这里会把类相关、程序启动类信息填充、引用计数都会讲下。想要深入了解OC的动态性，就必须去研究runtime的代码，所幸它是开源的，你可以在源码下载不同的版本。

typedef struct objc_class *Class;
typedef struct objc_object *id;

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags
    class_rw_t *data() {
        return bits.data();
    }
    ....省略
 }

struct objc_object {
private:
    isa_t isa;
public:
    // ISA() assumes this is NOT a tagged pointer object
    Class ISA();
    ....省略
}

根据上面的声明，凡是首地址是isa，都可以被认为是objc中的对象。运行时可以通过isa获取到该对象是属于什么类(Class)。那类对象有所属的类吗？有，称之为元类(Metaclass)。

class_isa_image

上面这张图可以把isa和superclass表现的很清楚。

结构体 isa_t

接下来看下isa的结构：

#if (!__LP64__  ||  TARGET_OS_WIN32  ||  TARGET_OS_SIMULATOR)
#   define SUPPORT_PACKED_ISA 0
#else
#   define SUPPORT_PACKED_ISA 1
#endif

union isa_t 
{
    isa_t() { }
    isa_t(uintptr_t value) : bits(value) { }

    Class cls;
    uintptr_t bits;

#if SUPPORT_PACKED_ISA
# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
#   define ISA_MAGIC_MASK  0x000003f000000001ULL
#   define ISA_MAGIC_VALUE 0x000001a000000001ULL
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 19;
#       define RC_ONE   (1ULL<<45)
#       define RC_HALF  (1ULL<<18)
    };

# elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL
#   define ISA_MAGIC_MASK  0x001f800000000001ULL
#   define ISA_MAGIC_VALUE 0x001d800000000001ULL
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 44; // MACH_VM_MAX_ADDRESS 0x7fffffe00000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 8;
#       define RC_ONE   (1ULL<<56)
#       define RC_HALF  (1ULL<<7)
    };

...省略
#if SUPPORT_INDEXED_ISA
...

isa指针是一个联合体,在不同环境下,结构体中属性值稍有差别。

• SUPPORT_INDEXED_ISA：通过一些资料查询，它是用于手表的；
• nonpointer：它为1时，表示使用了优化的isa指针，包含了引用计数，析构状态等；
• has_assoc: 是否包含关联对象，如果没有，会更快的释放内存；
• has_cxx_dtor：是否包含析构函数，如果没有，会更快的释放内存；
• shiftcls：类的指针；
• magic：用于判断是否完成初始化；
• weakly_referenced：对象是否指向一个弱引用对象，没有弱引用对象可以更快的被释放；
• deallocating：对象是否正在销毁；
• has_sidetable_rc：对象的引用计数太大了，存不下；
• extra_rc：引用计数，但是比retaincount小1；

用一张霜神的图片，可以更容易看清楚：

isa_内存布局

在x86_64架构上，可以看到类的指针用了47位来表示，因为类的指针要按照8字节对齐，所以后3位都是0，而为了减小内存的消耗，那3位可以表示为别的值，所以类指针真正有效的就只有44位。

inline Class  objc_object::ISA() 
{
#if SUPPORT_INDEXED_ISA
    if (isa.nonpointer) {
        uintptr_t slot = isa.indexcls;
        return classForIndex((unsigned)slot);
    }
    return (Class)isa.bits;
#else
    return (Class)(isa.bits & ISA_MASK);
#endif
}

通过isa.bits & ISA_MASK进行位运算，只取其中[3 - 47]，然后返回Class。

inline void objc_object::initIsa(Class cls, bool nonpointer, bool hasCxxDtor) 
{ 
    if (!nonpointer) {
        isa.cls = cls;
    } else {
        isa_t newisa(0);
        newisa.bits = ISA_MAGIC_VALUE;
        // isa.magic is part of ISA_MAGIC_VALUE
        // isa.nonpointer is part of ISA_MAGIC_VALUE
        newisa.has_cxx_dtor = hasCxxDtor;
        newisa.shiftcls = (uintptr_t)cls >> 3;
        
        isa = newisa;
    }
}

根据上面的图片，把ISA_MAGIC_VALUE的值放入64位空间里面，可以知道是给nonpointer和magic进行了赋值，has_cxx_dtor、shiftcls一样的操作。

cache_t

cache_t cache从字面的是缓存的意思，肯定是优化性能的，先看下它的结构：

struct cache_t {
    struct bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
}

struct bucket_t {
private:
    cache_key_t _key;
    IMP _imp;
}

• _buckets是一个散列表，用来存储方法缓存。bucket_t是一个key和IMP的结构体。
• _mask分配bucket的总数。
• _occupied表示目前占用缓存bucket的个数。

