很早之前看到一篇block原理的blog，里面介紹可以通過clang查看一個文件的c的實現(xiàn)，當(dāng)時就看了看轉(zhuǎn)化后的代碼，發(fā)現(xiàn)特別長，就大概瞄了幾眼就關(guān)了，這幾天研究runtime的時候就仔細(xì)的研究了下。
編譯器對類的轉(zhuǎn)化
@interface KKPerson : NSObject
{
@public
int age;
char *name;
}
@property (nonatomic, copy ) NSString *k_name;
- (void)foo;
+ (void)manager;
@end
定義一個很簡單的類，有實例變量、屬性、實例方法、類方法，通過clang查看它轉(zhuǎn)換成什么。
#ifndef _REWRITER_typedef_KKPerson
#define _REWRITER_typedef_KKPerson
typedef struct objc_object KKPerson;
typedef struct {} _objc_exc_KKPerson;
#endif
extern "C" unsigned long OBJC_IVAR_$_KKPerson$age;
extern "C" unsigned long OBJC_IVAR_$_KKPerson$name;
extern "C" unsigned long OBJC_IVAR_$_KKPerson$_k_name;
struct KKPerson_IMPL {
struct NSObject_IMPL NSObject_IVARS;
int age;
char *name;
NSString *_k_name;
};
// @property (nonatomic, copy ) NSString *k_name;
// - (void)foo;
// + (void)manager;
/* @end */
// @implementation KKPerson
static void _I_KKPerson_foo(KKPerson * self, SEL _cmd) {}
static void _C_KKPerson_manager(Class self, SEL _cmd) {}
static NSString * _I_KKPerson_k_name(KKPerson * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_KKPerson$_k_name)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
static void _I_KKPerson_setK_name_(KKPerson * self, SEL _cmd, NSString *k_name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct KKPerson, _k_name), (id)k_name, 0, 1); }
通過上面可以很容易看出來，KKPerson就是struct objc_object的別名，編譯器又生成了struct KKPerson_IMPL，它存儲著KKPerson的實例變量，這樣就可以很容易得出它占多大空間了。
接著它把oc方法直接轉(zhuǎn)換成c的方法，通過前綴_I_KKPerson_、_C_KKPerson_可以很方便地辨識出它是類方法還是實例方法，而且也能知道它是那個類的方法。
屬性
編譯器生成了_I_KKPerson_k_name、_I_KKPerson_setK_name_方法，能看出來屬性就是由實例變量、set方法、get方法構(gòu)成。get方法就簡單就是通過偏移量來獲取的，set方法是通過調(diào)用objc_setProperty方法來實現(xiàn)的。
void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy)
{
bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
bool mutableCopy = (shouldCopy == MUTABLE_COPY);
reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}
static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
if (offset == 0) {
object_setClass(self, newValue);
return;
}
id oldValue;
id *slot = (id*) ((char*)self + offset);
if (copy) {
newValue = [newValue copyWithZone:nil];
} else if (mutableCopy) {
newValue = [newValue mutableCopyWithZone:nil];
} else {
if (*slot == newValue) return;
newValue = objc_retain(newValue);
}
if (!atomic) {
oldValue = *slot;
*slot = newValue;
} else {
spinlock_t& slotlock = PropertyLocks[slot];
slotlock.lock();
oldValue = *slot;
*slot = newValue;
slotlock.unlock();
}
objc_release(oldValue);
}
通過它的4個參數(shù)也能推斷出它的大概實現(xiàn)，第一個參數(shù)偏移量，是拿舊值的，第二個參數(shù)新值，是賦值用的，第三個參數(shù)原子性，判斷是否加鎖，第四個參數(shù)拷貝，判斷是否拷貝?？瓷厦娴膶崿F(xiàn)也都是圍繞這4個參數(shù)來的，通過偏移量拿到舊值地址，再賦新值。
類的存儲
類的方法、實例變量都有了，那是通過什么數(shù)據(jù)結(jié)構(gòu)來存儲哪些東西呢？
先來看下類、方法、實例變量、分類他們的數(shù)據(jù)結(jié)構(gòu)。
struct _prop_t {
const char *name;
const char *attributes;
};
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;
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;
};
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;
};
這些數(shù)據(jù)結(jié)構(gòu)跟
runtime中的數(shù)據(jù)結(jié)構(gòu)稍有不同，而且在應(yīng)用啟動的時候會轉(zhuǎn)化的，這些數(shù)據(jù)結(jié)構(gòu)是存儲在section中的，具體哪些存儲到哪個位置，可以查看符號表，里面很清楚。
聲明完類的相關(guān)數(shù)據(jù)結(jié)構(gòu)后，就可以定義一個具體的類，這樣的話每一個類都有一個固定的地址，在整個運行過程中都不會變，因為在編譯期就決定了，除非改代碼再編譯。
extern "C" __declspec(dllexport) struct _class_t OBJC_CLASS_$_KKPerson __attribute__ ((used, section ("__DATA,__objc_data"))) = {
