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03.对象本质探究与isa

一、结构分析

我们知道,OC底层是C++,我们先将下面的代码还原成C++代码再进行下一步。
新建一个空项目在main函数中写入测试代码。
@interface RKObject : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) int age;
@end
@implementation RKObject
@end
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
NSLog(@"Hello, World!");
}
return 0;
}

1.1 还原成C++

执行clang -rewrite-objc main.m -o main.cpp,就能得到C++文件。
或者:
// 模拟器
xcrun -sdk iphonesimulator clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp
// 真机
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o mainarm64.cpp
我们找到和我们定义的对象相关的地方:

1.2 一些关键内容

a. 定义

这里是一些类型定义
#ifndef _REWRITER_typedef_RKObject
#define _REWRITER_typedef_RKObject
typedef struct objc_object RKObject;
typedef struct {} _objc_exc_RKObject;
#endif

b. 结构体

这里我们可以看到,是一个结构体。
而我们知道,结构体是不能继承的。这里通过一个NSObject_IVARS,实现了OC的继承
同样也可以看到我们定义的属性相对应的成员变量。
extern "C" unsigned long OBJC_IVAR_$_RKObject$_name;
extern "C" unsigned long OBJC_IVAR_$_RKObject$_age;
struct RKObject_IMPL {
struct NSObject_IMPL NSObject_IVARS;
int _age;
NSString *_name;
};
// @property (nonatomic, copy) NSString *name;
// @property (nonatomic, assign) int age;
/* @end */

c. 函数

这里是成员变量相关的set/get方法:
// @implementation RKObject
static NSString * _I_RKObject_name(RKObject * self, SEL _cmd) { return (*(NSString **)((char *)self + OBJC_IVAR_$_RKObject$_name)); }
extern "C" __declspec(dllimport) void objc_setProperty (id, SEL, long, id, bool, bool);
static void _I_RKObject_setName_(RKObject * self, SEL _cmd, NSString *name) { objc_setProperty (self, _cmd, __OFFSETOFIVAR__(struct RKObject, _name), (id)name, 0, 1); }
static int _I_RKObject_age(RKObject * self, SEL _cmd) { return (*(int *)((char *)self + OBJC_IVAR_$_RKObject$_age)); }
static void _I_RKObject_setAge_(RKObject * self, SEL _cmd, int age) { (*(int *)((char *)self + OBJC_IVAR_$_RKObject$_age)) = age; }
// @end
静态的实例方法
  • 我们发现它是static的,他不是实例方法么?
  • 我们再看,它包含了两个参数,这两个参数在我们使用的时候是隐含的:
    • 对象
    • Selector
  • 这样我们知道,实例方法本身也是静态的,只不过有两个隐藏的参数罢了。
get方法如何获取成员变量的
  • name的get方法的实现中看到
    • return (*(NSString **)((char *)self + OBJC_IVAR_$_RKObject$_name));
  • 实例首地址 + 成员变量的offset = 成员变量的指针

d. isa

在上面我们知道,struct NSObject_IMPL NSObject_IVARS,是OC继承关系的核心。
那么NSObject_IMPL是什么呢?
我们很快就能在文件中找到:
struct NSObject_IMPL {
Class isa;
};
它就是isa

e. Class

在文件中我们找到,Class的定义。本质是一个objc_class结构体指针的别名。
typedef struct objc_class *Class;
struct objc_object {
Class _Nonnull isa __attribute__((deprecated));
};

f. id类型

再往下我们还能发现一个,它也是一个objc_object *,结构体指针。
这也解答了,为什么我们平时使用id类型的时候没有*
typedef struct objc_object *id;

二、isa

alloc与字节对齐一文中我们有聊到initInstanceIsa,将isa和class进行绑定。
我们从这里开始继续研究isa

2.1 源码

inline void
objc_object::initIsa(Class cls, bool nonpointer, UNUSED_WITHOUT_INDEXED_ISA_AND_DTOR_BIT bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
isa_t newisa(0);
if (!nonpointer) {
newisa.setClass(cls, this);
} else {
ASSERT(!DisableNonpointerIsa);
ASSERT(!cls->instancesRequireRawIsa());
#if SUPPORT_INDEXED_ISA
ASSERT(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
# if ISA_HAS_CXX_DTOR_BIT
newisa.has_cxx_dtor = hasCxxDtor;
# endif
newisa.setClass(cls, this);
#endif
newisa.extra_rc = 1;
}
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}

2.2 isa的本质 isa_t

从源码中我们看到isa的类型为isa_t,我们找到isa_t的源码:
union isa_t {
// 构造方法
isa_t() { }
isa_t(uintptr_t value) : bits(value) { }
uintptr_t bits;
private:
// Accessing the class requires custom ptrauth operations, so
// force clients to go through setClass/getClass by making this
// private.
Class cls;
public:
#if defined(ISA_BITFIELD)
struct {
ISA_BITFIELD; // defined in isa.h
};
bool isDeallocating() {
return extra_rc == 0 && has_sidetable_rc == 0;
}
void setDeallocating() {
extra_rc = 0;
has_sidetable_rc = 0;
}
#endif
void setClass(Class cls, objc_object *obj);
Class getClass(bool authenticated);
Class getDecodedClass(bool authenticated);
};
破案了,他是个联合体!

