iOS底层-NSObject对象本质与isa

2021年6月14日 写评论

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。

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
  • 在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

在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位
      • 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_t的BITFIELD中的shiftcls
    • 纯isa
      • 会被直接赋值到isa_t的cls
    • 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

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