创建类_类对象何去何从

2021-08-25 本文已影响0人 Priders

创建类_类对象何去何从

在Objective-C 中，普通的对象是类的实例，而类也是一个对象，他是objc_class的一个实例，元类(MetaClass)也是一个对象，也是objc_class的一个实例
我们通常认识的，对象的isa 指向类，类的isa指向其元类，元类的isa指向根元类。
我们创建一个类文件的时候，我们就意味着认为它将在后面可以直接的去使用。那么它怎么来的，怎么加载到，怎么去获取里面的方法？

先看 Objc_Clang_after.cpp 这个文件经过clang 编译成CPP来的，实际上我们Xcode也帮我们处理好。
在编译阶段就如这个文件出现大部分的结构体把我们代码中声明的这些描述清楚。

这些_prop_t、_protocol_t 、_objc_method 、_protocol_t 、
_ivar_t、_class_ro_t、_class_t、_category_t为我们整个文件的描述后续基础的准备。
在这个阶段
1、把一些方法、协议、属性、成员变量，装进ro这个定义的结构体里面；
2、把我们处理好的ro结构体放进_class_t之中去，等等为什么isa、superclass、cache、vtable。！！！都是0，特别是isa,superclass、明明就有isa与父类。
3、原来在后面会有一个OBJC_CLASS_SETUP_$_XXX的静态函数会帮我们把（2）中缺少isa、superclass、cache等进行处理。
4、那到底谁负责调用这个函数呢？
留意到.objc_inithooks$B猜想由hooks名字可知会在objc_init中的某个时刻（readClass/realized之前）调用这个函数指针数组（因为每个类都有，所以多个就成了数组） $B或许还有$ A。暂且留下思考。
5、我们处理好的两个_class_t ，存储到mach-o的section信息（__objc_classlist）
留下伏笔就是为了前面文章说到的readClass 到时候进行读取。

提示点：
定义一个静态全局struct，声明不被优化，存储到mach-o的section信息中
如下存储的参数名为(__objc_classlist)，后期根据此名称取值

static struct _class_t *L_OBJC_LABEL_CLASS_$ [2] 
__attribute__((used, section ("__DATA, __objc_classlist,regular,no_dead_strip")))= {

__declspec(allocate("segname")) 告诉编译器分配一个名字为segname的段
在实际使用时，还需要#pragma section事先告诉编译器，我们要使用的段名

#pragma section(".objc_inithooks$B", long, read, write)
__declspec(allocate(".objc_inithooks$B")) static void *OBJC_CLASS_SETUP[] =

何时读取

关于存起来的这些那些的存储到mach-o数据段内容，什么时候读取到呢？不是每次我都去mach-o里面拿吧？

mach-o文件会在exec_mach_imgact这个阶段加载进内存，那么到时候去mach-o映射进内存的部分里面拿，取出来存在全局的gdb_objc_realized_classes里。

在调用map_images_nolock 中 _read_Images ==>中的discover classes流程中readClass来读取类信息


    classref_t const *classlist = _getObjc2ClassList(hi, &count);
//_getObjc2ClassList ==> GETSECT(_getObjc2ClassList,  classref_t const, "__objc_classlist");

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

        for (i = 0; i < count; i++) {
            Class cls = (Class)classlist[i];
            Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
        ......
        }

readClass来读取类信息
- 其中涉及addNamedClass 把我们所有的Class 都存进一个gdb_objc_realized_classes的map中
- addClassTableEntry 已经分配了才进入在所有类的表中添加一个类。如果addMeta为真，也自动添加类的元类。这里是不会进入的
- 注意点这里仅仅！！！map 并没有做其他的处理处理部分在于RealizeClass部分
- RealizeClass：实现；认识到；了解，。。。变现。。这里可以理解为将class从仅仅的一个我们不为所知的class变现为我们可以认识的class，我们认识到的class 那必须正常化把里面需要的信息都进行处理 _read_images 的markdown有部分总结后期对此有不同的认识在进行总结