2.3 ISA_BITFIELD isa的位域

arm64下的ISA_BITFIELD:
# define ISA_BITFIELD \
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 unused : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 19
  • nonpointer
    • 是否是纯isa指针
      • 0:纯isa指针,只包含类对象地址
      • 1:还包含了类的信息、对象的引用计数
  • has_assoc
    • 是否存在关联对象的标志位
  • has_cxx_dtor
    • 该对象是否有C++或者Objc的析构器
      • 如果有析构函数,则需要做析构逻辑
      • 如果没有,就可以快速的释放对象
  • shiftcls
    • 类指针的值
      • 开启指针优化的情况下,arm64中用33位来存
  • magic
    • 用于判断当前对象是已经初始化的对象
    • 还是没有初始化的空间
  • weakly_referenced
    • 是否被弱引用
    • 没有弱引用的对象可以更快的释放
  • unused
    *
  • has_sidetable_rc
    • 当引用计数大于10时,需要借用该变量存储进位
  • extra_rc
    • 引用计数
    • 实际的值是引用计数-1,即:
      • 引用计数为10,extra_rc = 9

2.4 获取class对象: getClass

inline Class
isa_t::getClass(MAYBE_UNUSED_AUTHENTICATED_PARAM bool authenticated) {
#if SUPPORT_INDEXED_ISA
return cls;
#else
uintptr_t clsbits = bits;
# if __has_feature(ptrauth_calls)
# if ISA_SIGNING_AUTH_MODE == ISA_SIGNING_AUTH
// Most callers aren't security critical, so skip the
// authentication unless they ask for it. Message sending and
// cache filling are protected by the auth code in msgSend.
if (authenticated) {
// Mask off all bits besides the class pointer and signature.
clsbits &= ISA_MASK;
if (clsbits == 0)
return Nil;
clsbits = (uintptr_t)ptrauth_auth_data((void *)clsbits, ISA_SIGNING_KEY, ptrauth_blend_discriminator(this, ISA_SIGNING_DISCRIMINATOR));
} else {
// If not authenticating, strip using the precomputed class mask.
clsbits &= objc_debug_isa_class_mask;
}
# else
// If not authenticating, strip using the precomputed class mask.
clsbits &= objc_debug_isa_class_mask;
# endif
# else
clsbits &= ISA_MASK;
# endif
return (Class)clsbits;
#endif
}
分析源码实现,我们知道了。是通过isa的bits再与上一个掩码ISA_MASK来获取到类对象的地址的。
  • ARM64: define ISA_MASK 0x0000000ffffffff8ULL
  • __x86_64__: define ISA_MASK 0x00007ffffffffff8ULL

a.验证

用我们上面的demo进行验证,注意是Mac环境,不是arm64。
1
(lldb) p/x RKObject.class
(Class) $0 = 0x00000001000081f8 RKObject
(lldb) x/4gx obj
0x1007576a0: 0x011d8001000081fd 0x0000000000000000
0x1007576b0: 0x0000000000000000 0x0000000000000000
(lldb) p/x 0x011d8001000081fd & 0x00007ffffffffff8ULL
(unsigned long long) $2 = 0x00000001000081f8
这里我们发现计算的结果和直接获取的结果一致。

b. 不使用掩码,通过位运算计算出Class

以Mac下为例,isa指针的结构是一定的。在一个isa指针的空间内,class的位置是相对固定的。
# define ISA_BITFIELD \
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 unused : 1; \
uintptr_t has_sidetable_rc : 1; \
uintptr_t extra_rc : 8
结构如下: B + Class + A(小端模式,从右向左)
  • A:1+1+1 = 3
  • Class:44
  • B:6+1+1+1+8 = 17
(lldb) x/4gx obj
0x1007576a0: 0x011d8001000081fd 0x0000000000000000
0x1007576b0: 0x0000000000000000 0x0000000000000000
(lldb) p/t 0x011d8001000081fd
(long) $11 = 0b00000001000111011 00000000000000100000000000000001000000111111 101
计算:
  • 左移3位,使A从右边溢出,左边多的位会补0
    • 现在的结构:000 + B + Class
(lldb) p/x 0x011d8001000081fd >> 3
(long) $7 = 0x0023b0002000103f
(lldb) p/t 0x0023b0002000103f
(long) $12 = 0b000 00000001000111011 00000000000000100000000000000001000000111111
  • 现在需要去掉B,右移3+17=20
    • 现在的结构:Class + 20个0
(lldb) p/x 0x0023b0002000103f << 20
(long) $8 = 0x0002000103f00000
(lldb) p/t 0x0002000103f00000
(long) $13 = 0b00000000000000100000000000000001000000111111 00000000000000000000
  • 回到原位,右移17位:
    • 现在的结构:17个0 + Class + 3个0
(lldb) p/x 0x0002000103f00000 >> 17
(long) $9 = 0x00000001000081f8
(lldb) p/t 0x00000001000081f8
(long) $14 = 0b00000000000000000 00000000000000100000000000000001000000111111 000