Cache的作用主要是优化方法调用的性能，当对象收到消息后，会优先去Cache中查找，如果Cache中没有才会去类的方法列表找。如果没有Cache，我们每次调用方法都去类的方法找，会严重影响性能。

class_data_bits_t

一个类的方法，属性和遵循的协议都在class_data_bits_t里面。我们先看下代码：

struct class_data_bits_t {
    // Values are the FAST_ flags above.
    uintptr_t bits;
    class_rw_t* data() {
        return (class_rw_t *)(bits & FAST_DATA_MASK);
    }
}

data()方法来返回class_rw_t *指针，里面存储的是类的信息，它跟objc_object::ISA()方法是一样的。

class_rw_t和class_ro_t

通过名字都能看出来class_rw_t是可读写的，class_ro_t是仅仅可读的，他们里面存储的都是类的相关信息，属性、方法还有遵循的协议等。

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint32_t version;

    const class_ro_t *ro;

    method_array_t methods;
    property_array_t properties;
    protocol_array_t protocols;

    Class firstSubclass;
    Class nextSiblingClass;

    char *demangledName;
}

其中有一个指向常量的指针ro，它存储了当前类在编译期就已经确定的属性、方法以及遵循的协议。

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;
#ifdef __LP64__
    uint32_t reserved;
#endif

    const uint8_t * ivarLayout;
    
    const char * name;
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;

    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;

    method_list_t *baseMethods() const {
        return baseMethodList;
    }
};

这个结构跟通过clang -rewrite-objc之后产出的类结构完全一样：

struct _class_ro_t {
    unsigned int flags;
    unsigned int instanceStart;
    unsigned int instanceSize;
    unsigned int reserved;
    const unsigned char *ivarLayout;
    const char *name;
    const struct _method_list_t *baseMethods;
    const struct _objc_protocol_list *baseProtocols;
    const struct _ivar_list_t *ivars;
    const unsigned char *weakIvarLayout;
    const struct _prop_list_t *properties;
};

struct _class_t {
    struct _class_t *isa;
    struct _class_t *superclass;
    void *cache;
    void *vtable;
    struct _class_ro_t *ro;
};

这样的类结构跟这里的类结构还有一点不一样就是class_rw_t，那它是怎么调整的呢？什么时候操作呢？还有运行期类的信息什么时候赋值呢？

map_images_nolock

通过调式，发现Runtime的入口是 objc-os.mm 里面的函数 _objc_init。在这个函数里面，动态库注册了3个函数回调map_2_images、load_images、unmap_image。

void _objc_init(void)
└──const char *map_2_images(...)
    └──const char *map_images_nolock(...)
        └──void _read_images(header_info **hList, uint32_t hCount)

void map_images_nolock(unsigned mhCount, const char * const mhPaths[],
                  const struct mach_header * const mhdrs[])
{
    static bool firstTime = YES;
    header_info *hList[mhCount];
    uint32_t hCount;
    size_t selrefCount = 0;
    if (firstTime) {
        preopt_init();
    }
    int totalClasses = 0;
    int unoptimizedTotalClasses = 0;
    {
        uint32_t i = mhCount;
        while (i--) {
            const headerType *mhdr = (const headerType *)mhdrs[I];
            // 将mhdr通过appendHeader函数添加到链表中 由FirstHeader,LastHeader和HeaderCount维护
            auto hi = addHeader(mhdr, mhPaths[i], totalClasses, unoptimizedTotalClasses);
            if (!hi) {
                // no objc data in this entry
                continue;
            }
            
            if (mhdr->filetype == MH_EXECUTE) {
                // Size some data structures based on main executable's size
#if __OBJC2__
                size_t count;
                _getObjc2SelectorRefs(hi, &count);
                selrefCount += count;
                //Fix up old objc_msgSend_fixup call sites 基本上都为0
                _getObjc2MessageRefs(hi, &count);
                selrefCount += count;
#else
                _getObjcSelectorRefs(hi, &selrefCount);
#endif
            }
            
            hList[hCount++] = hi;
        }
    }

    if (firstTime) {
        //将被引用的sel数量存到静态变量SelrefCount中;
        //注册一些sel到namedSelectors中
        sel_init(selrefCount);
        //初始化AutoreleasePoolPage用于autorelease机制
        // 初始化SideTable用于__weak变量引用管理
        arr_init();
    }

    if (hCount > 0) {
        _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
    }
    firstTime = NO;
}

