
    auto *map = remappedClasses(NO);
    if (!map)
        return cls;
    auto iterator = map->find(cls);
    if (iterator == map->end())
        return cls;
    return std::get<1>(*iterator);
}

// Attach categories 附加类别 -- 疑问：ro中也有方法列表 rw中也有方法列表，下面这个方法可以说明
//将ro数据写入到rw
methodizeClass(cls, previously);
return cls;

static void methodizeClass(Class cls, Class previously)
{
    runtimeLock.assertLocked();
    bool isMeta = cls->isMetaClass();
    auto rw = cls->data(); // 初始化一个rw
    auto ro = rw->ro();
    auto rwe = rw->ext();
    ...
    // Install methods and properties that the class implements itself.
    //将属性列表、方法列表、协议列表等贴到rw中
    // 将ro中的方法列表加入到rw中
    method_list_t *list = ro->baseMethods();//获取ro的baseMethods
    if (list) {
        prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));//methods进行排序
        if (rwe) rwe->methods.attachLists(&list, 1);//对rwe进行处理
    }
    // 加入属性
    property_list_t *proplist = ro->baseProperties;
    if (rwe && proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }
    // 加入协议
    protocol_list_t *protolist = ro->baseProtocols;
    if (rwe && protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }
    // Root classes get bonus method implementations if they don't have 
    // them already. These apply before category replacements.
    if (cls->isRootMetaclass()) {
        // root metaclass
        addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
    }
    // Attach categories.
    // 加入分类中的方法
    if (previously) {
        if (isMeta) {
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_METACLASS);
        } else {
            // When a class relocates, categories with class methods
            // may be registered on the class itself rather than on
            // the metaclass. Tell attachToClass to look for those.
            objc::unattachedCategories.attachToClass(cls, previously,
                                                     ATTACH_CLASS_AND_METACLASS);
        }
    }
    objc::unattachedCategories.attachToClass(cls, cls,
                                             isMeta ? ATTACH_METACLASS : ATTACH_CLASS);
    ....
}

static void 
prepareMethodLists(Class cls, method_list_t **addedLists, int addedCount,
                   bool baseMethods, bool methodsFromBundle)
{
    ...
    // Add method lists to array.
    // Reallocate un-fixed method lists.
    // The new methods are PREPENDED to the method list array.
    for (int i = 0; i < addedCount; i++) {
        method_list_t *mlist = addedLists[i];
        ASSERT(mlist);
        // Fixup selectors if necessary
        if (!mlist->isFixedUp()) {
            fixupMethodList(mlist, methodsFromBundle, true/*sort*/);//排序
        }
    }
    ...
}

static void 
fixupMethodList(method_list_t *mlist, bool bundleCopy, bool sort)
{
    runtimeLock.assertLocked();
    ASSERT(!mlist->isFixedUp());
    // fixme lock less in attachMethodLists ?
    // dyld3 may have already uniqued, but not sorted, the list
    if (!mlist->isUniqued()) {
        mutex_locker_t lock(selLock);
        // Unique selectors in list.
        for (auto& meth : *mlist) {
            const char *name = sel_cname(meth.name);
            meth.name = sel_registerNameNoLock(name, bundleCopy);
        }
    }
    // Sort by selector address.根据sel地址排序
    if (sort) {
        method_t::SortBySELAddress sorter;
        std::stable_sort(mlist->begin(), mlist->end(), sorter);
    }
    // Mark method list as uniqued and sorted
    mlist->setFixedUp();
}

void attachToClass(Class cls, Class previously, int flags)
{
    runtimeLock.assertLocked();
    ASSERT((flags & ATTACH_CLASS) ||
           (flags & ATTACH_METACLASS) ||
           (flags & ATTACH_CLASS_AND_METACLASS));
    const char *mangledName  = cls->mangledName();
    const char *LGPersonName = "LGPerson";
    if (strcmp(mangledName, LGPersonName) == 0) {
        bool kc_isMeta = cls->isMetaClass();
        auto kc_rw = cls->data();
        auto kc_ro = kc_rw->ro();
        if (!kc_isMeta) {
            printf("%s: 这个是我要研究的 %s \n",__func__,LGPersonName);
        }
    }
    auto &map = get();
    auto it = map.find(previously);//找到一个分类进来一次，即一个个加载分类，不要混乱
    if (it != map.end()) {//这里会走进来：当主类没有实现load，分类开始加载，迫使主类加载，会走到if流程里面
        category_list &list = it->second;
        if (flags & ATTACH_CLASS_AND_METACLASS) {//判断是否是元类
            int otherFlags = flags & ~ATTACH_CLASS_AND_METACLASS;
            attachCategories(cls, list.array(), list.count(), otherFlags | ATTACH_CLASS);//实例方法
            attachCategories(cls->ISA(), list.array(), list.count(), otherFlags | ATTACH_METACLASS);//类方法
        } else {
            //如果不是元类，则只走一次 attachCategories
            attachCategories(cls, list.array(), list.count(), flags);
        }
        map.erase(it);
    }
}