三、isa和类对象的绑定

这里是objc_object::initIsa
inline void
objc_object::initIsa(Class cls, bool nonpointer, UNUSED_WITHOUT_INDEXED_ISA_AND_DTOR_BIT bool hasCxxDtor)
{
ASSERT(!isTaggedPointer());
isa_t newisa(0);
if (!nonpointer) { // 如果是纯isa就直接setClass
newisa.setClass(cls, this);
} else {
ASSERT(!DisableNonpointerIsa);
ASSERT(!cls->instancesRequireRawIsa());
#if SUPPORT_INDEXED_ISA
ASSERT(cls->classArrayIndex() > 0);
newisa.bits = ISA_INDEX_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
newisa.has_cxx_dtor = hasCxxDtor;
newisa.indexcls = (uintptr_t)cls->classArrayIndex();
#else
newisa.bits = ISA_MAGIC_VALUE;
// isa.magic is part of ISA_MAGIC_VALUE
// isa.nonpointer is part of ISA_MAGIC_VALUE
# if ISA_HAS_CXX_DTOR_BIT
newisa.has_cxx_dtor = hasCxxDtor;
# endif
newisa.setClass(cls, this);
#endif
newisa.extra_rc = 1;
}
// This write must be performed in a single store in some cases
// (for example when realizing a class because other threads
// may simultaneously try to use the class).
// fixme use atomics here to guarantee single-store and to
// guarantee memory order w.r.t. the class index table
// ...but not too atomic because we don't want to hurt instantiation
isa = newisa;
}

3.1 class绑定流程图

1

3.2 中间有个判断 Nonpointer isa

下面我们具体研究下

四、Nonpointer isa

在上面我们有多次看到Nonpointer isa出现。现在我们深入的研究一下

4.1 isa_t

在上面我们知道isa是一个联合体,而联合体成员是互斥的。
这个就是Nonpointer isa区别于纯isa的核心!

4.2 isa_t::setClass

这里是设置Class的核心实现
// Set the class field in an isa. Takes both the class to set and
// a pointer to the object where the isa will ultimately be used.
// This is necessary to get the pointer signing right.
//
// Note: this method does not support setting an indexed isa. When
// indexed isas are in use, it can only be used to set the class of a
// raw isa.
inline void
isa_t::setClass(Class newCls, UNUSED_WITHOUT_PTRAUTH objc_object *obj)
{
// Match the conditional in isa.h.
#if __has_feature(ptrauth_calls) || TARGET_OS_SIMULATOR
# if ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_NONE
// No signing, just use the raw pointer.
uintptr_t signedCls = (uintptr_t)newCls;
# elif ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_ONLY_SWIFT
// We're only signing Swift classes. Non-Swift classes just use
// the raw pointer
uintptr_t signedCls = (uintptr_t)newCls;
if (newCls->isSwiftStable())
signedCls = (uintptr_t)ptrauth_sign_unauthenticated((void *)newCls, ISA_SIGNING_KEY, ptrauth_blend_discriminator(obj, ISA_SIGNING_DISCRIMINATOR));
# elif ISA_SIGNING_SIGN_MODE == ISA_SIGNING_SIGN_ALL
// We're signing everything
uintptr_t signedCls = (uintptr_t)ptrauth_sign_unauthenticated((void *)newCls, ISA_SIGNING_KEY, ptrauth_blend_discriminator(obj, ISA_SIGNING_DISCRIMINATOR));
# else
# error Unknown isa signing mode.
# endif
shiftcls_and_sig = signedCls >> 3;
#elif SUPPORT_INDEXED_ISA
// Indexed isa only uses this method to set a raw pointer class.
// Setting an indexed class is handled separately.
cls = newCls;
#else // Nonpointer isa, no ptrauth
shiftcls = (uintptr_t)newCls >> 3;
#endif
}

4.3 Nonpointer isa 和 纯isa的区别

  • Nonpointer isa
    • 类对象会被赋值到isa_tBITFIELD中的shiftcls
  • isa
    • 会被直接赋值到isa_tcls
  • 两者互斥
  • 默认创建的对象都是Nonpointer isa
    • 可通过修改Xcode环境变量改成使用纯isa