这个函数主要就是通过mhdrs生成hList，然后传递给_read_images函数。

_read_images

这个方法是处理类的相关信息的，有些长，一点一点来分析。

类信息

for (EACH_HEADER) {
        if (! mustReadClasses(hi)) {
            // Image is sufficiently optimized that we need not call readClass()
            continue;
        }

        bool headerIsBundle = hi->isBundle();
        bool headerIsPreoptimized = hi->isPreoptimized();

        classref_t *classlist = _getObjc2ClassList(hi, &count);
        for (i = 0; i < count; i++) {
            Class cls = (Class)classlist[I];
            Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);

            ...
        }
    }

bool mustReadClasses(header_info *hi)
{
    const char *reason;

    // If the image is not preoptimized then we must read classes.
    if (!hi->isPreoptimized()) {
        reason = nil; // Don't log this one because it is noisy.
        goto readthem;
    }

    assert(!hi->isBundle());  // no MH_BUNDLE in shared cache

    // If the image may have missing weak superclasses then we must read classes
    if (!noMissingWeakSuperclasses()) {
        reason = "the image may contain classes with missing weak superclasses";
        goto readthem;
    }

    // If there are unresolved future classes then we must read classes.
    if (haveFutureNamedClasses()) {
        reason = "there are unresolved future classes pending";
        goto readthem;
    }

    // readClass() does not need to do anything.
    return NO;

 readthem:
    return YES;
}

bool header_info::isPreoptimized() const
{
    // preoptimization disabled for some reason
    if (!preoptimized) return NO;

    // image not from shared cache, or not fixed inside shared cache
    if (!info()->optimizedByDyld()) return NO;

    return YES;
}

在map_images_nolock函数中调用了preopt_init();，通过调式，它会把preoptimized置为NO，所以isPreoptimized函数也是一直返回NO的，因此mustReadClasses会返回YES。

_getObjc2ClassList函数会调用getsectiondata来返回所有的类列表，这是读取二进制文件也就是mach-o得到的，具体的之前专门文章介绍。

Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
    const char *mangledName = cls->mangledName();
   ...省略
   addNamedClass(cls, mangledName, replacing);
    // 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;
}

static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
    runtimeLock.assertWriting();
    Class old;
    if ((old = getClass(name))  &&  old != replacing) {
        inform_duplicate(name, old, cls);
        // getNonMetaClass uses name lookups. Classes not found by name 
        // lookup must be in the secondary meta->nonmeta table.
        addNonMetaClass(cls);
    } else {
        NXMapInsert(gdb_objc_realized_classes, name, cls);
    }
}

readClass函数主要就是调用addNamedClass函数，通过NXMapInsert把类插入进去，把类跟类名做一个关联。

方法信息

static size_t UnfixedSelectors;
    sel_lock();
    for (EACH_HEADER) {
        if (hi->isPreoptimized()) continue;

        bool isBundle = hi->isBundle();
        SEL *sels = _getObjc2SelectorRefs(hi, &count);
        UnfixedSelectors += count;
        for (i = 0; i < count; i++) {
            const char *name = sel_cname(sels[i]);
            sels[i] = sel_registerNameNoLock(name, isBundle);
        }
    }
    sel_unlock();

这个就比较简单了，就是把方法名插入到namedSelectors表里面。

协议信息

struct protocol_t : objc_object {
    const char *mangledName;
    ...
}
// Discover protocols. Fix up protocol refs.
    for (EACH_HEADER) {
        extern objc_class OBJC_CLASS_$_Protocol;
        Class cls = (Class)&OBJC_CLASS_$_Protocol;
        assert(cls);
        NXMapTable *protocol_map = protocols();
        bool isPreoptimized = hi->isPreoptimized();
        bool isBundle = hi->isBundle();

        protocol_t **protolist = _getObjc2ProtocolList(hi, &count);
        for (i = 0; i < count; i++) {
            readProtocol(protolist[i], cls, protocol_map, 
                         isPreoptimized, isBundle);
        }
    }

    static void readProtocol(protocol_t *newproto, Class protocol_class,
             NXMapTable *protocol_map, 
             bool headerIsPreoptimized, bool headerIsBundle)
    {
            auto insertFn = headerIsBundle ? NXMapKeyCopyingInsert : NXMapInsert;
            ...
            newproto->initIsa(protocol_class);  // fixme pinned
            insertFn(protocol_map, newproto->mangledName, newproto);
            ...
    }