Class

objc 关于类方面的思考为什么class 结构体相关的居然可以继承？
oc的结构体可以继承吗？不可以。

在开头写到的objc_class这一个结构体 struct objc_class : objc_object,与此同时struct objc_object， objc_object也是一个结构体，大家有没有好奇到为什么结构体可以继承。这里的头文件，可以说用于C++之中，C++支持结构体继承，故在oc里会对应着一个Class这么一个结构体指针，而在oc里只知道他是这么一个东西，C++在runtime源码里面就会用到Class 里面具体的objc_class相关的内容。个人认为不能把这个搞混了，在头文件创建一个结构体继承（objc_class模拟这样子去使用的时候）之后没问题，一旦在.m文件用到这个继承就会报错（.mm没问题支持C++嘛）。

struct objc_class;
struct objc_object;

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

Objc_Clang_after.cpp

#ifndef _REWRITER_typedef_Singer
#define _REWRITER_typedef_Singer
typedef struct objc_object Singer;
typedef struct {} _objc_exc_Singer;
#endif

extern "C" unsigned long OBJC_IVAR_$_Singer$_name;
extern "C" unsigned long OBJC_IVAR_$_Singer$_age;
struct Singer_IMPL {
    struct NSObject_IMPL NSObject_IVARS;
    NSString * _Nonnull _name;
    int * _Nonnull _age;
};

// @property (nonatomic,copy)NSString *name;
// @property (nonatomic,assign)int *age;
/* @end */



#ifndef _REWRITER_typedef_Eason
#define _REWRITER_typedef_Eason
typedef struct objc_object Eason;
typedef struct {} _objc_exc_Eason;
#endif

extern "C" unsigned long OBJC_IVAR_$_Eason$_musicAblums;
struct Eason_IMPL {
    struct Singer_IMPL Singer_IVARS;
    NSMutableArray * _Nonnull _musicAblums;
};

// @property (nonatomic,strong)NSMutableArray *musicAblums;
/* @end */

#pragma clang assume_nonnull end

// @implementation Singer

static void _C_Singer_load(Class self, SEL _cmd) {
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_w__fpzqhmrd08dczx4bdjqly8040000gn_T_Singer_2e3388_mi_0);
}

static void _I_Singer_instanceFunction(Singer * self, SEL _cmd) {
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_w__fpzqhmrd08dczx4bdjqly8040000gn_T_Singer_2e3388_mi_1,__func__);
}

static void _C_Singer_classFunction(Class self, SEL _cmd) {
    NSLog((NSString *)&__NSConstantStringImpl__var_folders_w__fpzqhmrd08dczx4bdjqly8040000gn_T_Singer_2e3388_mi_2);
}

static NSString * _Nonnull _I_Singer_name(Singer * self, SEL _cmd) { return (*(NSString * _Nonnull *)((char *)self + OBJC_IVAR_$_Singer$_name)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);

static void _I_Singer_setName_(Singer * self, SEL _cmd, NSString * _Nonnull name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct Singer, _name), (id)name, 0, 1); }

static int * _Nonnull _I_Singer_age(Singer * self, SEL _cmd) { return (*(int * _Nonnull *)((char *)self + OBJC_IVAR_$_Singer$_age)); }
static void _I_Singer_setAge_(Singer * self, SEL _cmd, int * _Nonnull age) { (*(int * _Nonnull *)((char *)self + OBJC_IVAR_$_Singer$_age)) = age; }
// @end