0, // &OBJC_METACLASS_$_KKPerson,
0, // &OBJC_CLASS_$_NSObject,
0, // (void *)&_objc_empty_cache,
0, // unused, was (void *)&_objc_empty_vtable,
&_OBJC_CLASS_RO_$_KKPerson,
};
static struct _class_ro_t _OBJC_CLASS_RO_$_KKPerson __attribute__ ((used, section ("__DATA,__objc_const"))) = {
0, __OFFSETOFIVAR__(struct KKPerson, age), sizeof(struct KKPerson_IMPL),
(unsigned int)0,
0,
"KKPerson",
(const struct _method_list_t *)&_OBJC_$_INSTANCE_METHODS_KKPerson,
0,
(const struct _ivar_list_t *)&_OBJC_$_INSTANCE_VARIABLES_KKPerson,
0,
(const struct _prop_list_t *)&_OBJC_$_PROP_LIST_KKPerson,
};
可以看出類真正的數(shù)據(jù)都是在存在_OBJC_CLASS_RO_$_KKPerson里面的，里面有類名、類大小、起始地址(繼承)、實例方法、實例變量、屬性列表。
看到這有個疑問，那類方法存到哪了呢，答案是存在元類里面了。
extern "C" __declspec(dllexport) struct _class_t OBJC_METACLASS_$_KKPerson __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_$_KKPerson,
};
static struct _class_ro_t _OBJC_METACLASS_RO_$_KKPerson __attribute__ ((used, section ("__DATA,__objc_const"))) = {
1, sizeof(struct _class_t), sizeof(struct _class_t),
(unsigned int)0,
0,
"KKPerson",
(const struct _method_list_t *)&_OBJC_$_CLASS_METHODS_KKPerson,
0,
0,
0,
0,
};
可以看出_OBJC_METACLASS_RO_$_KKPerson里面存儲著_OBJC_$_CLASS_METHODS_KKPerson地址，里面存儲的都是類方法。
static struct /*_ivar_list_t*/ {
unsigned int entsize; // sizeof(struct _prop_t)
unsigned int count;
struct _ivar_t ivar_list[3];
} _OBJC_$_INSTANCE_VARIABLES_KKPerson __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_ivar_t),
3,
{{(unsigned long int *)&OBJC_IVAR_$_KKPerson$age, "age", "i", 2, 4},
{(unsigned long int *)&OBJC_IVAR_$_KKPerson$name, "name", "*", 3, 8},
{(unsigned long int *)&OBJC_IVAR_$_KKPerson$_k_name, "_k_name", "@\"NSString\"", 3, 8}}
};
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[3];
} _OBJC_$_INSTANCE_METHODS_KKPerson __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
3,
{{(struct objc_selector *)"foo", "v16@0:8", (void *)_I_KKPerson_foo},
{(struct objc_selector *)"k_name", "@16@0:8", (void *)_I_KKPerson_k_name},
{(struct objc_selector *)"setK_name:", "v24@0:8@16", (void *)_I_KKPerson_setK_name_}}
};
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CLASS_METHODS_KKPerson __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"manager", "v16@0:8", (void *)_C_KKPerson_manager}}
};
這里只列出實例列表、實例方法列表、類方法列表，其他的都差不多。
每一個方法都有方法名、方法參數(shù)符號、實現(xiàn)地址（IMP），從這也可以看出來，調(diào)用一個實例方法，都是先從類方法列表里面查找，找到了就拿出相應(yīng)的實現(xiàn)地址，然后調(diào)用。
細(xì)心的人能看到OBJC_CLASS_$_KKPerson結(jié)構(gòu)體，前面幾個值都為0，那是什么時候又重新賦值呢？
static void OBJC_CLASS_SETUP_$_KKPerson(void ) {
OBJC_METACLASS_$_KKPerson.isa = &OBJC_METACLASS_$_NSObject;
OBJC_METACLASS_$_KKPerson.superclass = &OBJC_METACLASS_$_NSObject;
OBJC_METACLASS_$_KKPerson.cache = &_objc_empty_cache;
OBJC_CLASS_$_KKPerson.isa = &OBJC_METACLASS_$_KKPerson;
OBJC_CLASS_$_KKPerson.superclass = &OBJC_CLASS_$_NSObject;
OBJC_CLASS_$_KKPerson.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_$_KKPerson,
};
可以看到是通過OBJC_CLASS_SETUP_$_KKPerson這個方法來重新賦值的，而且還有一個void *的數(shù)組，里面存的是OBJC_CLASS_SETUP_$_KKPerson方法地址。能夠推測出在編譯的某個階段通過這個數(shù)組來調(diào)用里面的方法，然后再對類進(jìn)行重新賦值，至于是什么時候，目前還沒研究出來。
typedef struct objc_object KKPerson;
struct KKPerson_IMPL {
struct NSObject_IMPL NSObject_IVARS;
int age;
char *name;
NSString *_k_name;
};
#define __OFFSETOFIVAR__(TYPE, MEMBER) ((long long) &((TYPE *)0)->MEMBER)
extern "C" unsigned long int OBJC_IVAR_$_KKPerson$age __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct KKPerson, age);