    // Fix up @protocol references
    // Preoptimized images may have the right 
    // answer already but we don't know for sure.
    for (EACH_HEADER) {
        protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
        for (i = 0; i < count; i++) {
            remapProtocolRef(&protolist[I]);
        }
    }
    
static void remapProtocolRef(protocol_t **protoref)
{
    runtimeLock.assertLocked();

    protocol_t *newproto = remapProtocol((protocol_ref_t)*protoref);
    if (*protoref != newproto) {
        *protoref = newproto;
        UnfixedProtocolReferences++;
    }
}

通过查看protocol_t结构，发现它本质上也是一个对象，它的isa就是(Class)&OBJC_CLASS_$_Protocol，然后把协议都插入到protocol_map表里。

通过*protoref的名字，如果能在protocol_map找到，而且地址不一样，就进行替换。

non-lazy classes

// Realize non-lazy classes (for +load methods and static instances)
    for (EACH_HEADER) {
        classref_t *classlist = 
            _getObjc2NonlazyClassList(hi, &count);
        for (i = 0; i < count; i++) {
            Class cls = remapClass(classlist[i]);
            if (!cls) continue;
            realizeClass(cls);
        }
    }

_getObjc2NonlazyClassList是获取有重写+load方法的类，对应的是_OBJC_LABEL_NONLAZY_CLASS_$[]。这个方法的重点是realizeClass。

realizeClass

static Class realizeClass(Class cls)
{
    runtimeLock.assertWriting();
    const class_ro_t *ro;
    class_rw_t *rw;
    Class supercls;
    Class metacls;
    bool isMeta;

    if (!cls) return nil;
    if (cls->isRealized()) return cls;

    ro = (const class_ro_t *)cls->data();
    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 {
        // Normal class. Allocate writeable class data.
        rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
        rw->ro = ro;
        rw->flags = RW_REALIZED|RW_REALIZING;
        cls->setData(rw);
    }

    isMeta = ro->flags & RO_META;

    rw->version = isMeta ? 7 : 0;  // old runtime went up to 6
    // Choose an index for this class.
    // Sets cls->instancesRequireRawIsa if indexes no more indexes are available
    cls->chooseClassArrayIndex();
    // 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.
    supercls = realizeClass(remapClass(cls->superclass));
    metacls = realizeClass(remapClass(cls->ISA()));

...省略

    // Update superclass and metaclass in case of remapping
    cls->superclass = 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();
        }
    }

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

    // Attach categories
    methodizeClass(cls);
    return cls;
}

ro = (const class_ro_t *)cls->data();，从这行代码可以看出从二进制读取到的class_ro_t结构，而不是class_rw_t。

rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
rw->ro = ro;
rw->flags = RW_REALIZED|RW_REALIZING;
cls->setData(rw);

// class_t->data is class_rw_t, not class_ro_t
#define RW_REALIZED           (1<<31)

// class has started realizing but not yet completed it
#define RW_REALIZING          (1<<19)

这几行代码就是给rw结构进行赋值，并对flag进行改变，表示开始realizing，但还没完成。

if (!cls) return nil;
if (cls->isRealized()) return cls;
    
supercls = realizeClass(remapClass(cls->superclass));
metacls = realizeClass(remapClass(cls->ISA()));

通过这4行代码，递归对类进行realize，从这也能看出类的superclass和isa结构，跟这个图进行认证。

class_isa_image

cls->initClassIsa(metacls);这行代码可以说明类的isa就是metacls。

methodizeClass这个方法主要就是对rw的方法、协议、属性进行赋值。这里有一点比较奇怪的是为啥就仅仅对重写+load方法的类进行realizeClass，而不是所有的类？我就全局搜索都有那些地方调用这个方法，发现有好多地方会调用，如lookUpImpOrForward、look_up_class等，他们都会进行判断，如果类没有isRealized，就会调用这个方法。这样做的目的可能是为了性能考虑，让启程尽快启动。