// @implementation Eason


static NSMutableArray * _Nonnull _I_Eason_musicAblums(Eason * self, SEL _cmd) { return (*(NSMutableArray * _Nonnull *)((char *)self + OBJC_IVAR_$_Eason$_musicAblums)); }
static void _I_Eason_setMusicAblums_(Eason * self, SEL _cmd, NSMutableArray * _Nonnull musicAblums) { (*(NSMutableArray * _Nonnull *)((char *)self + OBJC_IVAR_$_Eason$_musicAblums)) = musicAblums; }
// @end

struct _prop_t {
    const char *name;
    const char *attributes;
};

struct _protocol_t;

struct _objc_method {
    struct objc_selector * _cmd;
    const char *method_type;
    void  *_imp;
};

struct _protocol_t {
    void * isa;  // NULL
    const char *protocol_name;
    const struct _protocol_list_t * protocol_list; // super protocols
    const struct method_list_t *instance_methods;
    const struct method_list_t *class_methods;
    const struct method_list_t *optionalInstanceMethods;
    const struct method_list_t *optionalClassMethods;
    const struct _prop_list_t * properties;
    const unsigned int size;  // sizeof(struct _protocol_t)
    const unsigned int flags;  // = 0
    const char ** extendedMethodTypes;
};

struct _ivar_t {
    unsigned long int *offset;  // pointer to ivar offset location
    const char *name;
    const char *type;
    unsigned int alignment;
    unsigned int  size;
};

struct _class_ro_t {
    unsigned int flags;
    unsigned int instanceStart;
    unsigned int instanceSize;
    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;
};

struct _category_t {
    const char *name;
    struct _class_t *cls;
    const struct _method_list_t *instance_methods;
    const struct _method_list_t *class_methods;
    const struct _protocol_list_t *protocols;
    const struct _prop_list_t *properties;
};
extern "C" __declspec(dllimport) struct objc_cache _objc_empty_cache;
#pragma warning(disable:4273)

extern "C" unsigned long int OBJC_IVAR_$_Singer$_name __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct Singer, _name);
extern "C" unsigned long int OBJC_IVAR_$_Singer$_age __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct Singer, _age);

static struct /*_ivar_list_t*/ {
    unsigned int entsize;  // sizeof(struct _prop_t)
    unsigned int count;
    struct _ivar_t ivar_list[2];
} _OBJC_$_INSTANCE_VARIABLES_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_ivar_t),
    2,
    {{(unsigned long int *)&OBJC_IVAR_$_Singer$_name, "_name", "@\"NSString\"", 3, 8},
     {(unsigned long int *)&OBJC_IVAR_$_Singer$_age, "_age", "^i", 3, 8}}
};

static struct /*_method_list_t*/ {
    unsigned int entsize;  // sizeof(struct _objc_method)
    unsigned int method_count;
    struct _objc_method method_list[9];
} _OBJC_$_INSTANCE_METHODS_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_objc_method),
    9,
    {{(struct objc_selector *)"instanceFunction", "v16@0:8", (void *)_I_Singer_instanceFunction},
    {(struct objc_selector *)"name", "@16@0:8", (void *)_I_Singer_name},
    {(struct objc_selector *)"setName:", "v24@0:8@16", (void *)_I_Singer_setName_},
    {(struct objc_selector *)"age", "^i16@0:8", (void *)_I_Singer_age},
    {(struct objc_selector *)"setAge:", "v24@0:8^i16", (void *)_I_Singer_setAge_},
    {(struct objc_selector *)"name", "@16@0:8", (void *)_I_Singer_name},
    {(struct objc_selector *)"setName:", "v24@0:8@16", (void *)_I_Singer_setName_},
    {(struct objc_selector *)"age", "^i16@0:8", (void *)_I_Singer_age},
    {(struct objc_selector *)"setAge:", "v24@0:8^i16", (void *)_I_Singer_setAge_}}
};

static struct /*_method_list_t*/ {
    unsigned int entsize;  // sizeof(struct _objc_method)
    unsigned int method_count;
    struct _objc_method method_list[2];
} _OBJC_$_CLASS_METHODS_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_objc_method),
    2,
    {{(struct objc_selector *)"load", "v16@0:8", (void *)_C_Singer_load},
    {(struct objc_selector *)"classFunction", "v16@0:8", (void *)_C_Singer_classFunction}}
};

