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();//重点在这里
}

----------第一层-----------------------
class_rw_t* data() {
return (class_rw_t *)(bits & FAST_DATA_MASK);
}
------------第二层---------------------
//1.表示是否swift

define FAST_IS_SWIFT (1UL<<0)

//2.当前类或者父类是否有retain、release等方法

define FAST_HAS_DEFAULT_RR (1UL<<1)

// class's instances requires raw isa

//3.类是否需要初始化isa

define FAST_REQUIRES_RAW_ISA (1UL<<2)

// data pointer

//4.数据段指针 1111111111 1111111111 1111111111 1111111111 1111000

define FAST_DATA_MASK 0x00007ffffffffff8UL

----第三层 到此找到 rw-------
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;///RO很重要

method_array_t methods;
property_array_t properties;
protocol_array_t protocols;

Class firstSubclass;
Class nextSiblingClass;

char *demangledName;

-----------------到此已经找到了类的所有结构体可以对类初始化---------
//1.把ro中的方法添加到rw中
method_list_t *list = ro->baseMethods();
if (list) {
prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
rw->methods.attachLists(&list, 1);
}

//2.把ro中的属性添加到rw中
property_list_t *proplist = ro->baseProperties;
if (proplist) {
    rw->properties.attachLists(&proplist, 1);
}

//3.把ro中的协议添加到rw中
protocol_list_t *protolist = ro->baseProtocols;
if (protolist) {
    rw->protocols.attachLists(&protolist, 1);
}

//把分类中的方法、属性、协议添加到类中去
category_list *cats = unattachedCategoriesForClass(cls, true /*realizing*/);
attachCategories(cls, cats, false /*don't flush caches*/);

------------------到此完成了一个类的初始化------------------