分类

for (EACH_HEADER) {
        category_t **catlist = 
            _getObjc2CategoryList(hi, &count);
        bool hasClassProperties = hi->info()->hasCategoryClassProperties();

        for (i = 0; i < count; i++) {
            category_t *cat = catlist[I];
            Class cls = remapClass(cat->cls);
            ...
            // Process this category. 
            // First, register the category with its target class. 
            // Then, rebuild the class's method lists (etc) if 
            // the class is realized. 
            bool classExists = NO;
            if (cat->instanceMethods ||  cat->protocols  
                ||  cat->instanceProperties) 
            {
                addUnattachedCategoryForClass(cat, cls, hi);
                if (cls->isRealized()) {
                    remethodizeClass(cls);
                    classExists = YES;
                }
            }

            if (cat->classMethods  ||  cat->protocols  
                ||  (hasClassProperties && cat->_classProperties)) 
            {
                addUnattachedCategoryForClass(cat, cls->ISA(), hi);
                if (cls->ISA()->isRealized()) {
                    remethodizeClass(cls->ISA());
                }
            }
        }
    }

struct category_t {
    const char *name;
    classref_t cls;
    struct method_list_t *instanceMethods;
    struct method_list_t *classMethods;
    struct protocol_list_t *protocols;
    struct property_list_t *instanceProperties;
    // Fields below this point are not always present on disk.
    struct property_list_t *_classProperties;

    method_list_t *methodsForMeta(bool isMeta) {
        if (isMeta) return classMethods;
        else return instanceMethods;
    }

    property_list_t *propertiesForMeta(bool isMeta, struct header_info *hi);
};

addUnattachedCategoryForClass方法把分类的相关信息存到category_map表里面。接下来如果类已经isRealized过了，就调用remethodizeClass方法，它首先在分类表里根据cls把相关信息删除，再调用attachCategories方法。这个方法就是把分类的方法、属性、协议加入到类里面。

static void  attachCategories(Class cls, category_list *cats, bool flush_caches)
{
    if (!cats) return;
    if (PrintReplacedMethods) printReplacements(cls, cats);

    bool isMeta = cls->isMetaClass();

    // fixme rearrange to remove these intermediate allocations
    method_list_t **mlists = (method_list_t **)
        malloc(cats->count * sizeof(*mlists));
    property_list_t **proplists = (property_list_t **)
        malloc(cats->count * sizeof(*proplists));
    protocol_list_t **protolists = (protocol_list_t **)
        malloc(cats->count * sizeof(*protolists));

    // Count backwards through cats to get newest categories first
    int mcount = 0;
    int propcount = 0;
    int protocount = 0;
    int i = cats->count;
    bool fromBundle = NO;
    while (i--) {
        auto& entry = cats->list[I];

        method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
        if (mlist) {
            mlists[mcount++] = mlist;
            fromBundle |= entry.hi->isBundle();
        }

        property_list_t *proplist = 
            entry.cat->propertiesForMeta(isMeta, entry.hi);
        if (proplist) {
            proplists[propcount++] = proplist;
        }

        protocol_list_t *protolist = entry.cat->protocols;
        if (protolist) {
            protolists[protocount++] = protolist;
        }
    }

    auto rw = cls->data();

    prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
    rw->methods.attachLists(mlists, mcount);
    free(mlists);
    if (flush_caches  &&  mcount > 0) flushCaches(cls);

    rw->properties.attachLists(proplists, propcount);
    free(proplists);

    rw->protocols.attachLists(protolists, protocount);
    free(protolists);
}

这个方法比较简单，就是从cats获取所有的分类信息，然后对rw进行赋值。

这里面第一个回调函数map_images就没了，接下来是load_images函数，它主要是操作+load方法。

load_images

void load_images(const char *path __unused, const struct mach_header *mh)
{
    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;
    recursive_mutex_locker_t lock(loadMethodLock);
    // Discover load methods
    {
        rwlock_writer_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }
    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}

通过prepare_load_methods函数找到所有重写+load方法的类和分类，然后通过call_load_methods调用他们。

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, I;

    runtimeLock.assertWriting();

    classref_t *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[I];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        realizeClass(cls);
        assert(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}

_getObjc2NonlazyClassList前面已经讲过，_getObjc2NonlazyCategoryList它获取的是重写+load的所有分类，它对应的是_OBJC_LABEL_NONLAZY_CATEGORY_$[]。