static struct /*_prop_list_t*/ {
    unsigned int entsize;  // sizeof(struct _prop_t)
    unsigned int count_of_properties;
    struct _prop_t prop_list[2];
} _OBJC_$_PROP_LIST_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_prop_t),
    2,
    {{"name","T@\"NSString\",C,N,V_name"},
    {"age","T^i,N,V_age"}}
};

static struct _class_ro_t _OBJC_METACLASS_RO_$_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    1, sizeof(struct _class_t), sizeof(struct _class_t),
    0,
    "Singer",
    (const struct _method_list_t *)&_OBJC_$_CLASS_METHODS_Singer,
    0,
    0,
    0,
    0,
};

static struct _class_ro_t _OBJC_CLASS_RO_$_Singer __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    0, __OFFSETOFIVAR__(struct Singer, _name), sizeof(struct Singer_IMPL),
    0,
    "Singer",
    (const struct _method_list_t *)&_OBJC_$_INSTANCE_METHODS_Singer,
    0,
    (const struct _ivar_list_t *)&_OBJC_$_INSTANCE_VARIABLES_Singer,
    0,
    (const struct _prop_list_t *)&_OBJC_$_PROP_LIST_Singer,
};

extern "C" __declspec(dllimport) struct _class_t OBJC_METACLASS_$_NSObject;

extern "C" __declspec(dllexport) struct _class_t OBJC_METACLASS_$_Singer __attribute__ ((used, section ("__DATA,__objc_data"))) = {
    0, // &OBJC_METACLASS_$_NSObject,
    0, // &OBJC_METACLASS_$_NSObject,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_METACLASS_RO_$_Singer,
};

extern "C" __declspec(dllimport) struct _class_t OBJC_CLASS_$_NSObject;

extern "C" __declspec(dllexport) struct _class_t OBJC_CLASS_$_Singer __attribute__ ((used, section ("__DATA,__objc_data"))) = {
    0, // &OBJC_METACLASS_$_Singer,
    0, // &OBJC_CLASS_$_NSObject,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_CLASS_RO_$_Singer,
};
static void OBJC_CLASS_SETUP_$_Singer(void ) {
    OBJC_METACLASS_$_Singer.isa = &OBJC_METACLASS_$_NSObject;
    OBJC_METACLASS_$_Singer.superclass = &OBJC_METACLASS_$_NSObject;
    OBJC_METACLASS_$_Singer.cache = &_objc_empty_cache;
    OBJC_CLASS_$_Singer.isa = &OBJC_METACLASS_$_Singer;
    OBJC_CLASS_$_Singer.superclass = &OBJC_CLASS_$_NSObject;
    OBJC_CLASS_$_Singer.cache = &_objc_empty_cache;
}

extern "C" unsigned long int OBJC_IVAR_$_Eason$_musicAblums __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct Eason, _musicAblums);

static struct /*_ivar_list_t*/ {
    unsigned int entsize;  // sizeof(struct _prop_t)
    unsigned int count;
    struct _ivar_t ivar_list[1];
} _OBJC_$_INSTANCE_VARIABLES_Eason __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_ivar_t),
    1,
    {{(unsigned long int *)&OBJC_IVAR_$_Eason$_musicAblums, "_musicAblums", "@\"NSMutableArray\"", 3, 8}}
};

static struct /*_method_list_t*/ {
    unsigned int entsize;  // sizeof(struct _objc_method)
    unsigned int method_count;
    struct _objc_method method_list[4];
} _OBJC_$_INSTANCE_METHODS_Eason __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_objc_method),
    4,
    {{(struct objc_selector *)"musicAblums", "@16@0:8", (void *)_I_Eason_musicAblums},
    {(struct objc_selector *)"setMusicAblums:", "v24@0:8@16", (void *)_I_Eason_setMusicAblums_},
    {(struct objc_selector *)"musicAblums", "@16@0:8", (void *)_I_Eason_musicAblums},
    {(struct objc_selector *)"setMusicAblums:", "v24@0:8@16", (void *)_I_Eason_setMusicAblums_}}
};