extern "C" unsigned long int OBJC_IVAR_$_KKPerson$name __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct KKPerson, name);
extern "C" unsigned long int OBJC_IVAR_$_KKPerson$_k_name __attribute__ ((used, section ("__DATA,__objc_ivar"))) = __OFFSETOFIVAR__(struct KKPerson, _k_name);
我這邊還有一個疑惑，在定義實例變量結(jié)構(gòu)體的時候，需要偏移量，通過上面的代碼也能看出來，__OFFSETOFIVAR__這個宏定義就是求一個結(jié)構(gòu)體某個變量的偏移量，但是__OFFSETOFIVAR__(struct KKPerson, age)里面的第一個參數(shù)竟然是struct KKPerson，不應(yīng)該是struct KKPerson_IMPL嗎？這點很不理解，我推測在之后的階段又替換為struct KKPerson_IMPL了，不然沒法解釋，希望某個大佬能給我解答下。
/// RewriteIvarOffsetComputation - This routine synthesizes computation of
/// ivar offset.
void RewriteModernObjC::RewriteIvarOffsetComputation(ObjCIvarDecl *ivar,
std::string &Result) {
Result += "__OFFSETOFIVAR__(struct ";
Result += ivar->getContainingInterface()->getNameAsString();
if (LangOpts.MicrosoftExt)
Result += "_IMPL";
Result += ", ";
if (ivar->isBitField())
ObjCIvarBitfieldGroupDecl(ivar, Result);
else
Result += ivar->getNameAsString();
Result += ")";
}
這個疑問我通過分析clang的源碼后，發(fā)現(xiàn)LangOpts.MicrosoftExt為真的時候，就是struct KKPerson_IMPL。至于LangOpts.MicrosoftExt，它是clang對Microsoft的c++擴(kuò)展。
這里只是把一個類編譯后結(jié)構(gòu)稍微講解了下，那這些結(jié)構(gòu)在應(yīng)用啟動后，在內(nèi)存是什么樣的，就需要看runtime的代碼了，下一篇開始看runtime。
最后貼一張符號表，能夠清晰地看到每個結(jié)構(gòu)的地址。
# Sections:
# Address Size Segment Section
0x100000C80 0x00000203 __TEXT __text
0x100000E84 0x0000002A __TEXT __stubs
0x100000EB0 0x00000056 __TEXT __stub_helper
0x100000F06 0x0000000B __TEXT __objc_classname
0x100000F11 0x00000048 __TEXT __objc_methname
0x100000F59 0x0000002B __TEXT __objc_methtype
0x100000F84 0x0000002A __TEXT __cstring
0x100000FB0 0x00000048 __TEXT __unwind_info
0x100001000 0x00000010 __DATA __nl_symbol_ptr
0x100001010 0x00000038 __DATA __la_symbol_ptr
0x100001048 0x00000020 __DATA __cfstring
0x100001068 0x00000008 __DATA __objc_classlist
0x100001070 0x00000008 __DATA __objc_imageinfo
0x100001078 0x00000198 __DATA __objc_const
0x100001210 0x00000030 __DATA __objc_selrefs
0x100001240 0x00000008 __DATA __objc_classrefs
0x100001248 0x00000018 __DATA __objc_ivar
0x100001260 0x00000050 __DATA __objc_data
# Symbols:
# Address Size File Name
0x100000C80 0x00000010 [ 1] -[KKPerson foo]
0x100000C90 0x00000010 [ 1] +[KKPerson manager]
0x100000CA0 0x00000030 [ 1] -[KKPerson k_name]
0x100000CD0 0x00000040 [ 1] -[KKPerson setK_name:]
0x100000D10 0x00000040 [ 1] -[KKPerson .cxx_destruct]
0x100000D50 0x00000133 [ 1] _main
0x100000E84 0x00000006 [ 2] _objc_autoreleasePoolPop
0x100000E8A 0x00000006 [ 2] _objc_autoreleasePoolPush
0x100000E90 0x00000006 [ 2] _objc_getProperty
0x100000E96 0x00000006 [ 2] _objc_msgSend
0x100000E9C 0x00000006 [ 2] _objc_retainAutoreleasedReturnValue
0x100000EA2 0x00000006 [ 2] _objc_setProperty_nonatomic_copy
0x100000EA8 0x00000006 [ 2] _objc_storeStrong
0x100000EB0 0x00000010 [ 0] helper helper
0x100000EC0 0x0000000A [ 2] _objc_autoreleasePoolPop
0x100000ECA 0x0000000A [ 2] _objc_autoreleasePoolPush
0x100000ED4 0x0000000A [ 2] _objc_getProperty
0x100000EDE 0x0000000A [ 2] _objc_msgSend
0x100000EE8 0x0000000A [ 2] _objc_retainAutoreleasedReturnValue
0x100000EF2 0x0000000A [ 2] _objc_setProperty_nonatomic_copy
0x100000EFC 0x0000000A [ 2] _objc_storeStrong
0x100000F06 0x00000009 [ 1] literal string: KKPerson
0x100000F0F 0x00000002 [ 1] literal string: !