// class +load has been called
#define RW_LOADED             (1<<23)

static void schedule_class_load(Class cls)
{
    if (!cls) return;
    if (cls->data()->flags & RW_LOADED) return;
    schedule_class_load(cls->superclass);
    add_class_to_loadable_list(cls);
    cls->setInfo(RW_LOADED); 
}

static struct loadable_category *loadable_categories = nil;
static int loadable_categories_used = 0;
static int loadable_categories_allocated = 0;

struct loadable_category {
    Category cat;  // may be nil
    IMP method;
};

void add_class_to_loadable_list(Class cls)
{
    IMP method;
    loadMethodLock.assertLocked();
    method = cls->getLoadMethod();
    if (!method) return;  // Don't bother if cls has no +load method
    if (loadable_classes_used == loadable_classes_allocated) {
        loadable_classes_allocated = loadable_classes_allocated*2 + 16;
        loadable_classes = (struct loadable_class *)
            realloc(loadable_classes,
                              loadable_classes_allocated *
                              sizeof(struct loadable_class));
    }
    
    loadable_classes[loadable_classes_used].cls = cls;
    loadable_classes[loadable_classes_used].method = method;
    loadable_classes_used++;
}

IMP objc_class::getLoadMethod()
{
    runtimeLock.assertLocked();

    const method_list_t *mlist;
    mlist = ISA()->data()->ro->baseMethods();
    if (mlist) {
        for (const auto& meth : *mlist) {
            const char *name = sel_cname(meth.name);
            if (0 == strcmp(name, "load")) {
                return meth.imp;
            }
        }
    }
    return nil;
}

schedule_class_load(cls->superclass);这行代码递归调用直至到NSObject，因为NSObject的父类为nil，然后通过add_class_to_loadable_list方法把有+load方法加入到loadable_classes，最后把当前类的flag置为+load已经调用的状态。从这也可以看到是先从父类的+load开始算起的。

void add_category_to_loadable_list(Category cat)
{
    IMP method;
    loadMethodLock.assertLocked();
    method = _category_getLoadMethod(cat);

    // Don't bother if cat has no +load method
    if (!method) return;
    
    if (loadable_categories_used == loadable_categories_allocated) {
        loadable_categories_allocated = loadable_categories_allocated*2 + 16;
        loadable_categories = (struct loadable_category *)
            realloc(loadable_categories,
                              loadable_categories_allocated *
                              sizeof(struct loadable_category));
    }

    loadable_categories[loadable_categories_used].cat = cat;
    loadable_categories[loadable_categories_used].method = method;
    loadable_categories_used++;
}

分类的+load存取是直接从分类的方法找到有+load的，然后放到loadable_categories。

call_load_methods

void call_load_methods(void)
{
    static bool loading = NO;
    bool more_categories;

    loadMethodLock.assertLocked();
    if (loading) return;
    loading = YES;

    void *pool = objc_autoreleasePoolPush();
    do {
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
            call_class_loads();
        }

        // 2. Call category +loads ONCE
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    objc_autoreleasePoolPop(pool);
    loading = NO;
}

static void call_class_loads(void)
{
    int I;
    
    // Detach current loadable list.
    struct loadable_class *classes = loadable_classes;
    int used = loadable_classes_used;
    loadable_classes = nil;
    loadable_classes_allocated = 0;
    loadable_classes_used = 0;
    
    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Class cls = classes[i].cls;
        load_method_t load_method = (load_method_t)classes[i].method;
        if (!cls) continue; 
        (*load_method)(cls, SEL_load);
    }
    if (classes) free(classes);
}

调用+load方法也是先调用类的，再调用分类的，类的调用就是从loadable_classes获取所有的类的+load实现，然后直接调用。

* call_load_methods
* Call all pending class and category +load methods.
* Class +load methods are called superclass-first. 
* Category +load methods are not called until after the parent class's +load.

这段注释也能很好的诠释这些。

static bool call_category_loads(void)
{
    int i, shift;
    bool new_categories_added = NO;
    
    // Detach current loadable list.
    struct loadable_category *cats = loadable_categories;
    int used = loadable_categories_used;
    int allocated = loadable_categories_allocated;
    loadable_categories = nil;
    loadable_categories_allocated = 0;
    loadable_categories_used = 0;

    // Call all +loads for the detached list.
    for (i = 0; i < used; i++) {
        Category cat = cats[i].cat;
        load_method_t load_method = (load_method_t)cats[i].method;
        Class cls;
        if (!cat) continue;

        cls = _category_getClass(cat);
        if (cls  &&  cls->isLoadable()) {
            (*load_method)(cls, SEL_load);
            cats[i].cat = nil;
        }
    }