static void
attachCategories(Class cls, const locstamped_category_t *cats_list, uint32_t cats_count,
                 int flags)
{
    if (slowpath(PrintReplacedMethods)) {
        printReplacements(cls, cats_list, cats_count);
    }
    if (slowpath(PrintConnecting)) {
        _objc_inform("CLASS: attaching %d categories to%s class '%s'%s",
                     cats_count, (flags & ATTACH_EXISTING) ? " existing" : "",
                     cls->nameForLogging(), (flags & ATTACH_METACLASS) ? " (meta)" : "");
    }
    /*
     * Only a few classes have more than 64 categories during launch.
     * This uses a little stack, and avoids malloc.
     *
     * Categories must be added in the proper order, which is back
     * to front. To do that with the chunking, we iterate cats_list
     * from front to back, build up the local buffers backwards,
     * and call attachLists on the chunks. attachLists prepends the
     * lists, so the final result is in the expected order.
     */
    constexpr uint32_t ATTACH_BUFSIZ = 64;
    method_list_t   *mlists[ATTACH_BUFSIZ];
    property_list_t *proplists[ATTACH_BUFSIZ];
    protocol_list_t *protolists[ATTACH_BUFSIZ];
    uint32_t mcount = 0;
    uint32_t propcount = 0;
    uint32_t protocount = 0;
    bool fromBundle = NO;
    bool isMeta = (flags & ATTACH_METACLASS);
    /*
     rwe的创建，
     那么为什么要在这里进行`rwe的初始化`？因为我们现在要做一件事：往`本类`中`添加属性、方法、协议`等
     */
    auto rwe = cls->data()->extAllocIfNeeded();
    //mlists 是一个二维数组
    for (uint32_t i = 0; i < cats_count; i++) {
        auto& entry = cats_list[i];
        method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
        if (mlist) {
            if (mcount == ATTACH_BUFSIZ) {//mcount = 0，ATTACH_BUFSIZ= 64，不会走到if里面的流程
                prepareMethodLists(cls, mlists, mcount, NO, fromBundle);//准备排序
                rwe->methods.attachLists(mlists, mcount);
                mcount = 0;
            }
            mlists[ATTACH_BUFSIZ - ++mcount] = mlist;
            fromBundle |= entry.hi->isBundle();
        }
        property_list_t *proplist =
            entry.cat->propertiesForMeta(isMeta, entry.hi);
        if (proplist) {
            if (propcount == ATTACH_BUFSIZ) {
                rwe->properties.attachLists(proplists, propcount);
                propcount = 0;
            }
            proplists[ATTACH_BUFSIZ - ++propcount] = proplist;
        }
        protocol_list_t *protolist = entry.cat->protocolsForMeta(isMeta);
        if (protolist) {
            if (protocount == ATTACH_BUFSIZ) {
                rwe->protocols.attachLists(protolists, protocount);
                protocount = 0;
            }
            protolists[ATTACH_BUFSIZ - ++protocount] = protolist;
        }
    }
    if (mcount > 0) {
        prepareMethodLists(cls, mlists + ATTACH_BUFSIZ - mcount, mcount, NO, fromBundle);//排序
        rwe->methods.attachLists(mlists + ATTACH_BUFSIZ - mcount, mcount);//mlists + ATTACH_BUFSIZ - mcount 为内存平移
        if (flags & ATTACH_EXISTING) flushCaches(cls);
    }
    rwe->properties.attachLists(proplists + ATTACH_BUFSIZ - propcount, propcount);
    rwe->protocols.attachLists(protolists + ATTACH_BUFSIZ - protocount, protocount);
}