0x100000F11 0x00000008 [ 1] literal string: manager
0x100000F19 0x00000004 [ 1] literal string: foo
0x100000F1D 0x0000000E [ 1] literal string: .cxx_destruct
0x100000F2B 0x00000007 [ 1] literal string: k_name
0x100000F32 0x0000000B [ 1] literal string: setK_name:
0x100000F3D 0x00000004 [ 1] literal string: age
0x100000F41 0x00000005 [ 1] literal string: name
0x100000F46 0x00000008 [ 1] literal string: _k_name
0x100000F4E 0x00000006 [ 1] literal string: alloc
0x100000F54 0x00000005 [ 1] literal string: init
0x100000F59 0x00000008 [ 1] literal string: v16@0:8
0x100000F61 0x00000008 [ 1] literal string: @16@0:8
0x100000F69 0x0000000B [ 1] literal string: v24@0:8@16
0x100000F74 0x00000002 [ 1] literal string: I
0x100000F76 0x00000002 [ 1] literal string: *
0x100000F78 0x0000000C [ 1] literal string: @"NSString"
0x100000F84 0x00000007 [ 1] literal string: k_name
0x100000F8B 0x0000001A [ 1] literal string: T@"NSString",C,N,V_k_name
0x100000FA5 0x00000004 [ 1] literal string: adf
0x100000FA9 0x00000005 [ 1] literal string: xiao
0x100000FB0 0x00000048 [ 0] compact unwind info
0x100001000 0x00000008 [ 0] non-lazy-pointer-to-local: dyld_stub_binder
0x100001008 0x00000008 [ 0] non-lazy-pointer
0x100001010 0x00000008 [ 2] _objc_autoreleasePoolPop
0x100001018 0x00000008 [ 2] _objc_autoreleasePoolPush
0x100001020 0x00000008 [ 2] _objc_getProperty
0x100001028 0x00000008 [ 2] _objc_msgSend
0x100001030 0x00000008 [ 2] _objc_retainAutoreleasedReturnValue
0x100001038 0x00000008 [ 2] _objc_setProperty_nonatomic_copy
0x100001040 0x00000008 [ 2] _objc_storeStrong
0x100001048 0x00000020 [ 1] CFString
0x100001068 0x00000008 [ 1] anon
0x100001070 0x00000008 [ 0] objc image info
0x100001078 0x00000020 [ 1] l_OBJC_$_CLASS_METHODS_KKPerson
0x100001098 0x00000048 [ 1] l_OBJC_METACLASS_RO_$_KKPerson
0x1000010E0 0x00000068 [ 1] l_OBJC_$_INSTANCE_METHODS_KKPerson
0x100001148 0x00000068 [ 1] l_OBJC_$_INSTANCE_VARIABLES_KKPerson
0x1000011B0 0x00000018 [ 1] l_OBJC_$_PROP_LIST_KKPerson
0x1000011C8 0x00000048 [ 1] l_OBJC_CLASS_RO_$_KKPerson
0x100001210 0x00000008 [ 1] pointer-to-literal-cstring
0x100001218 0x00000008 [ 1] pointer-to-literal-cstring
0x100001220 0x00000008 [ 1] pointer-to-literal-cstring
0x100001228 0x00000008 [ 1] pointer-to-literal-cstring
0x100001230 0x00000008 [ 1] pointer-to-literal-cstring
0x100001238 0x00000008 [ 1] pointer-to-literal-cstring
0x100001240 0x00000008 [ 1] objc-class-ref
0x100001248 0x00000008 [ 1] _OBJC_IVAR_$_KKPerson._k_name
0x100001250 0x00000008 [ 1] _OBJC_IVAR_$_KKPerson.age
0x100001258 0x00000008 [ 1] _OBJC_IVAR_$_KKPerson.name
0x100001260 0x00000028 [ 1] _OBJC_METACLASS_$_KKPerson
0x100001288 0x00000028 [ 1] _OBJC_CLASS_$_KKPerson