    // Compact detached list (order-preserving)
    shift = 0;
    for (i = 0; i < used; i++) {
        if (cats[i].cat) {
            cats[i-shift] = cats[I];
        } else {
            shift++;
        }
    }
    used -= shift;

    // Copy any new +load candidates from the new list to the detached list.
    new_categories_added = (loadable_categories_used > 0);
    for (i = 0; i < loadable_categories_used; i++) {
        if (used == allocated) {
            allocated = allocated*2 + 16;
            cats = (struct loadable_category *)
                realloc(cats, allocated *
                                  sizeof(struct loadable_category));
        }
        cats[used++] = loadable_categories[I];
    }

    // Destroy the new list.
    if (loadable_categories) free(loadable_categories);

    // Reattach the (now augmented) detached list. 
    // But if there's nothing left to load, destroy the list.
    if (used) {
        loadable_categories = cats;
        loadable_categories_used = used;
        loadable_categories_allocated = allocated;
    } else {
        if (cats) free(cats);
        loadable_categories = nil;
        loadable_categories_used = 0;
        loadable_categories_allocated = 0;
    }

    return new_categories_added;
}

这个方法我是有点疑惑的，不太明白又加层机制是为什么？首先cls->isLoadable()是一直返回yes的，cls不存在的可能性我认为不存在（这点有待探讨）。Re-entrant可重入问题，在call_load_methods里面已经解决了。

// Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

至于在+load方法里面动态生成一个分类，那为啥类的时候不考虑那种情况呢，再说loadable_categories是在prepare_load_methods方法里面确定好的，没有那个地方再对它进行修改。综合几种情况下来，还是不太明白为啥那样做的原因。

引用计数

这里简单的说下引用计数的原理，引用计数跟objc_storeStrong这个函数紧密相连，引用计数的增加减少都是通过它，这个函数是编译阶段由编译器在合适的位置自己增加的，所以才叫自动引用计数。还有一种情况是直接调用objc_retain增加引用计数的，但是减少的话都是通过objc_storeStrong。（这段话可能不太准确，但是我测试好几种情况都是这样的，但是不管还有那种情况，最后肯定都是调用retain、release来进行增加减少引用计数的）

void objc_storeStrong(id *location, id obj)
{
    id prev = *location;
    if (obj == prev) {
        return;
    }
    objc_retain(obj);
    *location = obj;
    objc_release(prev);
}

下面看2个例子比较容易明白，其中第2个例子就是直接调用objc_retain来增加引用计数的。

#1：
{
    NSObject *obj1 = [[NSObject alloc]init];
    NSObject *obj2 = [[NSObject alloc]init];
    obj2 = obj1;
}

#2：
{
    NSObject *obj1 = [[NSObject alloc]init];
    NSObject *obj2 = obj1;
}

当出括号时，会调用2次objc_storeStrong，当调用第2个时，会调用dealloc。

objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
    if (isTaggedPointer()) return (id)this;

    bool sideTableLocked = false;
    bool transcribeToSideTable = false;

    isa_t oldisa;
    isa_t newisa;

    do {
        transcribeToSideTable = false;
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            if (tryRetain) return sidetable_tryRetain() ? (id)this : nil;
            else return sidetable_retain();
        }
        // don't check newisa.fast_rr; we already called any RR overrides
        if (slowpath(tryRetain && newisa.deallocating)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            return nil;
        }
        uintptr_t carry;
        newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc++

        if (slowpath(carry)) {
            // newisa.extra_rc++ overflowed
            if (!handleOverflow) {
                ClearExclusive(&isa.bits);
                return rootRetain_overflow(tryRetain);
            }
            // Leave half of the retain counts inline and 
            // prepare to copy the other half to the side table.
            if (!tryRetain && !sideTableLocked) sidetable_lock();
            sideTableLocked = true;
            transcribeToSideTable = true;
            newisa.extra_rc = RC_HALF;
            newisa.has_sidetable_rc = true;
        }
    } while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));

    if (slowpath(transcribeToSideTable)) {
        // Copy the other half of the retain counts to the side table.
        sidetable_addExtraRC_nolock(RC_HALF);
    }

    if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
    return (id)this;
}

最主要的是newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry); // extra_rc++这行代码，它是增加引用计数的，上面讲述isa时，知道最后的8位是存引用计数的，如果引用计数太大，8位不够存时（超过255），就用sideTable表结构，这一块就不讲了，因为感觉用不到，255已经够大了。

static ALWAYS_INLINE uintptr_t 
addc(uintptr_t lhs, uintptr_t rhs, uintptr_t carryin, uintptr_t *carryout)
{
    return __builtin_addcl(lhs, rhs, carryin, carryout);
}

static ALWAYS_INLINE uintptr_t 
subc(uintptr_t lhs, uintptr_t rhs, uintptr_t carryin, uintptr_t *carryout)
{
    return __builtin_subcl(lhs, rhs, carryin, carryout);
}