class_rw_ext_t *extAllocIfNeeded() {
    auto v = get_ro_or_rwe();
    if (fastpath(v.is<class_rw_ext_t *>())) { //判断rwe是否存在
        return v.get<class_rw_ext_t *>();//如果存在，则直接获取
    } else {
        return extAlloc(v.get<const class_ro_t *>());//如果不存在则进行开辟
    }
}
👇//extAlloc源码实现
class_rw_ext_t *
class_rw_t::extAlloc(const class_ro_t *ro, bool deepCopy)
{
    runtimeLock.assertLocked();
    //此时只有rw，需要对rwe进行数据添加，即0-1的过程
    auto rwe = objc::zalloc<class_rw_ext_t>();//创建
    rwe->version = (ro->flags & RO_META) ? 7 : 0;
    method_list_t *list = ro->baseMethods();
    if (list) {
        if (deepCopy) list = list->duplicate();
        rwe->methods.attachLists(&list, 1);
    }
    // See comments in objc_duplicateClass
    // property lists and protocol lists historically
    // have not been deep-copied
    //
    // This is probably wrong and ought to be fixed some day
    property_list_t *proplist = ro->baseProperties;
    if (proplist) {
        rwe->properties.attachLists(&proplist, 1);
    }
    protocol_list_t *protolist = ro->baseProtocols;
    if (protolist) {
        rwe->protocols.attachLists(&protolist, 1);
    }
    set_ro_or_rwe(rwe, ro);
    return rwe;
}

struct method_list_t : entsize_list_tt<method_t, method_list_t, 0x3> 
struct property_list_t : entsize_list_tt<property_t, property_list_t, 0> 
struct protocol_list_t {
    // count is pointer-sized by accident.
    uintptr_t count;
    protocol_ref_t list[0]; // variable-size
    size_t byteSize() const {
        return sizeof(*this) + count*sizeof(list[0]);
    }
    protocol_list_t *duplicate() const {
        return (protocol_list_t *)memdup(this, this->byteSize());
    }
    ...
}

void attachLists(List* const * addedLists, uint32_t addedCount) {
    if (addedCount == 0) return;
    if (hasArray()) {
        // many lists -> many lists
        //计算数组中旧lists的大小
        uint32_t oldCount = array()->count;
        //计算新的容量大小 = 旧数据大小+新数据大小
        uint32_t newCount = oldCount + addedCount;
        //根据新的容量大小，开辟一个数组，类型是 array_t，通过array()获取
        setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
        //设置数组大小
        array()->count = newCount;
        //旧的数据从 addedCount 数组下标开始 存放旧的lists，大小为 旧数据大小 * 单个旧list大小
        memmove(array()->lists + addedCount, array()->lists, 
                oldCount * sizeof(array()->lists[0]));
        //新数据从数组 首位置开始存储，存放新的lists，大小为 新数据大小 * 单个list大小
        memcpy(
               array()->lists, addedLists, 
               addedCount * sizeof(array()->lists[0]));
    }
    else if (!list  &&  addedCount == 1) {
        // 0 lists -> 1 list
        list = addedLists[0];//将list加入mlists的第一个元素，此时的list是一个一维数组
    } 
    else {
        // 1 list -> many lists 有了一个list，有往里加很多list
        //新的list就是分类，来自LRU的算法思维，即最近最少使用
        //获取旧的list
        List* oldList = list;
        uint32_t oldCount = oldList ? 1 : 0;
        //计算容量和 = 旧list个数+新lists的个数
        uint32_t newCount = oldCount + addedCount;
        //开辟一个容量和大小的集合，类型是 array_t，即创建一个数组，放到array中，通过array()获取
        setArray((array_t *)malloc(array_t::byteSize(newCount)));
        //设置数组的大小
        array()->count = newCount;
        //判断old是否存在，old肯定是存在的，将旧的list放入到数组的末尾
        if (oldList) array()->lists[addedCount] = oldList;
        // memcpy（开始位置，放什么，放多大） 是内存平移，从数组起始位置存入新的list
        //其中array()->lists 表示首位元素位置
        memcpy(array()->lists, addedLists, 
               addedCount * sizeof(array()->lists[0]));
    }
}