static struct /*_prop_list_t*/ {
    unsigned int entsize;  // sizeof(struct _prop_t)
    unsigned int count_of_properties;
    struct _prop_t prop_list[1];
} _OBJC_$_PROP_LIST_Eason __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    sizeof(_prop_t),
    1,
    {{"musicAblums","T@\"NSMutableArray\",&,N,V_musicAblums"}}
};

static struct _class_ro_t _OBJC_METACLASS_RO_$_Eason __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    1, sizeof(struct _class_t), sizeof(struct _class_t),
    0,
    "Eason",
    0,
    0,
    0,
    0,
    0,
};

static struct _class_ro_t _OBJC_CLASS_RO_$_Eason __attribute__ ((used, section ("__DATA,__objc_const"))) = {
    0, __OFFSETOFIVAR__(struct Eason, _musicAblums), sizeof(struct Eason_IMPL),
    0,
    "Eason",
    (const struct _method_list_t *)&_OBJC_$_INSTANCE_METHODS_Eason,
    0,
    (const struct _ivar_list_t *)&_OBJC_$_INSTANCE_VARIABLES_Eason,
    0,
    (const struct _prop_list_t *)&_OBJC_$_PROP_LIST_Eason,
};

extern "C" __declspec(dllexport) struct _class_t OBJC_METACLASS_$_Singer;
extern "C" __declspec(dllimport) struct _class_t OBJC_METACLASS_$_NSObject;

extern "C" __declspec(dllexport) struct _class_t OBJC_METACLASS_$_Eason __attribute__ ((used, section ("__DATA,__objc_data"))) = {
    0, // &OBJC_METACLASS_$_NSObject,
    0, // &OBJC_METACLASS_$_Singer,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_METACLASS_RO_$_Eason,
};

extern "C" __declspec(dllexport) struct _class_t OBJC_CLASS_$_Singer;

extern "C" __declspec(dllexport) struct _class_t OBJC_CLASS_$_Eason __attribute__ ((used, section ("__DATA,__objc_data"))) = {
    0, // &OBJC_METACLASS_$_Eason,
    0, // &OBJC_CLASS_$_Singer,
    0, // (void *)&_objc_empty_cache,
    0, // unused, was (void *)&_objc_empty_vtable,
    &_OBJC_CLASS_RO_$_Eason,
};
static void OBJC_CLASS_SETUP_$_Eason(void ) {
    OBJC_METACLASS_$_Eason.isa = &OBJC_METACLASS_$_NSObject;
    OBJC_METACLASS_$_Eason.superclass = &OBJC_METACLASS_$_Singer;
    OBJC_METACLASS_$_Eason.cache = &_objc_empty_cache;
    OBJC_CLASS_$_Eason.isa = &OBJC_METACLASS_$_Eason;
    OBJC_CLASS_$_Eason.superclass = &OBJC_CLASS_$_Singer;
    OBJC_CLASS_$_Eason.cache = &_objc_empty_cache;
}
#pragma section(".objc_inithooks$B", long, read, write)
__declspec(allocate(".objc_inithooks$B")) static void *OBJC_CLASS_SETUP[] = {
    (void *)&OBJC_CLASS_SETUP_$_Singer,
    (void *)&OBJC_CLASS_SETUP_$_Eason,
};
static struct _class_t *L_OBJC_LABEL_CLASS_$ [2] __attribute__((used, section ("__DATA, __objc_classlist,regular,no_dead_strip")))= {
    &OBJC_CLASS_$_Singer,
    &OBJC_CLASS_$_Eason,
};
static struct _class_t *_OBJC_LABEL_NONLAZY_CLASS_$[] = {
        &OBJC_CLASS_$_Singer,
};
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

__declspec(allocate("segname")) 告诉编译器分配一个名字为segname的段
在实际使用时，还需要#pragma section事先告诉编译器，我们要使用的段名

创建类_类对象何去何从

创建类_类对象何去何从

何时读取

Class

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