这2个函数是LLVM提供的，就是简单的数相加，不过如果超过最大值或者最小值时carryout就不为0了。从这也能看出来为啥实际的引用计数比extra_rc加1的原因，alloc的时候是不增加引用计数的。

ALWAYS_INLINE bool 
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
    if (isTaggedPointer()) return false;

    bool sideTableLocked = false;

    isa_t oldisa;
    isa_t newisa;

 retry:
    do {
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;
        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (sideTableLocked) sidetable_unlock();
            return sidetable_release(performDealloc);
        }
        // don't check newisa.fast_rr; we already called any RR overrides
        uintptr_t carry;
        newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc--
        if (slowpath(carry)) {
            // don't ClearExclusive()
            goto underflow;
        }
    } while (slowpath(!StoreReleaseExclusive(&isa.bits, 
                                             oldisa.bits, newisa.bits)));

    if (slowpath(sideTableLocked)) sidetable_unlock();
    return false;

 underflow:
    // newisa.extra_rc-- underflowed: borrow from side table or deallocate

    // abandon newisa to undo the decrement
    newisa = oldisa;

    if (slowpath(newisa.has_sidetable_rc)) {
        if (!handleUnderflow) {
            ClearExclusive(&isa.bits);
            return rootRelease_underflow(performDealloc);
        }

        // Transfer retain count from side table to inline storage.

        if (!sideTableLocked) {
            ClearExclusive(&isa.bits);
            sidetable_lock();
            sideTableLocked = true;
            // Need to start over to avoid a race against 
            // the nonpointer -> raw pointer transition.
            goto retry;
        }

        // Try to remove some retain counts from the side table.        
        size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF);

        // To avoid races, has_sidetable_rc must remain set 
        // even if the side table count is now zero.

        if (borrowed > 0) {
            // Side table retain count decreased.
            // Try to add them to the inline count.
            newisa.extra_rc = borrowed - 1;  // redo the original decrement too
            bool stored = StoreReleaseExclusive(&isa.bits, 
                                                oldisa.bits, newisa.bits);
            if (!stored) {
                // Inline update failed. 
                // Try it again right now. This prevents livelock on LL/SC 
                // architectures where the side table access itself may have 
                // dropped the reservation.
                isa_t oldisa2 = LoadExclusive(&isa.bits);
                isa_t newisa2 = oldisa2;
                if (newisa2.nonpointer) {
                    uintptr_t overflow;
                    newisa2.bits = 
                        addc(newisa2.bits, RC_ONE * (borrowed-1), 0, &overflow);
                    if (!overflow) {
                        stored = StoreReleaseExclusive(&isa.bits, oldisa2.bits, 
                                                       newisa2.bits);
                    }
                }
            }

            if (!stored) {
                // Inline update failed.
                // Put the retains back in the side table.
                sidetable_addExtraRC_nolock(borrowed);
                goto retry;
            }

            // Decrement successful after borrowing from side table.
            // This decrement cannot be the deallocating decrement - the side 
            // table lock and has_sidetable_rc bit ensure that if everyone 
            // else tried to -release while we worked, the last one would block.
            sidetable_unlock();
            return false;
        }
        else {
            // Side table is empty after all. Fall-through to the dealloc path.
        }
    }

    // Really deallocate.

    if (slowpath(newisa.deallocating)) {
        ClearExclusive(&isa.bits);
        if (sideTableLocked) sidetable_unlock();
        return overrelease_error();
        // does not actually return
    }
    newisa.deallocating = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;

    if (slowpath(sideTableLocked)) sidetable_unlock();

    __sync_synchronize();
    if (performDealloc) {
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return true;
}

减少引用计数后，if (slowpath(carry)) goto underflow;}，直接就开始((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);。

runtime相关就讲到这里，如果有什么错误，请相互交流指正。